Blood Banking / Immunohematology

The A antigen and the B antigen. These are carbohydrate (sugar) antigens built on a common precursor (H substance), and an individual's ABO type is named for which of these antigens are present on the red cells.

Because ABO antibodies are naturally occurring, predictable, and clinically significant IgM that activate complement, an ABO-incompatible transfusion can cause an immediate, potentially fatal intravascular hemolytic reaction.

They are predominantly IgM. IgM antibodies are large pentamers that agglutinate red cells efficiently at room temperature and readily activate complement, which is why ABO mismatches cause acute intravascular hemolysis.

Anti-B. Landsteiner's rule states that healthy individuals produce antibodies against the ABO antigens they LACK. A group A person lacks B antigen and therefore makes anti-B.

Anti-A, anti-B, and anti-A,B. Group O cells lack both A and B antigens, so per Landsteiner's rule the plasma contains antibodies to both, plus the cross-reacting anti-A,B.