CD4 and infection: The early childhood exposures to infection were associated with the proliferation and expansion of B or T cell clones (Olszak et al., 2012), and early common infections before the maturity of CD4 T cells are likely to adjust the constitution of symbiotic gut microbiota and contribute to an immune tolerance state toward some antigens with the aid of regulatory T cells and sIgA from mothers (Torow et al., 2015).