The ABO blood group system classifies blood into four main types — A, B, AB, and O — based on the presence or absence of specific antigens (A and B antigens) on the surface of red blood cells, and corresponding antibodies in the plasma. Blood type A has A antigens and anti-B antibodies; type B has B antigens and anti-A antibodies; type AB has both antigens and no antibodies; type O has neither antigen but both antibodies.

ABO blood typing is essential before any blood transfusion or organ transplant to prevent potentially fatal immune reactions. Research has also linked blood type to varying risks of certain conditions including cardiovascular disease, certain cancers, and susceptibility to specific infections — though blood type alone is not a strong clinical risk factor for most conditions.