**3. Adaptive immune response**

#### **3.1 T cell**

Research by Rank et al. in athymic mice demonstrated the importance of T lymphocytes for chlamydial immunity. In this study, after inoculation of *C. muridarum* intravaginally in mice, chronic infection occurred in athymic mice, while wild-type controls were able to eliminate the infection within 20 days [32].

T cells cannot recognize pathogen antigens without MHC molecules. MHC II molecules are only found on professional antigen presenting cells, including DC, macrophage, B cell, while MHC I molecules are expressed on the surface of all nucleated cells. CD4 T cells recognize antigens presented in MHC class II and CD8 T cells are activated by MHC class I antigen complexes [72]. In fact, both T cell subsets have been shown to recognize *C. trachomatis* antigens such as outer membrane protein 2 (Omp2), polymorphic outer membrane protein D (POMP-D), MOMP, heat shock protein 60 (HSP60), chlamydial protease-activating factor (CPAF), PmpG, PmpF, and RpIF [58].
