The many types into which an individual’s blood may be classified, based on the presence or absence of certain antigenic carbohydrates (agglutinogens) on the surface of the red blood cells. Individuals with blood of one group have antibodies in their serum that react against the agglutinogens on the cells of other groups. These antibodies are produced in response to exposure to bacteria that have very similar carbohydrate groups on their surface. Incompatibility between groups results in clumping of cells (agglutination), so knowledge of blood groups is important for blood transfusions. In humans, the two most important blood group systems are the ABO system and the system involving the rhesus factor.