REVY, ISM / SCIENCE PHOTO LIBRARY REVY, ISM / SCIENCE PHOTO LIBRARY
Blood group test showing a reaction between a blood type and an antibody serum. There are four human blood types: A, B, AB and O. Type A blood has type A antigens (surface proteins) on its red 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 a serum containing anti-A antigens causes an agglutination reaction, where the red blood cells become bound together in clumps by the antibodies (dense red particles). AB blood has no antibodies, but has both A and B antigens on its cells. Type O blood has both antibodies but no antigens. Identifying the blood type prevents patients from having dangerous immune system reactions when receiving donated blood.
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