bloodgroup

A blood group is a classification of blood based on the presence or absence of specific antigens on the surface of red blood cells. The most clinically important systems are ABO and Rh. Antigens are molecules that can trigger an immune response, and antibodies against these antigens can cause red cell destruction if blood is mismatched during transfusion.

The ABO system relies on carbohydrate antigens A and B. The ABO gene encodes enzymes that attach these antigens to cell surface carbohydrates. Individuals with type A blood have A antigens and anti-B antibodies in their plasma; type B have B antigens and anti-A antibodies; type AB have both A and B antigens but no anti-A or anti-B antibodies; type O have neither antigen and both anti-A and anti-B antibodies. In transfusion, type O red cells can be given to any ABO type (universal donor), while type AB recipients can receive red cells from any ABO type (universal recipient). Incompatibility can cause acute hemolytic reactions.

The Rh system centers on the D antigen. Rh positive means the D antigen is present; Rh negative means it is absent. Anti-D antibodies typically form after exposure to Rh-positive blood and are of particular importance in pregnancy, because maternal anti-D can cause hemolytic disease of the newborn if the fetus is Rh-positive.

Clinical testing includes blood typing and crossmatching to determine compatibility before transfusion. Molecular methods can supplement serology in certain cases. Blood group status influences transfusion practice, obstetric risk management, and forensic investigations, and the distribution of ABO and Rh variants varies among populations.