Blood group test. Pipette being used to add serum containing blood group A antibodies to samples of blood being tested for their blood group. The 'A' antibodies will react with any 'A' antigens on the red blood cells in the samples, making the cells stick together (agglutinate). Blood is grouped by the antigens it has. The major blood groups are A, B, AB and O. Blood is further classified as Rhesus (Rh) positive or negative. The group is identified to prevent potentially fatal immune reactions in the patient receiving the blood.