Blood Type (ABO System)

The ABO blood group system classifies human blood based on the presence or absence of specific antigens—molecules on the surface of red blood cells. There are four main types: A, B, AB, and O. Type A has A antigens; B has B antigens; AB has both; and O has neither. These antigens determine compatibility for transfusions, as the immune system reacts against incompatible types. Blood also contains antibodies that target the antigens not present on one's own red blood cells. Proper matching prevents immune reactions and ensures safe blood transfusions.