Blood group test. Image 4 of 4. Positive result of a test for blood group B antigens. 'B' antibodies were added to the blood samples, and have reacted with 'B' antigens on the red blood cells. This has made the cells stick together (agglutinate) in the clumps seen. Blood is grouped by its antigens. The major blood groups are A, B, AB and O. Only B and AB blood contains 'B' antigens. Blood is further classified as Rhesus (Rh) positive or negative. The group must be identified to avoid potentially fatal immune reactions in the patient receiving the blood. For a sequence showing a blood test for the 'B' antigens, see images M530/480 to M530/483.