When the antigens on the surface of red blood cells react with antibodies in the serum, the cells clump together. this process is called:
clotting
agglutination
hemorrhage
anemia

Agglutination

Explanation:

Agglutination is the reaction of an antibody naturally present or produced in plasma - agglutinin - with certain antigens on the red cell membrane - agglutinogen - forming a cluster of small cell masses.

This reaction can be used to identify a person's blood type and has consequences when blood transfusions are performed. Because an individual may or may not contain antigens in their red blood cells, the agglutination process will occur when the donor's blood has an antigen that is not present in the recipient's blood. In this case, the immune system will recognize this foreign body and react by agglutinating red blood cells, leading to death.

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