Growing Up at Home

Interaction of a “naïve” T lymphocyte with an appropriately presented antigen induces a primary cell–mediated immune response. If the antigen is encountered in skin, the naïve T cells in lymph nodes become effector memory T cells. They then upregulate skin-homing addressins, migrate into skin, and, with any luck, work there to destroy persistent threatening antigens, such as those associated with infectious agents. After the battle is finished, the skin contains many tissue-resident memory T cells (T_RM cells), even in skin that never encountered the antigen. These skin-resident memory cells stay in skin to establish an on-site defense against similar antigens that might breach the skin barrier at some future date. However, other, phenotypic…