CD4 and infection: Macrophages engulf the bacteria and produce pro-inflammatory factors (45); IFN-γ secreted by NK cells not only kills infected host cells but also induces an immune response to Th1 (46, 47); When infection leads to the development of antigen-specific immunity, CD4+ T cells along with B cells produce immunity where chlamydia-specific Th1 CD4+ T cells and antibodies are considered protective (48, 49), whereas CD8+ T cell response is considered non-essential or even pathogenic (50, 51).