Anatomy and Chemistry Basics
The Cardiovascular System
Various glycoproteins and lipoproteins are embedded in the surfaces of red blood cells. These proteins are inherited, and their structures may vary from one individual to another. If during a transfusion an individual receives blood containing RBCs with proteins that the individual does not carry, then these proteins may be recognized as antigens (foreign substances) by the immune system. If so, antibodies are produced that bind to the antigens and cause agglutination (clumping) and subsequent destruction of the foreign RBCs.
There are over 30 common groups of RBC proteins (referred to as antigens, isoantigens, or agglutinogens). 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.
-
ABO blood group. The gene responsible for this group has three alleles. One allele produces an “A” antigen, a second produces a “B” antigen, and a third produces no antigens (“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 antigens. 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 types, but not of the O type. (The O type does not carry any foreign antigens.)
-
Rh blood group. This is a complex group defined by antigens 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 antigen) and an Rh− allele (producing no Rh antigen). Thus, individuals are either Rh+ if they inherit one or two Rh+ alleles or Rh− if they inherit two Rh- alleles.
