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Military technician reviewing blood typing test of a donor to determine who they can donate their blood to. Human blood groups are categorized into four types: A, B, AB and O. Blood typing tests mix patients' blood with antibodies against type A and type B blood. The blood reacts (agglutinates) producing red, dense dots if it has reacted to the antibodies. Blood group A blood agglutinates when added to anti-A antibodies because it contains B antigens (protein molecules on the surface of red blood cells). Blood group B blood agglutinates when added to anti-B because it contains B antigens. AB blood type reacts to both anti-A and anti-B antibodies because it has both A and B antigens. Blood group O doesn't react to anti-A and anti-B antibodies because it doesn't have A or B antigens. People who have Blood group O are universal donors because they can donate their blood to all blood types. People who have blood group A can only receive O or A blood. People who have blood group B can only receive O or B blood. People who have blood group AB can receive all blood group types. Blood group O can only receive blood from blood group O donors. Photographed on 20 December 2019. (KEYSTONE/SCIENCE PHOTO LIBRARY/US AIR FORCE/SCIENCE PHOTO LIBRARY)