Antigens

Antigens in the blood are marker substances, often proteins or carbohydrates, found on the surface of red blood cells that identify the blood type. Your immune system uses these markers to distinguish your own cells from foreign ones; for example, a blood type A person has A antigens on their red blood cells and B antibodies in their plasma, while a person with type O blood has no A or B antigens but both A and B antibodies in their plasma.