CD4 and infection: Following infection, naive CD4+ T cells can differentiate into distinct T-helper (Th) subsets, including Th1, T follicular helper (Tfh), Th2, Th17, and regulatory T cells as well as memory cell precursors.1 Naive and newly activated CD4+ T cells sense changes in the microenvironment and integrate those signals through the upregulation of lineage-defining transcriptional factors.