Blood group test. Packet containing a blood group test to enable people to determine their blood group at home. Blood group (ABO) is determined by the presence or absence of certain inherited substances (antigens) on the surface of red blood cells. Blood of one group contains antibodies in the serum that react against the cells of other groups, causing agglutination (clumping of red blood cells). The Rhesus (Rh) gene also codes for an antigen on the surface of red blood cells. People are either Rh positive (+) if they have the antigen, or Rh negative (-) if they don't. If the incorrect blood group is used in a blood transfusion it can prove fatal.