Cellular basis of immune response to antigens of ABO blood-group system. Capacity to provide help during response to T-cell-dependent ABO-system antigens is restricted to individuals of blood group O.

It is postulated that the near-identity of the A and B antigens in man makes it impossible for individuals of groups A, A2, or B to provide T-cell help during a response to T-dependent non-self antigens by B2 lymphocytes. Thus, people of these groups are unable to produce immune IgG antibody. The presence of naturally occurring IgM antibody to non-self antigen from the age of 6 months is due to a separate system of T-cell-independent B1 cells which have arisen during evolution to protect the host against polysaccharide-encapsulated bacterial pathogens.