precipitin

A precipitin is an antibody that produces a visible precipitate when it binds its soluble antigen. In serology, precipitating antibodies form a lattice with soluble antigens, causing turbidity or a discernible precipitate under appropriate conditions. The reaction is most readily observed near the equivalence zone of antigen and antibody concentrations, where cross-linking of multiple antigen molecules by antibodies yields an insoluble network.

Precipitin reactions underlie several immunodiagnostic techniques, notably immunodiffusion methods such as the Ouchterlony double-diffusion assay and radial immunodiffusion. In these assays, antigen and antiserum diffuse through a gel; at points where their concentrations are optimal, a visible line or arc of precipitate forms, enabling identification of antigens or assessment of antigenic relatedness. The principle is also used to determine the species origin of proteins in forensic and clinical contexts.

Historically, precipitin tests were among the early tools in serology for characterizing serum proteins and antigen–antibody relationships. Although many modern diagnostics rely on other modalities (enzyme-linked immunosorbent assays, agglutination, nephelometry), precipitin concepts remain foundational in understanding immune complex formation and are still used in some laboratories as qualitative analyses.

See also: immunodiffusion, precipit

itin test.