Antigens & Antibodies of the ABO System

Blood Type	Genotypes	Antigens on RBCs	Antibodies in Blood	Safe Transfusions To	Safe Transfusions From
A	AA , AO	A	anti-B	A , AB	A , O
B	BB , BO	B	anti-A	B , AB	B , O
AB	AB	A , B	-	AB	A , B , AB , O
O	OO	-	anti-A, B	A , B , AB , O	O

Molecular & Immuno-Genetics of the ABO Blood Type system

    The ABO blood type is produced by a locus on the long arm of Chromosome 9 (9q32.4). There are a variety of alleles (designated I^A and I^B) that generate one of two antigenic proteins, A or B, respectively, on the surface of Red Blood Cells (RBCs). A third class of alleles (designated i) does not generate an antigen. Blood type is determined by the presence of one or the other, both, or neither antigen. Genotypes I^AI^A and I^Ai both produce blood type A, I^BI^B and I^Bi both produce type B. Alleles I^A and I^B are therefore genetically dominant to i. Genotype I^AI^B produces blood type AB with both antigens: the alleles I^A and I^B are therefore genetically co-dominant. Genotype ii produces neither antigen and produces blood type O.

    The immune system generate antibodies, anti-A and anti-B, in the circulating blood that react against blood type B or A antigens, respectively. In the absence of one or the other antigen, antibodies are produced to the opposite type. In the absence of neither [i.e., the presence of both] antigen, no antibodies are produced, as in type AB. Finally, in the absence of both [i.e., the presence of neither], both antibodies are produced, as in type O. All of this means that a person's immune system does not react to their own blood type antigens, but does react to foreign blood type antigens.

    Stated another way, a person with blood type A produces A antigens and anti-B antibodies. They can donate to another type A recipient, but cannot receive transfusion from a type B donor

    Blood type testing consists of mixing whole blood from the person tested with anti-A, anti-B, or both antibodies: agglutination indicates blood types B, A, and AB, respectively, because of the antibody-antigen reaction. Absence of agglutination indicates blood type O, because there are no antigens present to react.

    Blood type determines success of transfusion of whole blood between persons, as indicated in the table above. A person may receive blood safely only from someone with the same blood type, or from Type O, because the latter lack antigens. Thus, Type O persons are "Universal Donors" [hence the common order in medical dramas, "Hang two units of Type O, stat!"]. Type AB persons are "Universal Recipients", because they lack antibodies. Although the conditions for transfusion to a Type O person are the most restrictive, because Type O is the most common blood type in most western populations, banked blood and donors are readily available or recruitable during emergencies. Type B is more common in Asian populations. Although Type AB is the rarest blood type, such persons may receive blood from any source.