Note: Hemagglutination may be either direct, in which erythrocyte antigens are reactants, or indirect (passive) for coated antigen or, in the case of reverse (passive) assays, coated antibody. One of the most common uses of hemagglutination is to quantitate the number of hemagglutinating viruses (cf. influenza) or their soluble hemagglutinating surface subunits. It can also be used for blood typing.