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Blood crossmatching. Gloved hands pipetting blood during a crossmatching test. The test is done to check that the blood group of the donated blood (in bags at right) matches that of the recipient and so it is safe to transfuse. The presence or absence of various proteins called antigens on the surface of red blood cells determines the blood group. Here, the donated blood is group B and Rhesus (Rh) positive. Blood of one group contains antibodies that react against the cells of other groups, causing red blood cell clumping (haemagglutination). Blood from the recipient must therefore be added to the donated blood to check that this lethal clumping does not occur. (KEYSTONE/SCIENCE PHOTO LIBRARY/JIM VARNEY)