It is thought that antigen-specific CD4+ T cells can help control infection through their production of IFN-γ, as endogenous C. trachomatis-specific CD4+ T cells (14) and transgenic CD4+ T cells corresponding to both C. trachomatis and the mouse-adapted pathogen C. muridarum (8, 14, 15) have all been shown to produce IFN-γ. This evidence concerns the gene CD4 and infection.