Figure 1.

Heterogeneity in memory CD4 T cells. Memory CD4 T cells can be found in lymphoid organs, blood, and at tissue sites. Stem cell memory T cells (Tscm) and central memory T (Tcm) cells are found in lymphoid organs and in the blood. Both populations are relatively undifferentiated compared to other memory CD4 T cell subsets, and are long lived. Follicular helper memory T (Tfh) cells can also be found in the blood and lymphoid organs. They express the B cell follicle homing receptor, CXCR5, which can position them near B cell follicles to provide rapid B cell help upon reactivation. Effector memory (Tem) and tissue resident memory T (Trm) cells can both be found in peripheral tissues and are more differentiated than Tcm and Tscm. Tem are migratory, passing through tissues and the blood, while Trm are restricted to tissues. Both populations can respond rapidly to tissue invading pathogens (15, 17, 23–27).