The four major blood groups are group A, group B, group AB and group O. Group A has the A antigen in the red cells and A antibody in plasma, and B only has B antigens and B anti-bodies in the plasma. Group AB has both A and B antigens but neither anti-body in the plasma, while group O does not have A or B antibodies in the plasma.
People with type O blood are known as universal donors because their blood can be given to people with any of the ABO types. The Rh system indicates whether a person's blood is Rh-positive or negative. Rh-positive blood helps in people with Rh-negative blood, but a person with Rh-negative blood cannot get Rh-positive blood. Mismatches between blood types during a transfusion can cause serious or life-threatening reactions.
Blood consists of plasma, blood cells, red and white cells, and platelets. A person's blood group depends on what antigens occur on the surface of the person's red blood cells and their genetic makeup. Just in case a blood transfusion is needed, a blood test is recommended. A blood test is conducted by mixing different samples of plasma that contain different antibodies and watching how they react to the person's blood cells.