Blood group test showing reactions between blood types (down right) and their antibodies (across top). There are four human blood types: A, B, AB and O. Type A blood has type A antigens on its blood cells and anti-B antibodies in its serum. Type B blood is the reverse. Mixing type A blood with anti-A antibodies leads to an agglutination reaction, the result of which is seen as dense red dots. AB blood has both A and B antigens on its cells, but no antibodies. Type O blood has both antigens and no antibodies. Anti-D is an antibody used to test for Rhesus antigens, and it reacts with Rhesus positive blood. Typing is necessary to prevent reactions during blood transfusions.