Co-dominant blood & antigen compatibility in the ABO blood group system

     Blood-type in the ABO system is determined by three alleles  (I^A , I^B , I^O) at the Isoagglutinin locus on Chromosome 9. The I^A and I^B alleles determine antigenic proteins on the surface of red blood cells, which in turn cause the immune system to develop antibodies against the antigen not present. An individual who is I^AI^B has both antigens, and therefore does not generate antibodies to either: alleles I^A and I^B are thus co-dominant. Individuals who are I^AI^A or I^AI^O have the same blood type and generate anti-B antibodies: I^A is thus dominant to I^O.  The same principles apply to I^BI^B and I^BI^O, such that I^B is dominant to I^O. An individual who is I^OI^O has no antigens, and generates antibodies to both.

    The test for ABO blood type involves mixing purified A' (anti-A) and B' (anti-B) antibodies with the blood to be typed. Type A blood coagulates in the presence of A' antibody only, type B reacts with B' antigen only, type AB with neither, and type O with both. The same reaction would occur in vivo, which necessitates typing of blood prior to transfusion. Persons with an I^A or I^B allele cannot receive blood from a person with either an I^B or an I^A allele, respectively. That is, type A persons cannot receive Type B or AB blood, and vice versa.

    In TV medical shows, the frequent request for "O negative" blood reflects the fact that Type O blood is compatible with any ABO blood type: Type O is thus described as the 'universal donor'. Type AB is a 'universal recipient' due to the lack of antibody production, which is fortunate because the AB type is the rarest.

    "Negative" refers to the second major blood-type locus, the Rh (Rhesus ) factor, with two alleles Rh⁺ and Rh^-, where the first is a simple dominant to the second and produces an antigen absent in the second. Following the same principles as in ABO,  Rh^- blood can be safely transfused to any recipient. Note that Rh^- persons constitute only  ~15% of the North American population.