IL-10 inhibits the Th1 response by blocking IFNγ production (de Waal Malefyt et al., 1992) and an excessive IL-10 response is associated with persistence of chlamydial infection in humans (Natividad et al., 2008; Öhman et al., 2006). This evidence concerns the gene IFNG and chlamydia trachomatis infectious disease.