Suppressor T cells induced in vitro by high concentrations of antigen release suppressor factors into their medium if incubated for 24 hours with antigen. Since there are reports that extracts of suppressor cells are related to immune response gene function, the capacity of responder or nonresponder strains to produce suppressor factor (SF) was investigated. Both responder and nonresponder strains produced SF to (T,G)-A--L and GAT. This together with differences in the target and the lack of H-2 restrictions indicates that SF is quite distinct from suppressor cell extracts, despite both having I-J controlled determinants and an antigen combining site.

The structure of SF was analysed using serological means. Rabbit and mouse antisera were raised against SF purified by absorption and elution from antigen columns. Rabbit antisera were not specific for strain or antigen specificity of SF, but did not react with helper factor, and thus defined a “constant region.”

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