Agglutinin

An agglutinin is a substance capable of causing agglutination, the clumping together of particles such as red blood cells or bacteria. Agglutination occurs when the agglutinin binds to antigens on the surfaces of two or more particles, cross-linking them into visible aggregates. While the term is most commonly used for antibodies, other proteins, notably plant lectins, can also act as agglutinins.

Immunoglobulin agglutinins are antibodies that cross-link target cells or particles. IgM, with its pentameric structure, is especially efficient at agglutination because it has many binding sites. IgG can also cause agglutination but often with lower efficiency and may require higher antigen density or secondary mechanisms such as complement activation. In serology and transfusion medicine, naturally occurring agglutinins such as anti-A and anti-B antibodies recognize ABO antigens on red blood cells and produce characteristic agglutination patterns that determine a person's blood group.

Cold agglutinins are a subset that react at low temperatures and can cause red blood cell clumping in vitro or in vivo, sometimes contributing to autoimmune hemolytic anemia. Agglutination tests in the laboratory include direct agglutination tests, indirect (passive) agglutination, and latex agglutination, used to detect antibodies or antigens against bacteria, viruses, or other targets.

Non-immune agglutinins such as plant lectins (phytoagglutinins) bind specific carbohydrate residues on cell surfaces and can cross-link cells, a property exploited in research to study glycosylation patterns and cell recognition. The study of agglutinins spans immunology, hematology, microbiology, and clinical diagnostics.