What are antigens?
Blood type (also called a blood group) is determined by the presence or absence of certain molecules, called antigens, on the surface of red blood cells. An antigen is a molecule or substance that causes an immune response. Blood type antigens may be proteins or carbohydrates, depending on the blood group system. The antigens on a person’s own body cells are recognized by their immune system as “self” antigens, and their immune system does not attack them. However, if a person is exposed to a blood group antigen that is different from their own blood group, the person’s immune system will produce antibodies against the donor blood antigens. These antibodies can bind to antigens on the surfaces of transfused red blood cells (or other tissue cells), often leading to destruction of the cells by the immune system.