The red cells of an individual contain antigens on cell surfaces that correspond to their blood group. Antibodies in the serum that identify such antigen locate on the surfaces of red cells of another blood group. At present, 35 blood group systems representing over 300 antigens are listed by the International Society of Blood Transfusion (ISBT).
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What is blood group?
The presence or absence of certain protein, antigens located on the surface of the red blood cells, and antibodies in the blood plasma, lead to the different types of blood groups. Heretofore, there are more than 300 human blood groups, but only about 20 are genetically-determined. Among these 20 blood groups, a very minority are clinically significant transfusion reactions, in which ABO and Rh systems are the most common ones.
The ABO system
The ABO system is the most important blood-group system in human-blood transfusion, since any person above the age of 6 months possess clinically significant anti-A and/or anti-B antibodies in their serum. Blood group A contains antibody against blood group B in serum and vice-versa, while blood group O contains no A/B antigen but both their antibodies in serum. This discovery is a tremendous progress in clinical transfusion practice to prevent fatal danger by ABO-incompatible blood transfusion.
The Rh system
The Rh system (Rh meaning Rhesus) is the second most significant blood-group system in human-blood transfusion including antigens of D (or Rho), C, E, c, and e, among which the D antigen is the most significant one. Those possessing D antigen on the surface of RBCs are Rh-positive, while other who lack D antigen are Rh-negative, no matter what other Rh antigens are present. The presence or absence of the Rh(D) antigen is signified by the + or ? sign, so that for example the A? group is ABO type A and does not have the Rh (D) antigen.
Other clinically important blood group systems
In addition to the ABO antigens and Rh antigens, many other antigens are expressed on the red blood cell surface membrane. For example, an individual can be AB, D positive, and at the same time M and N positive (MNS system), K positive (Kell system), Lea or Leb negative (Lewis system), and so on, being positive or negative for each blood group system antigen.
What is blood group antibody?
Blood group antibodies in the serum are the clinically significant antibodies, which can specially identify antigens locate on the surfaces of red cells of another blood group, typically for transfusion purposes. As we all know, if mixing incompatible blood groups, blood clumping or agglutination will occur. That is to say, transfusion reactions or hemolytic disease of the fetus and newborn (HDFN) are usually resulted from alloantibodies produced by the exposure to a different blood group by transfusion or pregnancy.
The anti-A and anti-B activity prominently resides in IgM class, while only a few rests with IgG coating the RBCs with no effect on their viability but resulting in HDFN by crossing the placenta. Specially, those with O blood type regularly have more IgG anti-A and anti-B than any others.
As a monomer with 2 Fab sites and a Fc portion carrying macrophage receptor, IgG, requires high concentration to activate complement, but only to C3 immune complexes, which is able to amplify extravascular hemolysis. Most IgGs can bind with related antigen on the red blood cells around the normal body temperature of 37°C, which are defined as warm antibodies and most clinically significant antibodies. On the other side, IgM, as a pentamer with 10 Fab sites and without the macrophage receptor, is the cold antibody that binds with Ag at ambient temperature or colder temperatures. IgMs do not usually cause clinical problems, but they may be picked up on laboratory testing since the polymers allow complement activation to C9 and intravascular hemolysis if reactivate at 37°C.