There are over 30 common groups of RBC proteins, referred to as antigens, isoantigens, or agglutinogens (which is the preferred term). Generally, each group is controlled by a single gene, and for each gene, two alleles, or forms, of the gene are inherited (one allele from each parent). Each blood group gene may have two or more different alleles in the population. Although not all blood group proteins stimulate the immune response, two important ones do:
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ABO blood group. The gene responsible for this group has three alleles. One allele produces an “A” agglutinogen, a second produces a “B” agglutinogen, and a third produces no agglutinogens (“O”). Because individuals inherit two alleles, individuals may be of the A blood type, inheriting two A alleles (AA) or an A and an O allele (AO); the B blood type (BB or BO); the AB blood type (AB); or the O blood type (OO). The immune response is activated when an individual receives a transfusion with blood carrying nonself agglutinogens. For example, the immune system would respond if a person with A blood type (either AA or AO) receives blood of the B or AB blood type, but not of the O type. (The O type does not carry any foreign agglutinogens.)
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Rh blood group.This is a complex group defined by agglutinogens produced by three different genes. Each gene has two (or rarely, three) alleles. Because of the close linkage of the genes (they are positioned close to one another on the same chromosome), the expression of the group can be evaluated as if it were a single gene with two alleles, an Rh + allele (producing the Rh agglutinogen) and an Rh – allele (producing no Rh agglutinogen). Thus, individuals are either Rh + if they inherit one or two Rh + alleles or Rh – if they inherit two Rh – alleles. The Rh factor is typically called the D agglutinogen. It was originally found on the red blood cells of Rhesus monkeys (hence the “Rh” factor).