Blood group test. Technician's gloved hands taking a sample from a bag of donor blood and placing it in test tubes to be tested for its blood group. It is vital to identify the group of donor blood, as giving a patient blood of the wrong group can cause potentially fatal immune reactions. Blood is grouped according to the antigens it has on its red blood cells. The major blood groups are A, B, AB and O. Blood is further classified as Rhesus (Rh) positive or negative. Antibodies will be used to bind to the antigens and so identify the blood group. Antibodies are proteins that are part of the body's immune system.