Figure 1.

Population flows. In the thymus, progenitor cells express a unique functional T-cell receptor and either CD4 or CD8 co-receptors. These cells leave the thymus with a small clone size to enter their respective naive subpopulation (a). After exposure to cognate antigen, cells undergo clonal selection to proliferate and differentiate into their respective memory subpopulation. Homeostatic proliferation supplements these dynamic flows to maintain each subpopulation. Note that this figure does not show transient subpopulations such as effector cells. In (b), graphs illustrate changes with age: thymic output declines, the % naive declines, and the % memory increases correspondingly.