Studies using mouse models of genital chlamydial infection have shown that infection causes dendritic cells (DCs) to undergo maturation and strongly up-regulate major histocompatibility complex (MHC) antigens, and costimulatory molecules such as cluster of differentiation (CD) 40, CD80 and CD86, which are crucial for effective activation of the CD4+ T helper cell type 1 (Th1) cell response, especially the CD4 T cells and interferon (IFN) -γ production required for host defense [9–11]. This evidence concerns the gene CD4 and infection.