• WordNet 3.6
    • n precipitin an antibody that causes precipitation when it unites with its antigen
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Century Dictionary and Cyclopedia
    • n precipitin An adaptation-product which is formed in the blood-serum of an animal of one species when immunized with albumins or albuminous derivatives obtained from an animal of an alien species. When the sera of the two animals are brought together a precipitate results, in the formation of which the immunizing substance plays the active and the immune substance the passive rôle.
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In literature:

No white precipitin, as it is called, would form.
"The Silent Bullet" by Arthur B. Reeve
The precipitin test depends on a somewhat analogous reaction.
"Encyclopaedia Britannica, 11th Edition, Volume 4, Slice 1" by Various
The method of precipitin testing follows that outlined by Leone (1949).
"Myology and Serology of the Avian Family Fringillidae" by William B. Stallcup