In the ABO blood group system, naturally occurring IgM anti-A and anti-B are present in the serum in the absence of the corresponding red cell antigen.
Although described as naturally occurring (allo-) antibodies, anti-A and anti-B are not detectable in the blood of newborn infants. The antibodies become detectable at about 3 months of age following exposure to A and B like substances present in the environment e.g. in bacteria and some foods.
As a person gets older the concentration of naturally occurring anti-A and anti-B in the blood becomes less and these antibodies may be difficult to detect in the serum of some elderly patients.
Occasionally IgG hyperimmune anti-A and anti-B can be found in the serum of group O persons in response to stimulation by A and B like antigens in the environment, and following pregnancy, or the injection of some vaccines or toxoids. In tropical
countries it is common to find lytic IgG anti-A, anti-B, or both in the fresh serum of up to 50% group O persons. Lytic anti-A is also found in group B persons and lytic anti-B in group A persons (about
25% of sera).
Serious haemolytic reactions can occur when Group O whole blood containing anti-A and anti-B haemolysins is used to transfuse non-group O persons. Immune IgG lytic anti-A and anti-B can cross the placenta and cause ABO haemolytic
disease of the newborn (HDN).
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