ABSTRACT
Like B cells, T cells are selected by their specific antigen to take part in the immune response (clonal selection). They proliferate, producing a large number of cells (clonal expansion) and develop into effector cells, that is, Th cells or Tc cells, or into memory cells (Figure 1). The first memory cells to be produced (central memory, Tcm) have all the homing and chemokine receptors, like naive cells, to allow them to recirculate to the T-cell areas of the lymphoid tissues. These cells act as a clonal reservoir, are capable of proliferating on subsequent antigen activation, and give rise to effector memory Clonal expansion and development of memory T cells. Specific naive T cells are selected by antigen to proliferate (clonal selection and clonal expansion) and develop effector functions, that is, T-cell help or cytotoxicity (primary response). Central memory T cells (Tcm) are also produced that are capable of proliferation and recirculation through lymphoid tissues. On subsequent contacts with specific antigen (e.g., secondary response) they develop into effector memory cells (Tem) that rapidly acquire full function (within a few hours) and are capable of homing to sites of inflammation, where they mediate help or killing https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429212857/7842d445-aa7b-4eab-a2db-7351fa123550/content/fig1_F5.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> 142 cells (TEM). They rapidly develop effector function, for example cytotoxic activity, and are capable of migrating into sites of inflammation in nonlymphoid areas. On initial activation by antigen, Th cells produce IL-2, an autocrine growth factor for T-cell proliferation and also express IL-2 receptors (Figure 2). This results in clonal expansion. Other surface molecules induced by activation of T cells include CD40L (CD154), which through interaction with CD40 on dendritic cells leads to the production of cytokines (e.g., IL-1, IL-12) required for induction of Th subsets (see below).
