The so-called cotton antigen, a polymer of 5,7,3′,4′ tetrahydroxyflavan 3–4 diol (THF) extracted from cotton plant bracts, precipitated 58% IgG, 54% IgM and 15% IgA in a sample of human serum and concurrently antibody activity in this serum to three physicochemically distinct antigens was reduced by 50%, 52% and 62%, respectively. THF also reacted with five myeloma IgG at dilutions equal to, or higher than, comparable dilutions of human serum and gave positive gel diffusion reactions with H and L chains, Fab and Fc pieces of IgG.

On the basis of these results it is suggested that human immunoglobulins are nonspecifically precipitated by the tannin-like polymer.

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