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MODEL RELEASED. Blood group test. A technician pipettes diluted whole blood into test tubes which contain the antibodies against various blood groups. Blood group is determined by the presence or absence of certain inherited substances (antigens) on the surface of the red blood cells. Blood of one group contains antibodies in the serum that react against the cells of other groups, causing agglutination (clumping of red cells). An individual's blood group can therefore be determined by adding blood to solutions containing antibodies against each blood group. (KEYSTONE/SCIENCE PHOTO LIBRARY/ROBERT LONGUEHAYE, NIBSC)