CD4 and infection: Murine studies on Chlamydia natural history and immunity have demonstrated that only CD4+ T helper type 1 (Th1) responses (IFN-γ-mediated) are needed for clearance of initial infection and prevention of pathology, but both Th1 and antibody responses contribute to protection against reinfection (Morrison and Caldwell, 2002; Brunham and Rey-Ladino, 2005).