Abstract
The serum of lactating rats that have never been infected with the protozoan parasite Trypanosoma lewisi contains a rheumatoid factor-like immunoglobulin M (IgM). This IgM amplifies a specific immunoglobulin G (IgG) response to the parasite and accounts for the unusual resistance of previously uninfected lactating rats and their suckling pups to infection with T. lewisi. A similar rheumatoid factor-like IgM, which is induced late in the usual course of infection with T. lewisi in nonlactating rats, amplifies an earlier IgM response and terminates the infection. To our knowledge, this is the first description of a rheumatoid factor, which is classified as an autoimmune antibody, acting in a protective manner.
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