CD4 and infection: Subsequently, T cells, especially CD4+ Th1-type cells that were sensitized in the local lymph node and homing the infected lesion via the pulmonary circulation, can secrete Th1-typed cytokines such as IFN-γ, which activates macrophage phagocytosis, thereby killing intracellular bacteria, and TNF-α, which attracts more infiltrating macrophages and lymphocytes and promotes granuloma formation at the site of infection, ultimately leading to latent TB infection (LTBI).