agglutinins

Agglutinins are substances, typically antibodies, that cause agglutination—the clumping together of particles such as red blood cells, bacteria, or yeast—by cross-linking surface antigens on different particles. In immunology, the term most often refers to antibodies that bind antigens present on distinct cells or particles, producing visible aggregates.

In humans, agglutinins include natural antibodies and those formed after exposure. The best-known examples are isohemagglutinins against ABO blood group antigens. These are predominantly IgM antibodies that can cause rapid RBC agglutination upon contact with compatible antigens. IgG antibodies can also act as agglutinins, especially when the target cells display sufficient antigen density or after prior sensitization (alloantibodies or autoantibodies).

The mechanism involves cross-linking of antigenic determinants on separate particles, creating a lattice that results in visible clumping. IgM, a pentamer with ten antigen-binding sites, is highly effective at agglutination at low antigen densities, while IgG, typically a monomer with two binding sites, can mediate agglutination under favorable conditions. Some agglutinins are incomplete and require the antiglobulin (Coombs) concept to detect them in diagnostic testing.

Applications and clinical relevance include blood typing and crossmatching for transfusion safety, and the management of hemolytic disease of the newborn, where maternal agglutinins can target fetal red cells. Agglutination assays are used diagnostically to detect antibodies, antigens, or immune complexes, and in bacteriology to identify organisms by particle clumping.

The term agglutinin can also refer more broadly to any substance capable of causing agglutination, including certain plant lectins or other macromolecules that promote clumping of cells.