A blood type (also called a blood group) is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group system, and some of these antigens are also present on the surface of other types of cells of various tissues. Several of these red blood cell surface antigens, that stem from one allele (or very closely linked genes), collectively form a blood group system.
ABO blood group system
The ABO system is the most important blood-group system in human-blood transfusion. The associated anti-A antibodies and anti-B antibodies are usually "Immunoglobulin M", abbreviated IgM, antibodies. ABO IgM antibodies are produced in the first years of life by sensitization to environmental substances such as food, bacteria, and viruses. The "O" in ABO is often called "0" (zero/null) in other languages.
Rhesus blood group system
The Rhesus system is the second most significant blood-group system in human-blood transfusion. The most significant Rhesus antigen is the RhD antigen because it is the most immunogenic of the five main rhesus antigens. It is common for RhD-negative individuals not to have any anti-RhD IgG or IgM antibodies, because anti-RhD antibodies are not usually produced by sensitization against environmental substances. However, RhD-negative individuals can produce IgG anti-RhD antibodies following a sensitizing event: possibly a fetomaternal transfusion of blood from a fetus in pregnancy or occasionally a blood transfusion with RhD positive RBCs. Rh disease can develop in these cases.
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