Blood group test. Set of centrifuge tubes holding a mixture of a blood sample and an antibody being used to test its blood group. The mixture (red) is above a gel (pale blue) that will detect positive results to the test. The antibodies will react with certain antigens, making the red blood cells stick together (agglutinate). The tubes will be spun rapidly in a centrifuge to determine which samples have agglutinated. Blood is grouped by its antigens so as to prevent potentially fatal immune reactions in the patient who receives the blood. Antibodies are proteins that are part of the body's immune system. For the results of a centrifuged sample, see image M530/466.