The process in which red blood cells suspended in a solution are reduced to a solid pellet by a red cell antibody. This term is synonymous to the shorter “agglutination.” Hemagglutination is the core reaction in blood banking testing, as the majority of our testing has for years relied on detection of this process. Essentially, the presence of and characteristics of the hemagglutination reaction in-vitro are used to predict whether or not a particular antibody will cause a problem for a patient in-vivo.