During infection, naïve CD4+ T cells develop into distinct CD4+ Th cells, mainly including Th1, Th2, Th17, Th22, Tfh, and CD4+ cytotoxic T lymphocytes (CTLs), participating in pathogen clearance, by producing massive pro-inflammatory cytokines and chemokines [40,41]; exerting direct cytolysis [42,43]; and augmenting the functions of macrophages, CD8+ CTLs, B cell, and antibodies [7,42]. Here, CD4 is linked to infection.