When an antigen is present on the RBC, then the opposite antibody (also called agglutinin) is present in the plasma. For instance, type A blood has anti-type-B antibodies. Type B blood has anti-type-A antibodies. Type AB blood has no antibodies in the plasma, and type O blood has both anti-type-A and anti-type-B antibodies in the plasma. These antibodies are not present at birth but are formed spontaneously during infancy and last throughout life. In addition to the ABO blood group system, there is an Rh blood group system.