AZT action on HIV

Zidovudine (INN) or azidothymidine (AZT) (also called ZDV) is an antiretroviral drug, the first approved for treatment of HIV. It is also sold under the names Retrovir® and Retrovis®, and as an ingredient in Combivir® and Trizivir®. It is an analog of thymidine.

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Like other reverse transcriptase inhibitors, AZT works by inhibiting the action of reverse transcriptase, the enzyme that HIV uses to make a DNA copy of its RNA. The viral double-stranded DNA is subsequently spliced into the DNA of a target cell, where it is called a provirus.

Mode of action

The azido group increases the lipophilic nature of AZT, allowing it to cross cell membranes easily by diffusion and thereby also to cross the blood-brain barrier. Cellular enzymes convert AZT into the effective 5'-triphosphate form. Studies have shown that the termination of the formed DNA chains is the specific factor in the inhibitory effect.

The triphosphate form also has some ability to inhibit cellular DNA polymerase, which is used by normal cells as part of cell division. However, AZT has a 100- to 300-fold greater affinity for the HIV reverse transcriptase, as compared to the human DNA polymerase, accounting for its selective antiviral activity.A special kind of cellular DNA polymerase that replicates the DNA in mitochondria is relatively more sensitive to inhibition by AZT, and this accounts for certain toxicities such as damage to cardiac and other muscles

Viral resistance

AZT does not destroy the HIV infection, but only delays the progression of the disease and the replication of virus, even at very high doses. During prolonged AZT treatment HIV has the ability to gain an increased resistance to AZT by mutation of the reverse transcriptase. A study showed that AZT could not impede the resumption of virus production, and eventually cells treated with AZT produced viruses as much as the untreated cells. So as to slow the development of resistance, it is generally recommended that AZT be given in combination with another reverse transcriptase inhibitor and an antiretroviral from another group, such as a protease inhibitor or a non-nucleoside reverse transcriptase inhibitor.

Cytokinesis Swings into the Cell's Center

Joe Lutkenhaus describes how a β strand-to-α helix transition activates MinE and sets up oscillatory waves of MinE and MinD along the cell membrane



Interferons—Provoking Distinct Signals through the Same Receptor

Chris Garcia explains how the strength of the receptor-ligand interaction—not receptor conformational changes—tunes signaling duration.

New Way to Make Induced Pluripotent Stem Cells

Dr. Edward Morrisey at the University of Pennsylvania's Department of Medicine reports on a new way to make induced pluripotent stem cells with miRNA-based reprogramming via the miR302/367 cluster.


Gliadel Wafer Mechanism

Gliadel® Wafer (polifeprosan 20 with carmustine implant) is indicated in newly diagnosed patients with high-grade malignant glioma as an adjunct to surgery and radiation. Gliadel is also indicated in recurrent glioblastoma multiforme patients as an adjunct to surgery. Gliadel provides localized delivery of chemotherapy directly to the site of the tumor and is the only FDA approved brain cancer treatment capable of doing so.



Carmustine is one of the nitrosoureas indicated as palliative therapy as a single agent or in established combination therapy with other approved chemotherapeutic agents in treatment of brain tumors, multiple myeloma, Hodgkin's disease, and non-Hodgkin's lymphomas. Although it is generally agreed that Carmustine alkylates DNA and RNA, it is not cross resistant with other alkylators. As with other nitrosoureas, it may also inhibit several key enzymatic processes by carbamoylation of amino acids in proteins. For the treatment of brain tumors, multiple myeloma, Hodgkin's disease and Non-Hodgkin's lymphomas.

Mechanism Of Action:
Carmustine causes cross-links in DNA and RNA, leading to the inhibition of DNA synthesis, RNA production and RNA translation (protein synthesis). Carmustine also binds to and modifies (carbamoylates) glutathione reductase. This leads to cell death.

Electron transport chain

An electron transport chain associates electron carriers (such as NADH and FADH2) and mediating biochemical reactions that produce adenosine triphosphate (ATP), which is the energy currency of life. Only two sources of energy are available to living organisms: oxidation-reduction (redox) reactions and sunlight (used for photosynthesis). Organisms that use redox reactions to produce ATP are called chemotrophs. Organisms that use sunlight are called phototrophs. Both chemotrophs and phototrophs use electron transport chains to convert energy into ATP. This is achieved through a three-step process:



  • Gradually sap energy from high-energy electrons in a series of individual steps
  • Use that energy to forcibly unbalance the proton concentration across the membrane, creating an electrochemical gradient
  • Use the energy released by the drive to rebalance the proton distribution as a means of producing ATP.



Background
The Electron Transport Chain is also called the ETC. ATP is made by an enzyme called ATP synthase. The structure of this enzyme and its underlying genetic code is remarkably conserved in all known forms of life.

ATP synthase is powered by a transmembrane electrochemical potential gradient, usually in the form of a proton gradient. The function of the electron transport chain is to produce this gradient. In all living organisms, a series of redox reactions is used to produce a transmembrane electrochemical potential gradient.

Redox reactions are chemical reactions in which electrons are transferred from a donor molecule to an acceptor molecule. The underlying force driving these reactions is the Gibbs free energy of the reactants and products. The Gibbs free energy is the energy available ("free") to do work. Any reaction that decreases the overall Gibbs free energy of a system will proceed spontaneously.

The transfer of electrons from a high-energy molecule (the donor) to a lower-energy molecule (the acceptor) can be spatially separated into a series of intermediate redox reactions. This is an electron transport chain.

The fact that a reaction is thermodynamically possible does not mean that it will actually occur; for example, a mixture of hydrogen gas and oxygen gas does not spontaneously ignite. It is necessary either to supply an activation energy or to lower the intrinsic activation energy of the system, in order to make most biochemical reactions proceed at a useful rate. Living systems use complex macromolecular structures (enzymes) to lower the activation energies of biochemical reactions.

It is possible to couple a thermodynamically favorable reaction (a transition from a high-energy state to a lower-energy state) to a thermodynamically unfavorable reaction (such as a separation of charges, or the creation of an osmotic gradient), in such a way that the overall free energy of the system decreases (making it thermodynamically possible), while useful work is done at the same time. Biological macromolecules that catalyze a thermodynamically unfavorable reaction if and only if a thermodynamically favorable reaction occurs simultaneously underlie all known forms of life.

Electron transport chains capture energy in the form of a transmembrane electrochemical potential gradient. This energy can then be harnessed to do useful work. The gradient can be used to transport molecules across membranes. It can be used to do mechanical work, such as rotating bacterial flagella. It can be used to produce ATP high-energy molecules that are necessary for growth.

A small amount of ATP is available from substrate-level phosphorylation (for example, in glycolysis). Some organisms can obtain ATP exclusively by fermentation. In most organisms, however, the majority of ATP is generated by electron transport chains.

Electron transport chains in mitochondria
The cells of all eukaryotes (all animals, plants, fungi, algae, protozoa – in other words, all living things except bacteria and archaea) contain intracellular organelles called mitochondria, which produce ATP. Energy sources such as glucose are initially metabolized in the cytoplasm. The products are imported into mitochondria. Mitochondria continue the process of catabolism using metabolic pathways including the Krebs cycle, fatty acid oxidation, and amino acid oxidation.

The end result of these pathways is the production of two kinds of energy-rich electron donors, NADH and FADH2. Electrons from these donors are passed through an electron transport chain to oxygen, which is reduced to water. This is a multi-step redox process that occurs on the mitochondrial inner membrane. The enzymes that catalyze these reactions have the remarkable ability to simultaneously create a proton gradient across the membrane, producing a thermodynamically unlikely high-energy state with the potential to do work. Although electron transport occurs with great efficiency, a small percentage of electrons are prematurely leaked to oxygen, resulting in the formation of the toxic free-radical superoxide.

The similarity between intracellular mitochondria and free-living bacteria is striking. The known structural, functional, and DNA similarities between mitochondria and bacteria provide strong evidence that mitochondria evolved from intracellular prokaryotic symbionts that took up residence in primitive eukaryotic cells.


Four membrane-bound complexes have been identified in mitochondria. Each is an extremely complex transmembrane structure that is embedded in the inner membrane. Three of them are proton pumps. The structures are electrically connected by lipid-soluble electron carriers and water-soluble electron carriers. The overall electron transport chain




Complex I



Complex I (NADH dehydrogenase, also called NADH:ubiquinone oxidoreductase; EC 1.6.5.3) removes two electrons from NADH and transfers them to a lipid-soluble carrier, ubiquinone (Q). The reduced product, ubiquinol (QH2) is free to diffuse within the membrane. At the same time, Complex I moves four protons (H+) across the membrane, producing a proton gradient. Complex I is one of the main sites at which premature electron leakage to oxygen occurs, thus being one of main sites of production of a harmful free radical called superoxide.

The pathway of electrons occurs as follows:

NADH is oxidized to NAD+, reducing Flavin mononucleotide to FMNH2 in one two-electron step. The next electron carrier is a Fe-S cluster, which can only accept one electron at a time to reduce the ferric ion into a ferrous ion. In a convenient manner, FMNH2 can be oxidized in only two one-electron steps, through a semiquinone intermediate. The electron thus travels from the FMNH2 to the Fe-S cluster, then from the Fe-S cluster to the oxidized Q to give the free-radical (semiquinone) form of Q. This happens again to reduce the semiquinone form to the ubiquinol form, QH2. During this process, four protons are translocated across the inner mitochondrial membrane, from the matrix to the intermembrane space. This creates a proton gradient that will be later used to generate ATP through oxidative phosphorylation.


Complex II



Complex II (succinate dehydrogenase; EC 1.3.5.1) is not a proton pump. It serves to funnel additional electrons into the quinone pool (Q) by removing electrons from succinate and transferring them (via FAD) to Q. Complex II consists of four protein subunits: SDHA,SDHB,SDHC, and SDHD. Other electron donors (e.g., fatty acids and glycerol 3-phosphate) also funnel electrons into Q (via FAD), again without producing a proton gradient.


Complex III

Complex III (cytochrome bc1 complex; EC 1.10.2.2) removes in a stepwise fashion two electrons from QH2 and transfers them to two molecules of cytochrome c, a water-soluble electron carrier located within the intermembrane space. At the same time, it moves two protons across the membrane, producing a proton gradient (in total 4 protons: 2 protons are translocated and 2 protons are released from ubiquinol). When electron transfer is hindered (by a high membrane potential, point mutations or respiratory inhibitors such as antimycin A), Complex III may leak electrons to oxygen resulting in the formation of superoxide, a highly-toxic species, which is thought to contribute to the pathology of a number of diseases, including aging.

Complex IV

Complex IV (cytochrome c oxidase; EC 1.9.3.1) removes four electrons from four molecules of cytochrome c and transfers them to molecular oxygen (O2), producing two molecules of water (H2O). At the same time, it moves four protons across the membrane, producing a proton gradient.


Coupling with oxidative phosphorylation

The chemiosmotic coupling hypothesis, as proposed by Nobel Prize in Chemistry winner Peter D. Mitchell, explains that the electron transport chain and oxidative phosphorylation are coupled by a proton gradient across the inner mitochondrial membrane. The efflux of protons creates both a pH gradient and an electrochemical gradient. This proton gradient is used by the FOF1 ATP synthase complex to make ATP via oxidative phosphorylation. ATP synthase is sometimes regarded as complex V of the electron transport chain. The FO component of ATP synthase acts as an ion channel for return of protons back to mitochondrial matrix. During their return, the free energy produced during the generation of the oxidized forms of the electron carriers (NAD+ and FAD) is released. This energy is used to drive ATP synthesis, catalyzed by the F1 component of the complex.
Coupling with oxidative phosphorylation is a key step for ATP production. However, in certain cases, uncoupling may be biologically useful. The inner mitochondrial membrane of brown adipose tissue contains a large amount of thermogenin (an uncoupling protein), which acts as uncoupler by forming an alternative pathway for the flow of protons back to matrix. This results in consumption of energy in thermogenesis rather than ATP production. This may be useful in cases when heat production is required, for example in colds or during arise of hibernating animals. Synthetic uncouplers (e.g., 2,4-dinitrophenol) also exist, and, at high doses, are lethal.

Glycolysis


Glycolysis (from glycose, an older term for glucose + -lysis degradation) is the metabolic pathway that converts glucose, C6H12O6, into pyruvate, C3H5O3-. The free energy released in this process is used to form the high energy compounds, ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide).
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Glycolysis is a sequence of ten reactions involving ten intermediate compounds (one of the steps involves two intermediates). The intermediates provide entry points to glycolysis. For example, most monosaccharides, such as fructose, glucose, and galactose, can be converted to one of these intermediates. The intermediates may also be directly useful. For example, the intermediate dihydroxyacetone phosphate is a source of the glycerol that combines with fatty acids to form fat.
Glycolysis is the archetype of a universal metabolic pathway. It occurs, with variations, in nearly all organisms, both aerobic and anaerobic. The wide occurrence of glycolysis indicates that it is one of the most ancient metabolic pathways known.


The most common and well-known type of glycolysis is the Embden-Meyerhof pathway, which was elucidated by Gustav Embden and Otto Meyerhof.

Preparatory phase

The first five steps are regarded as the preparatory (or investment) phase since they consume energy to convert the glucose into two three-carbon sugar phosphate.

The first step in glycolysis is phosphorylation of glucose by a family of enzymes called hexokinases to form glucose 6-phosphate (G6P). This reaction consumes ATP, but it acts to keep the glucose concentration low, promoting continuous transport of glucose into the cell through the plasma membrane transporters. In addition, it blocks the glucose from leaking out - the cell lacks transporters for G6P. Glucose may alternatively be from the phosphorolysis or hydrolysis of intracellular starch or glycogen.

In animals, an isozyme of hexokinase called glucokinase is also used in the liver, which has a much lower affinity for glucose (Km in the vicinity of normal glycemia), and differs in regulatory properties. The different substrate affinity and alternate regulation of this enzyme are a reflection of the role of the liver in maintaining blood sugar levels.

Cofactors: Mg2+

G6P is then rearranged into fructose 6-phosphate (F6P) by glucose phosphate isomerase. Fructose can also enter the glycolytic pathway by phosphorylation at this point.

The change in structure is an isomerization, in which the G6P has been converted to F6P. The reaction requires an enzyme, phosphohexose isomerase, to proceed. This reaction is freely reversible under normal cell conditions. However, it is often driven forward because of a low concentration of F6P, which is constantly consumed during the next step of glycolysis. Under conditions of high F6P concentration this reaction readily runs in reverse. This phenomenon can be explained through Le Chatelier's Principle.

The energy expenditure of another ATP in this step is justified in 2 ways: The glycolytic process (up to this step) is now irreversible, and the energy supplied destabilizes the molecule. Because the reaction catalyzed by Phosphofructokinase 1 (PFK-1) is energetically very favorable, it is essentially irreversible, and a different pathway must be used to do the reverse conversion during gluconeogenesis. This makes the reaction a key regulatory point (see below).

The same reaction can also be catalysed by pyrophosphate dependent phosphofructokinase (PFP or PPi-PFK), which is found in most plants, some bacteria, archea and protists but not in animals. This enzyme uses pyrophosphate (PPi) as a phosphate donor instead of ATP. It is a reversible reaction, increasing the flexibility of glycolytic metabolism. A rarer ADP-dependent PFK enzyme variant has been identified in archaean species.

Cofactors: Mg2+

Destabilizing the molecule in the previous reaction allows the hexose ring to be split by aldolase into two triose sugars, dihydroxyacetone phosphate, a ketone, and glyceraldehyde 3-phosphate, an aldehyde. There are two classes of aldolases: class I aldolases, present in animals and plants, and class II aldolases which present in fungi and bacteria; the two classes use different mechanisms in cleaving the hexose ring.

Triosephosphate isomerase rapidly interconverts dihydroxyacetone phosphate with glyceraldehyde 3-phosphate (GADP) that proceeds further into glycolysis. This is advantageous, as it directs dihydroxyacetone phosphate down the same pathway as glyceraldehyde 3-phosphate, simplifying regulation.


Pay-off phase
The second half of glycolysis is known as the pay-off phase, characterised by a net gain of the energy-rich molecules ATP and NADH. Since glucose leads to two triose sugars in the preparatory phase, each reaction in the pay-off phase occurs twice per glucose molecule. This yields 2 NADH molecules and 4 ATP molecules, leading to a net gain of 2 NADH molecules and 2 ATP molecules from the glycolytic pathway per glucose.

The triose sugars are dehydrogenated and inorganic phosphate is added to them, forming 1,3-bisphosphoglycerate.

The hydrogen is used to reduce two molecules of NAD+, a hydrogen carrier, to give NADH + H+.

This step is the enzymatic transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP by phosphoglycerate kinase, forming ATP and 3-phosphoglycerate. At this step, glycolysis has reached the break-even point: 2 molecules of ATP were consumed, and 2 new molecules have now been synthesized. This step, one of the two substrate-level phosphorylation steps, requires ADP; thus, when the cell has plenty of ATP (and little ADP), this reaction does not occur. Because ATP decays relatively quickly when it is not metabolized, this is an important regulatory point in the glycolytic pathway.

Cofactors: Mg2+

Phosphoglycerate mutase now forms 2-phosphoglycerate. Notice that this enzyme is a mutase and not an isomerase. Whereas an isomerase changes the oxidation state of the carbons of the compound, a mutase does not.

Enolase next forms phosphoenolpyruvate from 2-phosphoglycerate.

Cofactors: 2 Mg2+: one "conformational" ion to coordinate with the carboxylate group of the substrate, and one "catalytic" ion which participates in the dehydration.

A final substrate-level phosphorylation now forms a molecule of pyruvate and a molecule of ATP by means of the enzyme pyruvate kinase. This serves as an additional regulatory step, similar to the phosphoglycerate kinase step.

Cofactors: Mg2+

Cancer Metastasis: CXCR4


CXCR4, also called fusin, is an alpha-chemokine receptor specific for stromal-derived-factor-1 (SDF-1 also called CXCL12), a molecule endowed with potent chemotactic activity for lymphocytes.
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Metastasis shares many similarities with leukocyte trafficking. Among those chemokine receptors thought to be involved in hemopoietic cell homing, stromal cell-derived factor-1 and its receptor CXC chemokine receptor-4 (CXCR4) have received considerable attention. Like hemopoietic cell homing, levels of stromal cell-derived factor-1 are high at sites of breast cancer metastasis including lymph node, lung, liver, and the marrow. Moreover, CXCR4 expression is low in normal breast tissues and high in malignant tumors, suggesting that a blockade of CXCR4 might limit tumor metastasis investigating the role of a synthetic antagonist 14-mer peptide (TN14003) in inhibiting metastasis in an animal model. Not only was TN14003 effective in limiting metastasis of breast cancer by inhibiting migration, but it may also prove useful as a diagnostic tool to identify CXCR4 receptor-positive tumor cells in culture and tumors in paraffin-embedded clinical samples.


CXC chemokine receptor 4 (CXCR4) has been shown to play a critical role in chemotaxis and homing, which are key steps in cancer metastasis. There is also increasing evidence that links this receptor to angiogenesis; however, its molecular basis remains elusive. Vascular endothelial growth factor (VEGF), one of the major angiogenic factors, promotes the formation of leaky tumor vasculatures that are the hallmarks of tumor progression. On investigating whether CXCR4 induces the expression of VEGF through the PI3K/Akt pathway. Results showed that CXCR4/CXCL12 induced Akt phosphorylation, which resulted in upregulation of VEGF at both the mRNA and protein levels. Conversely, blocking the activation of Akt signaling led to a decrease in VEGF protein levels; blocking CXCR4/CXCL12 interaction with a CXCR4 antagonist suppressed tumor angiogenesis and growth in vivo. Furthermore, VEGF mRNA levels correlated well with CXCR4 mRNA levels in patient tumor samples. In summary, our study demonstrates that the CXCR4/CXCL12 signaling axis can induce angiogenesis and progression of tumors by increasing expression of VEGF through the activation of PI3K/Akt pathway. Findings suggest that targeting CXCR4 could provide a potential new anti-angiogenic therapy to suppress the formation of both primary and metastatic tumors.

Color Blindness

Color blindness, a color vision deficiency, is the inability to perceive differences between some of the colors that others can distinguish. It is most often of genetic nature, but may also occur because of eye, nerve, or brain damage, or due to exposure to certain chemicals. The English chemist John Dalton published the first scientific paper on the subject in 1798, "Extraordinary facts relating to the vision of colours", after the realization of his own color blindness; because of Dalton's work, the condition is sometimes called daltonism, although this term is now used for a type of color blindness called deuteranopia.


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Color blindness is usually classed as disability; in certain situations, however, color blind people have an advantage over people with normal color vision. There are some studies which conclude that color blind individuals are better at penetrating certain camouflages.

Metastasis

Metastasis, is the spread of a disease from one organ or part to another non-adjacent organ or part. Only malignant tumor cells and infections have the capacity to metastasize.

Cancer cells can "break away", "leak", or "spill" from a primary tumor, enter lymphatic and blood vessels, circulate through the bloodstream, and settle down to grow within normal tissues elsewhere in the body. Metastasis is one of three hallmarks of malignancy (contrast benign tumors). Most tumors and other neoplasms can metastasize, although in varying degrees, barring a few exceptions (e.g., Glioma and Basal cell carcinoma never metastasize).

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When tumor cells metastasize, the new tumor is called a secondary or metastatic tumor, and its cells are like those in the original tumor. This means, for example, that, if breast cancer spreads (metastasizes) to the lung, the secondary tumor is made up of abnormal breast cells, not of abnormal lung cells. The tumor in the lung is then called metastatic breast cancer, not lung cancer.


Modes and sites of metastatic dispersal



Metastatic tumors are very common in the late stages of cancer. The spread of metastases may occur via the blood or the lymphatics or through both routes. The most common places for the metastases to occur are the adrenals, liver, brain, and the bones.[ There is also a propensity for certain tumors to seed in particular organs. This was first discussed as the "seed and soil" theory by Stephen Paget over a century ago in 1889. For example, prostate cancer usually metastasizes to the bones. In a similar manner, colon cancer has a tendency to metastasize to the liver. Stomach cancer often metastasizes to the ovary in women, then it is called a Krukenberg tumor. It is difficult for cancer cells to survive outside their region of origin, so in order to metastasize they must find a location with similar characteristics.

For example, breast tumor cells, which gather calcium ions from breast milk, metastasize to bone tissue, where they can gather calcium ions from bone. Malignant melanoma spreads to the brain, presumably because neural tissue and melanocytes arise from the same cell line in the embryo.

Cancer cells may spread to lymph nodes (regional lymph nodes) near the primary tumor. This is called nodal involvement, positive nodes, or regional disease. Localized spread to regional lymph nodes near the primary tumor is not normally counted as metastasis, although this is a sign of worse prognosis.

In addition to the above routes, metastasis may occur by direct seeding, e.g., in the peritoneal cavity or pleural cavity.



Factors involved



Metastasis is a complex series of steps in which cancer cells leave the original tumor site and migrate to other parts of the body via the bloodstream or the lymphatic system. To do so, malignant cells break away from the primary tumor and attach to and degrade proteins that make up the surrounding extracellular matrix (ECM), which separates the tumor from adjoining tissue. By degrading these proteins, cancer cells are able to breach the ECM and escape. When oral cancers metastasize, they commonly travel through the lymph system to the lymph nodes in the neck.

Cancer researchers studying the conditions necessary for cancer metastasis have discovered that one of the critical events required is the growth of a new network of blood vessels, called tumor angiogenesis.] It has been found that angiogenesis inhibitors would therefore prevent metastasis.


Metastasis and primary cancer



It is theorized that metastasis always coincides with a primary cancer, and, as such, is a tumor that started from a cancer cell or cells in another part of the body. However, over 10% of patients presenting to oncology units will have metastases without a primary tumor found. In these cases, doctors refer to the primary tumor as "unknown" or "occult," and the patient is said to have cancer of unknown primary origin (CUP) or Unknown Primary Tumors (UPT). It is estimated that 3% of all cancers are of unknown primary origin.Studies have shown that, if simple questioning does not reveal the cancer's source (coughing up blood -'probably lung', urinating blood - 'probably bladder'), complex imaging will not either. In some of these cases a primary may appear later.

The use of immunohistochemistry has permitted pathologists to give an identity to many of these metastases. However, imaging of the indicated area only occasionally reveals a primary. In rare cases (e.g., of melanoma), no primary tumor is found, even on autopsy. It is therefore thought that some primary tumors can regress completely, but leave their metastases behind.


Common sites of origin



  • Lung
  • Breast
  • Skin: Melanoma (other skin tumors rarely metastasize)
  • Colon
  • Kidney
  • Prostate
  • Pancreas


Diagnosis of primary and secondary tumors

The cells in a metastatic tumor resemble those in the primary tumor. Once the cancerous tissue is examined under a microscope to determine the cell type, a doctor can usually tell whether that type of cell is normally found in the part of the body from which the tissue sample was taken.

For instance, breast cancer cells look the same whether they are found in the breast or have spread to another part of the body. So, if a tissue sample taken from a tumor in the lung contains cells that look like breast cells, the doctor determines that the lung tumor is a secondary tumor. Still, the determination of the primary tumor can often be very difficult, and the pathologist may have to use several adjuvant techniques, such as immunohistochemistry, FISH (fluorescent in situ hybridization), and others. Despite the use of techniques, in some cases the primary tumor remains unidentified.

Metastatic cancers may be found at the same time as the primary tumor, or months or years later. When a second tumor is found in a patient that has been treated for cancer in the past, it is more often a metastasis than another primary tumor.

Deep brain stimulation & Parkinson's Disease

Deep brain stimulation (DBS) is a surgical treatment involving the implantation of a medical device called a brain pacemaker, which sends electrical impulses to specific parts of the brain. DBS in select brain regions has provided remarkable therapeutic benefits for otherwise treatment-resistant movement and affective disorders such as chronic pain, Parkinson’s disease, tremor and dystonia [1]. Yet, despite the long history of DBS [2], its underlying principles and mechanisms are still not clear. DBS directly changes brain activity in a controlled manner, its effects are reversible (unlike those of lesioning techniques) and is one of only a few neurosurgical methods that allows blinded studies.



The Food and Drug Administration (FDA) approved DBS as a treatment for essential tremor in 1997, for Parkinson's disease in 2002, and dystonia in 2003 . DBS is also routinely used to treat chronic pain and has been used to treat various affective disorders including clinical depression. While DBS has proven helpful for some patients , there is potential for serious complications and side effects.

The deep brain stimulation system consists of three components: the implanted pulse generator (IPG), the lead, and the extension. The IPG is a battery powered neurostimulator encased in a titanium housing, which sends electrical pulses to the brain to interfere with neural activity at the target site. The lead is a coiled wire insulated in polyurethane with four platinum iridium electrodes and is placed in one of three areas of the brain. The lead is connected to the IPG by the extension, an insulated wire that runs from the head, down the side of the neck, behind the ear to the IPG, which is placed subcutaneously below the clavicle or in some cases, the abdomen. The IPG can be calibrated by a neurologist, nurse or trained technician to optimize symptom suppression and control side effects.

DBS leads are placed in the brain according to the type of symptoms to be addressed. For essential tremor and Parkinsonian tremors, the lead is placed in the thalamus. For dystonia and symptoms associated with Parkinson's disease (rigidity, bradykinesia/akinesia and tremor), the lead may be placed in either the globus pallidus or subthalamic nucleus.

All three components are surgically implanted inside the body. The right side of the brain is stimulated to address symptoms on the left side of the body and vice versa.

Procedure
The procedure begins with preoperative identification of the neurosurgical target with computed tomography (CT), magnetic resonance imaging (MRI) or in earlier times ventriculography.[9]

During surgery, the patient is given local anesthesia and remains awake. A craniotomy is performed and a DBS lead is placed either unilaterally or bilaterally, depending on the patient's symptoms. Microelectrode recording may be used to more precisely locate the desired target within the brain. The IPG and extension are then implanted and connected to each lead.


Depending on the procedures of the medical facility, all components of the DBS system may not be implanted during a single surgery. After surgery is completed, the IPG is calibrated to maximize its effectiveness. Programming can take up to a year to achieve optimal settings.

Due to battery depletion, the IPG must be replaced—usually after three to five years, depending on the settings used. The entire unit is replaced to maintain an uncontaminated field within the body. Nevertheless, this is a minor surgical procedure involving only the shallow subclavicular pocket where the IPG resides. Remaining battery life may be reliably determined with a telemetric programmer so that arrangements can be made to replace the unit prior to battery failure.

Parkinson's disease
Parkinson's disease is a disorder that affects nerve cells, or neurons, in a part of the brain that controls muscle movement. In Parkinson's, neurons that make a chemical called dopamine die or do not work properly. Dopamine normally sends signals that help coordinate your movements. No one knows what damages these cells. Symptoms of Parkinson's disease may include



  • Trembling of hands, arms, legs, jaw and face
  • Stiffness of the arms, legs and trunk
  • Slowness of movement
  • Poor balance and coordination

As symptoms get worse, people with the disease may have trouble walking, talking or doing simple tasks. They may also have problems such as depression, sleep problems or trouble chewing, swallowing or speaking.

Parkinson's usually begins around age 60, but it can start earlier. It is more common in men than in women. There is no cure for Parkinson's disease. A variety of medicines sometimes help symptoms dramatically.

Nerve Impulse

The conduction of nerve impulses relies upon the movement of electrically charge ions across the nerve cell membrane. When a nerve is resting, or polorized, there are more potassium ions than sodium ions inside the cell, with an opposite ratio outside. Sodium ions are actively kept out of the cell by an energy consuming pump mechanism. This maintains a negative charge on the inside of the cell and a positive charge on the outside. When an impulse travels along the nerve, sodium ions flood into the cell and make the inside of the cell positive with respect to the outside. This produces a rise in the electrical potential across the cell membrane. After the impulse has passed, potassium ions leave the cell, restoring the negative charge within the cell and the positive charge outside it. While this resting situation is being restored another impulse cannot be generated.
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Ion Conductances

The generation of action potentials is mainly due to the changes of sodium (Na+) and potassium (K+) conductances. Figure 2 shows the concentrations of Na+ and K+ ions on both sides of a nerve membrane. For Na+, its concentration on the extracellular side is much higher than inside. We immediately notice that Na+ ions are far from electrochemical equilibrium -- both the electric force due to electric potential difference and the chemical force due to ion concentration difference are pointing inward. How could the nerve membrane maintain such a stable state? This is because the conductance of Na+ ions in the membrane is very small at the resting membrane potential. Although the inward driving force is large, the resulting Na+ influx is small. This small influx can be balanced by a slow ion transport process, the Na+-K+ pump, which moves the Na+ ions outward and simultaneously the K+ ions inward.



The conductance of Na+ ions may change dramatically with the membrane potential as demonstrated by voltage clamp experiments, in which the membrane potential is displaced to a new value and maintained there . Because ions carry charges, the movement of ions across the membrane will change the membrane potential. To maintain a constant membrane potential, the voltage clamp circuit must generate electric currents to neutralize the membrane potential change caused by the ionic flux. Thus, the ion current through the membrane is reflected in the electric current of the voltage clamp circuit outside the membrane

Alzheimer's disease

The first readily identified symptoms of Alzheimer's disease are usually short-term memory loss and visual-spatial confusion. These initial symptoms progress from seemingly simple and often fluctuating forgetfulness and difficulty orienting oneself in space such as in a traffic lane while driving, to a more pervasive loss of short-term memory and difficulty navigating through familiar areas such as one's neighborhood, then to loss of other familiar and well-known skills as well as recognition of objects and persons.


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Since family members are often the first to notice changes that might indicate the onset of Alzheimer's they should learn the early warning signs and serve as informants during initial evaluation of patients clinically. Aphasia, disorientation and disinhibition often accompany the loss of memory. Alzheimer's disease (AD) may also include behavioral changes, such as outbursts of violence or excessive passivity in people who have no previous history of such behavior.

In the later stages of the disease, deterioration of musculature and mobility, leading to bedfastness, inability to feed oneself, and incontinence, will be seen if death from some external cause (e.g. heart attack or pneumonia) does not intervene. Once identified, the average lifespan of patients living with Alzheimer's disease is approximately 7-10 years, although cases are known where reaching the final stage occurs within 4-5 years or at the other extreme they may survive up to 21 years.


Stages and symptoms
Mild — In the early stage of the disease, patients have a tendency to become less energetic or spontaneous, though changes in their behavior often go unnoticed even by the patients' immediate family. This stage of the disease has also been termed Minor Cognitive Impairment (MCI), when the patient does not meet the criteria for a diagnosis of dementia.
Moderate — As the disease progresses to the middle stage, patients might still be able to perform tasks independently (such as using the bathroom), but may need assistance with more complicated activities (such as paying bills).
Severe — As the disease progresses from the middle to the late stage, patients will not be able to perform even simple tasks independently and will require constant supervision. They become incontinent of bladder and then incontinent of bowel. They will eventually lose the ability to walk and eat without assistance. Language becomes severely disorganized, and then is lost altogether. They may eventually lose the ability to swallow food and fluid, and this can ultimately lead to death.


Diagnosis

No medical tests are available to diagnose Alzheimer's disease conclusively pre-mortem. A definitive diagnosis of Alzheimer's disease must await microscopic examination of brain tissue which generally occurs at autopsy therefore Alzheimer's disease (AD) is primarily a clinically diagnosed condition based on the presence of characteristic neurological and neuropsychological features and the absence of alternative diagnoses. Determination of neurological characteristics is made utilizing patient history and clinical observation, while neuropsychological evaluation includes memory testing and assessment of intellectual functioning over a series of weeks or months. Supplemental physical testing, including blood tests and neuroimaging, is utilized to rule out other diagnoses. Psychological testing, to include screening for depression and a mini mental state examination, can be helpful in establishing the presence and severity of dementia. Although certain clues from history may suggest a diagnosis of vascular dementias instead of, or in addition to, AD, the ability of certain neuroimaging modalities such as SPECT to differentiate vascular type from Alzheimer disease types of dementias, appears to be superior to clinical exam (PMID 15545324). Differential diagnosis should also include dementia with Lewy bodies and frontotemporal dementia.

Interviews with family members and/or caregivers are also utilized in the initial assessment of the disease, as a patient with Alzheimer's may tend to minimize his or her symptoms, or may undergo evaluation at a time when his or her symptoms are less apparent, as quotidian fluctuations ("good days and bad days") are a common feature of the disease. Observations noting that a patient's good memory function decreases over time plays a critical role in the diagnosis of Alzheimer's.

Biochemical characteristics

Alzheimer's disease has been identified as a protein misfolding disease, or proteopathy, due to the accumulation of abnormally folded A-beta and tau proteins in the brains of AD patients. A-beta, also written Aβ, is a short peptide that is a proteolytic byproduct of the transmembrane protein amyloid precursor protein (APP), whose function is unclear but thought to be involved in neuronal development. The presenilins are components of a proteolytic complex involved in APP processing and degradation. Although amyloid beta monomers are soluble and harmless, they undergo a dramatic conformational change at sufficiently high concentration to form a beta sheet-rich tertiary structure that aggregates to form amyloid fibrils that deposit outside neurons in dense formations known as senile plaques or neuritic plaques, in less dense aggregates as diffuse plaques, and sometimes in the walls of small blood vessels in the brain in a process called amyloid angiopathy or congophilic angiopathy.

AD is also considered a tauopathy due to abnormal aggregation of the tau protein, a microtubule-associated protein expressed in neurons that normally acts to stabilize microtubules in the cell cytoskeleton. Like most microtubule-associated proteins, tau is normally regulated by phosphorylation; however, in AD patients, hyperphosphorylated tau accumulates as paired helical filaments that in turn aggregate into masses inside nerve cell bodies known as neurofibrillary tangles and as dystrophic neurites associated with amyloid plaques.

Neuropathology

Both amyloid plaques and neurofibrillary tangles are clearly visible by microscopy in AD brains. At an anatomical level, AD is characterized by gross diffuse atrophy of the brain and loss of neurons, neuronal processes and synapses in the cerebral cortex and certain subcortical regions. This results in gross atrophy of the affected regions, including degeneration in the temporal lobe and parietal lobe, and parts of the frontal cortex and cingulate gyrus. Levels of the neurotransmitter acetylcholine are reduced. Levels of the neurotransmitters serotonin, norepinephrine, and somatostatin are also often reduced. Glutamate levels are usually elevated

Disease mechanism

Three major competing hypotheses exist to explain the cause of the disease. The oldest, on which most currently available drug therapies are based, is known as the "cholinergic hypothesis" and suggests that AD is due to reduced biosynthesis of the neurotransmitter acetylcholine. The medications that treat acetylcholine deficiency have served to only treat symptoms of the disease and have neither halted nor reversed it. The cholinergic hypothesis has not maintained widespread support in the face of this evidence, although cholingeric effects have been proposed to initiate large-scale aggregation leading to generalized neuroinflammation.

Research after 2000 includes hypotheses centered on the effects of the misfolded and aggregated proteins, amyloid beta and tau. The two positions differ with one stating that the tau protein abnormalities initiate the disease cascade, while the other states that amyloid beta (Aβ) deposits are the causative factor in the disease. The tau hypothesis is supported by the long-standing observation that deposition of amyloid plaques do not correlate well with neuron loss; however, a majority of researchers support the alternative hypothesis that Aβ is the primary causative agent.

The amyloid hypothesis is initially compelling because the gene for the amyloid beta precursor (APP) is located on chromosome 21, and patients with trisomy 21 (Down Syndrome) who thus have an extra gene copy almost universally exhibit AD-like disorders by 40 years of age. The traditional formulation of the amyloid hypothesis points to the cytotoxicity of mature aggregated amyloid fibrils, which are believed to be the toxic form of the protein responsible for disrupting the cell's calcium ion homeostasis and thus inducing apoptosis. A more recent and widely supported hypothesis suggests that the cytotoxic species is an intermediate misfolded form of Aβ, neither a soluble monomer nor a mature aggregated polymer but an oligomeric species. Relevantly, much early development work on lead compounds has focused on the inhibition of fibrillization, but the toxic-oligomer theory would imply that prevention of oligomeric assembly is the more important process or that a better target lies upstream, for example in the inhibition of APP processing to amyloid beta.

It should be noted further that ApoE4, the major genetic risk factor for AD, leads to excess amyloid build up in the brain before AD symptoms arise. Thus, Aβ deposition precedes clinical AD. Another strong support for the amyloid hypothesis, which looks at Aβ as the common initiating factor for Alzheimer's disease, is that transgenic mice solely expressing a mutant human APP gene develop first diffuse and then fibrillar amyloid plaques, associated with neuronal and microglial damage


Epidemiology

Alzheimer's disease is the most frequent type of dementia in the elderly and affects almost half of all patients with dementia. Correspondingly, advancing age is the primary risk factor for Alzheimer's. Among people aged 65, 2-3% show signs of the disease, while 25–50% of people aged 85 have symptoms of Alzheimer's and an even greater number have some of the pathological hallmarks of the disease without the characteristic symptoms. Every five years after the age of 65, the probability of having the disease doubles. The share of Alzheimer's patients over the age of 85 is the fastest growing segment of the Alzheimer's disease population in the US, although current estimates suggest the 75-84 population has about the same number of patients as the over 85 population

Prevention

Aging itself cannot be prevented, but the senescence of it can be mitigated. However, the evidence relating certain behaviors, dietary intakes, environmental exposures, and diseases to the likelihood of developing Alzheimer's varies in quality and its acceptance by the medical community. It is important to understand that interventions that reduce the risk of developing the disease in the first place may not alter its progression after symptoms become apparent. Due to their observational design, studies examining disease risk factors are often at risk from confounding variables. Several recent large randomized controlled trials—in particular the Women's Health Initiative—have called into question preventive measures based on cross-sectional studies. Some proposed preventive measures are even based on studies conducted solely in animals or in cell cultures but are not listed here.

Adults with damaged blood vessels in the brain or atrophy in their temporal lobe are more likely to develop Alzheimer's disease. It is known that blood vessel damage in the brain is more likely to occur in patients with high blood pressure, high cholesterol or diabetes. Therefore, prevention of these conditions can lower the risk of developing Alzheimer's, as well as heart attack and stroke.

Coronary Bypass Surgery

Coronary bypass surgery is a common procedure used to divert blood around blocked arteries in the heart. Coronary bypass surgery remains one of the gold standard surgical treatments for coronary artery disease.

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Just like all the other organs in your body, your heart needs blood and oxygen to do its job. Coronary arteries snake across the surface of your heart, delivering a constant supply of much-needed blood and oxygen to the heart muscle. When one or more of these arteries becomes narrowed or blocked, blood and oxygen are reduced and heart muscle is damaged.Coronary bypass surgery uses a healthy blood vessel harvested from your leg, arm, chest or abdomen and connects it to the other arteries in your heart so that blood is bypassed around the diseased or blocked area.



If lifestyle changes and medication haven't relieved your symptoms or if your narrowed coronary arteries put you at imminent risk of a heart attack, you and your doctor will need to consider whether coronary bypass surgery or another artery-opening procedure such as angioplasty is right for you.


Bypass surgery is an option if:

  • You have debilitating chest pain caused by narrowing of several of the arteries that supply your heart muscle, leaving the muscle short of blood during light exercise or at rest. Sometimes angioplasty and stent placement will bring relief in this situation, but for some, bypass is the best option.
  • You have more than one diseased coronary artery and the heart's main pump — the left ventricle — is functioning poorly.
  • Your left main coronary artery is severely narrowed or blocked. This artery feeds blood to the left ventricle.
  • You have an artery blockage for which angioplasty isn't appropriate, you've had a previous angioplasty or stent placement hasn't been successful, or you've had angioplasty but the artery has narrowed again (restenosis).

Coronary bypass surgery doesn't cure the underlying disease process called atherosclerosis or coronary artery disease. Even if you have bypass surgery, lifestyle changes are still necessary and an integral part of treatment after surgery. Lifestyle changes — especially smoking cessation — are crucial to reduce the chance of future blockages and heart attacks, even after successful bypass surgery. In addition, you will likely need to make other lifestyle changes, such as reducing certain types of fat in your diet, increasing physical activity, and controlling high blood pressure, diabetes and other risk factors for heart disease. Medications are routine after heart surgery to lower your blood cholesterol, reduce the risk of developing a blood clot and help your heart function as well as possible.

Rnai Discovery Animation

RNA interference (also called "RNA-mediated interference", abbreviated RNAi) is a mechanism for RNA-guided regulation of gene expression in which double-stranded ribonucleic acid inhibits the expression of genes with complementary nucleotide sequences. Conserved in most eukaryotic organisms, the RNAi pathway is thought to have evolved as a form of innate immunity against viruses and also plays a major role in regulating development and genome maintenance.

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The RNAi pathway is initiated by the enzyme dicer, which cleaves double-stranded RNA (dsRNA) to short double-stranded fragments of 20–25 base pairs. One of the two strands of each fragment, known as the guide strand, is then incorporated into the RNA-induced silencing complex (RISC) and base-pairs with complementary sequences. The most well-studied outcome of this recognition event is a form of post-transcriptional gene silencing. This occurs when the guide strand base pairs with a messenger RNA (mRNA) molecule and induces degradation of the mRNA by argonaute, the catalytic component of the RISC complex. The short RNA fragments are known as small interfering RNA (siRNA) which are perfectly complementary to the gene to which they are suppressing as they are derived from long dsRNA of that same gene or MicroRNA (miRNA) which are derived from the intragenic regions or an intron and are thus only partially complementary. The RNAi pathway has been particularly well-studied in certain model organisms such as the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the flowering plant Arabidopsis thaliana.
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The selective and robust effect of RNAi on gene expression makes it a valuable research tool, both in cell culture and in living organisms; synthetic dsRNA introduced into cells can induce suppression of specific genes of interest. RNAi may also be used for large-scale screens that systematically shut down each gene in the cell, which can help identify the components necessary for a particular cellular process or an event such as cell division. Exploitation of the pathway is also a promising tool in biotechnology and medicine.

Historically, RNA interference was known by other names, including post transcriptional gene silencing, transgene silencing, and quelling. Only after these apparently-unrelated processes were fully understood did it become clear that they all described the RNAi phenomenon. RNAi has also been confused with antisense suppression of gene expression, which does not act catalytically to degrade mRNA but instead involves single-stranded RNA fragments physically binding to mRNA and blocking translation. In 2006, Andrew Fire and Craig C. Mello shared the Nobel Prize in Physiology or Medicine for their work on RNA interference in the nematode worm C. elegans, which they published in 199

Transgenic Technology Animation

Transgenic technology used to deliberately alter the genome of an organism by the transfer of a gene or genes from another species or breed.


A genetically modified organism (GMO) is an organism whose genetic material has been altered using the genetic engineering techniques generally known as recombinant DNA technology. With recombinant DNA technology, DNA molecules from different sources are combined in vitro into one molecule to create a new gene. This modified DNA is then transferred into an organism causing the expression of modified or novel traits. The product is also known as an Genetically Engineered Organism or GEO.

Transgenic Technology Part 1



Transgenic Technology Part 2





The term "GMO" has historically been defined as organisms whose genetic makeup has been altered by conventional cross breeding or by "mutagenesis" breeding, as these methods predate the discovery of the recombinant DNA techniques. However, this term is now interchangeable with Genetically Engineered Organism.


Examples of GMOs are highly diverse, and include transgenic (genetically modified by recombinant DNA methods) animals such as mice, fish, transgenic plants, or various microbes, such as fungi and bacteria. The generation and use of GMOs has many reasons, chief among them are their use in research that addresses fundamental or applied questions in biology or medicine, for the production of pharmaceuticals and industrial enzymes, and for direct, and often controversial, applications aimed at improving human health (e.g., gene therapy) or agriculture (e.g., golden rice). The term "genetically modified organism" does not always imply, but can include, targeted insertions of genes from one into another species. For example, a gene from a jellyfish, encoding a fluorescent protein called GFP, can be physically linked and thus co-expressed with mammalian genes to identify the location of the protein encoded by the GFP-tagged gene in the mammalian cell. These and other methods are useful and indispensable tools for biologists in many areas of research, including those that study the mechanisms of human and other diseases or fundamental biological processes in eukaryotic or prokaryotic cells.

Transgenic animals

Transgenic animals are used as experimental models to perform phenotypic tests with genes whose function is unknown or to generate animals that are susceptible to certain compounds or stresses for testing in biomedical research.ther applications include the production of human hormones, such as insulin.

Frequently used in genetic research are transgenic fruit flies (Drosophila melanogaster) as genetic models to study the effects of genetic changes on development.Flies are often preferred over other animals for ethical reasons and ease of culture, and also because the fly genome is somewhat simpler than that of vertebrates.

Antibody Immune Response

Antibodies are Y-shaped proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses. They are made of a few basic structural units called chains; each antibody has two large heavy chains and two small light chains. There are several different types of antibody heavy chain, and several different kinds of antibodies, which are grouped into different isotypes based on which heavy chain they possess. Five different antibody isotypes are known in mammals, which perform different roles, and help direct the appropriate immune response for each different type of foreign object they encounter.
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Although the general structure of all antibodies is very similar, a small region at the tip of the protein is extremely variable, allowing millions of antibodies with slightly different tip structures to exist. Each of these variants can bind to a different target, known as an antigen. This huge diversity of antibodies allows the immune system to recognize an equally wide diversity of antigens. The unique part of the antigen recognized by an antibody is called an epitope. These epitopes fit precisely with their antibody, similar to a key fitting into a lock, in a highly specific interaction that allows antibodies to identify and bind only their unique antigen in the midst of the millions of different molecules that make up an organism. Recognition of an antigen by an antibody tags it for attack by other parts of the immune system. Antibodies can also neutralize targets directly by, for example, binding to a part of a pathogen that it needs to cause an infection.

The large and diverse population of antibodies is generated by random combinations of a set of gene segments that encode different antigen binding sites (or paratopes), followed by random mutations in this area of the antibody gene, which create further diversity. Antibody genes also re-organize in a process called class switching that changes the base of the heavy chain to another, creating a different isotype of the antibody that retains the antigen specific variable region. This allows a single antibody to be used by several different parts of the immune system.



Antibodies occur in two forms: a soluble form secreted into the blood and tissue fluids, and a membrane-bound form attached to the surface of a B cell that is called the B cell receptor (BCR). The BCR allows a B cell to detect when a specific antigen is present in the body and triggers B cell activation. Activated B cells differentiate into either antibody generating factories called plasma cells that secrete soluble antibody, or into memory cells that survive in the body for years afterwards to allow the immune system to remember an antigen and respond faster upon future exposures. Antibodies are, therefore, an essential component of the adaptive immune system that learns, adapts and remembers responses to invading pathogens. Production of antibodies is the main function of the humoral immune system.

Antibodies can come in different forms known as isotypes or classes. In mammals there are five antibody isotypes known as IgA, IgD, IgE,IgG and IgM. They are each named with an "Ig" prefix that stands for immunoglobulin, another name for antibody, and differ in their biological properties, functional locations and ability to deal with different antigens, as depicted in the table.


The antibody isotype of a B cell changes during the cell's development and activation. Immature B cells, which have never been exposed to antigen, are known as naïve B cells and express only the IgM isotype in a cell surface bound form. B cells begin to express both IgM and IgD when they reach maturity - the co-expression of both these immunoglobulin isotypes renders the B cell 'mature' and ready to respond to antigen. B cell activation follows engagement of the cell bound antibody molecule with an antigen, causing the cell to divide and differentiate into an antibody producing cell called a plasma cell. In this activated form, the B cell starts to produce antibody in a secreted form rather than a membrane-bound form. Some daughter cells of the activated B cells undergo isotype switching, a mechanism that causes the production of antiodies to change from IgM or IgD to the other antibody isotypes, IgE, IgA or IgG, that have defined roles in the immune system.


Structure

Antibodies are heavy globular plasma proteins that are also known as immunoglobulins. They have sugar chains added to some of their amino acid residues. In other words, antibodies are glycoproteins. The basic functional unit of each antibody is an immunoglobulin (Ig) monomer (containing only one Ig unit); secreted antibodies can also be dimeric with two Ig units as with IgA, tetrameric with four Ig units like teleost fish IgM, or pentameric with five Ig units, like mammalian IgM.


Immunoglobulin domains

The Ig monomer is a "Y"-shaped molecule that consists of four polypeptide chains; two identical heavy chains and two identical light chains connected by disulfide bonds. Each chain is composed of structural domains called Ig domains. These domains contain about 70-110 amino acids and are classified into different categories (for example, variable or IgV, and constant or IgC) according to their size and function. They possess a characteristic immunoglobulin fold in which two beta sheets create a “sandwich” shape, held together by interactions between conserved cysteines and other charged amino acids.


Function

Since antibodies exist freely in the bloodstream, they are said to be part of the humoral immune system. Circulating antibodies are produced by clonal B cells that specifically respond to only one antigen, a virus hull protein fragment, for example. Antibodies contribute to immunity in three main ways: they can prevent pathogens from entering or damaging cells by binding to them; they can stimulate removal of a pathogen by macrophages and other cells by coating the pathogen; and they can trigger direct pathogen destruction by stimulating other immune responses such as the complement pathway.


Medical applications

Disease diagnosis

Detection of particular antibodies is a very common form of medical diagnostics, and applications such as serology depend on these methods. For example, in biochemical assays for disease diagnosis, a titer of antibodies directed against Epstein-Barr virus or Lyme disease is estimated from the blood. If those antibodies are not present, either the person is not infected, or the infection occurred a very long time ago, and the B cells generating these specific antibodies have naturally decayed. In clinical immunology, levels of individual classes of immunoglobulins are measured by nephelometry (or turbidimetry) to characterize the antibody profile of patient. Elevations in different classes of immunoglobulins are sometimes useful in determining the cause of liver damage in patients whom the diagnosis is unclear. For example, elevated IgA indicates alcoholic cirrhosis, elevated IgM indicates viral hepatitis and primary biliary cirrhosis, while IgG is elevated in viral hepatitis, autoimmune hepatitis and cirrhosis. Autoimmune disorders can often be traced to antibodies that bind the body's own epitopes; many can be detected through blood tests. Antibodies directed against red blood cell surface antigens in immune mediated hemolytic anemia are detected with the Coombs test. The Coombs test is also used for antibody screening in blood transfusion preparation and also for antibody screening in antenatal women.Practically, several immunodiagnostic methods based on detection of complex antigen-antibody are used to diagnose infectious diseases, for example ELISA, immunofluorescence, Western blot, immunodiffusion, and immunoelectrophoresis.


Disease therapy

"Targeted" monoclonal antibody therapy is employed to treat diseases such as rheumatoid arthritis, multiple sclerosis,psoriasis, and many forms of cancer including non-Hodgkin's lymphoma,colorectal cancer, head and neck cancer and breast cancer.Some immune deficiencies, such as X-linked agammaglobulinemia and hypogammaglobulinemia, result in partial or complete lack of antibodies. These diseases are often treated by inducing a short term form of immunity called passive immunity. Passive immunity is achieved through the transfer of ready-made antibodies in the form of human or animal serum, pooled immunoglobulin or monoclonal antibodies, into the affected individual.