The process in which free red blood cells are bound together by an antibody and reduced to a visible pellet when centrifuged, most typically in test tubes. In short, when an antibody binds to an RBC antigen then binds to an antigen on a second RBC, the antibody links form “bridges” that lead to a visible aggregate of RBCs. Agglutination is the central reaction in blood banking, as most of our testing for decades has relied on its detection. The agglutinates are typically characterized on a 0-4+ scale, with 0 representing no reaction, and 4+ indicating a very strong reaction (see images above). Antibodies vary in their ability to cause agglutination, as IgM antibodies agglutinate RBCs carrying target antigens quite efficiently, while IgG antibodies typically bind to incompatible RBCs but do not directly agglutinate them. The characteristics of the agglutination reaction are used, usually reliably, to predict whether or not a particular antibody will cause a problem for a patient in-vivo. Agglutination is synonymous to the more precise but less commonly used “hemagglutination.”

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