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There are four basic blood groups-A, B, AB, and O. These different blood types are caused by the presence of a chemical marker-an antigen-on the surface of the type-A and type-B red blood cells. When mixed with the wrong blood type, these antigens are picked up by antibodies that cause the cells to clump. Someone with type-AB blood can receive any type blood with no ill effects, while people with type-O blood can only take their own type. People with type-A blood can receive A or O, and people with type-B blood can take B or O. This makes type-O blood the universal donor. Blood donors and recipients must be typed and matched very carefully before transfusions are given. |