There are four different ABO groups, determined by whether or not an individual’s red cells carry the A antigen (group A), the B antigen (group B), both A and B (group AB), or neither (group 0). Normal healthy individuals, from early in childhood, make antibodies against A or B antigens that are not present on their own cells.
If red cells of an incompatible ABO group are transfused, the recipient’s antibodies bind to the transfused red cells activating a number of processes which destroy the transfused cells in the circulation. This can in turn lead to low blood pressure, renal failure and sometimes death.
RhD antigen and antibody - Rh (commonly referred to a Rhesus) is another blood group system of which RhD is the most important. In a Caucasian population, about 15% will lack the RhD antigen and are termed RhD negative. The remainder possess the RhD antigen, and are termed RhD positive.
Antibodies to the RhD antigen occur only in individuals who are RhD negative, and follow transfusion or pregnancy. If a woman who is RhD negative develops anti RhD antibody during pregnancy, the antibodies can cross the placenta. If the fetus is RhD positive the antibodies can destroy the fetal red cells. This will cause haemolytic disease of fetus and newborn (HDFN).