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A reaction in which suspended particles are aggregated or clumped. It occurs upon the admixture of another type of particle, a change in the composition of the suspending fluid, or the addition of a soluble agent that acts as a bridge between two or more particles. The reaction is a secondary one in that the process resulting in agglutination occurs after the primary antigen-antibody linkage has taken place.
The particles undergoing agglutination may be either unicellular or microscopic multicellular organisms (such as bacteria and parasites), individual cells of multicellular organisms (such as erythrocytes and lymphocytes), or artificial particles (such as beads of plastic, glass, or polysaccharide). The immunological specificity of agglutination depends upon the uniqueness of the reaction between a marker substance on one type of particle and a receptor on either another type of particle or a specific antibody in solution. The marker can be a usual biological component of the surface of the particle or blood group substance on red cells. It can be an enzymatically or a chemically modified chemical group on the surface of biological particles. It can also be an adsorbed or a chemically attached substance. The attachment can be to biological particles or artificial ones. The receptor can be a biological component of the particle, an attached antibody, or antibody in solution. A reverse reaction is one in which the antibody is attached to a particle and the addition of the antigen causes the mixture to clump. Inhibition of agglutination can also be used to test for antigens, especially of low molecular weight, in a manner similar to that for agglutination itself. See Antigen-antibody reaction, Immunoassay