To determine whether innate immunity was also sufficient to fully protect FVB mice from infection in the absence of CD4+ T cell mediated immunity, mouse strains known to develop severe PcP following CD4-depletion (Balb/c, C.B-17, or C57BL/6) and FVB mice were treated with anti-CD4 monoclonal antibody and infected via intratracheal inoculation. This evidence concerns the gene CD4 and infection.