The presence or absence of genetically determined cell markers on the surfaces of the red blood cell membranes – called antigens, influences the type of blood group a person will have. The antigens are also known as agglutinogens. Each antigen has a corresponding antibody (agglutinin) within the plasma; however, no human has a similar antigen and antibody for the same blood group; it is incompatible with life.
The ABO blood has four blood groups, all determined by either antigen A, B, or both. When your red blood cells display antigen A, you have blood group A. If they display antigen B, you are B. if they display both antigens, you become AB. And if they display neither of the antigens, you are O.
Interestingly, the presence of antigen A implies that an individual has the opposite antibody, B, and vice versa. The presence of antibodies like the antigen on the red blood cells will lead to agglutination (clumping) of blood as the antibodies attack the red cells with the antigens, leading to hemolysis (breakdown of the red blood cells). It is one of the principles behind blood grouping in the laboratory.
Due to the variations in antigens in different blood groups, blood group AB is a universal recipient whereas O is a universal donor as illustrated below.
The ABO blood group system expands further by the rhesus factor, which increases the blood groups to eight. It is an independent blood-group system, extremely crucial to blood groups, crossmatching, and blood transfusion. The presence of a rhesus factor on the red blood cells assigns a positive on the specific blood group, and its absence, a negative. It results in the following blood groups – A+, A-, B+, B-, AB+, AB-, O+, and O-.
Besides its use in blood groups and crossmatch, the rhesus factor is crucial during pregnancy and childbirth. A rhesus-negative woman carrying a fetus with a rhesus-positive blood group may produce antibodies that will react with the subsequent fetus. The first child is always unaffected. The subsequent fetuses may suffer from hemolytic disease of the newborn. The degree of severity varies and can be fatal.