precipitin

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An antibody that reacts with a specific soluble antigen to produce a precipitate.

[PRECIPIT(ATE) + –IN.]

The visible result of the chemical interaction of antigen and antibody. Not all antibodies will result in precipitation, yet they may participate in agglutination reactions or add onto particulate antigens, and evidence for their occurrence together with precipitating antibody can be obtained for most sera. Precipitins may be noted qualitatively or be quantified by noting the end-point dilution (titer) of serum required to give a precipitate at the threshold of visibility, or the amount of antibody may be determined in milligrams or micrograms by analysis of the precipitate with correction for the antigen contained therein. See also Antibody; Antigen.

An antibody to soluble antigen that specifically aggregates the antigen in vivo or in vitro to give a visible precipitate.

• p. curve — a plot showing the amount of antibody precipitated with increasing amounts of antigen added in a precipitin reaction.
• p. reaction, p. test — a reaction involving the specific precipitation of an antigen in solution by mixing with a specific antiserum in the presence of electrolytes. See also lattice theory.