REVY, ISM / SCIENCE PHOTO LIBRARY REVY, ISM / SCIENCE PHOTO LIBRARY
Blood group test showing reactions between blood types (down right) and antibody serums (across top). The first column shows the blood's appearance prior to the tests. There are four human blood types: A, B, AB and O. Type A blood has type A antigens (surface proteins) on its blood cells and anti-B antibodies (proteins that bind to type B antigens) in its serum. Type B blood is the reverse. Mixing type A blood with anti-A serum (left column) causes an agglutination reaction, producing dense red dots. Type B blood undergoes the same reaction with anti-B serum (middle column) AB blood has no antibodies, but has both A and B antigens on its cells, so it agglutinates in the anti-A+B serum (right column). Type O blood has both antibodies but no antigens, so it does not react to the serums.
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