Production and Mode of Action of Killer T Cells (CTLs): A Summary Diagram
A sequence to better understand the production and mode of action of killer cells is presented in the form of a diagram summarizing the steps involved in this adaptive immunity. Killer cell production requires the involvement of several categories of immune cells: T4 lymphocytes and T8 lymphocytes. These two categories of lymphocytes carry receptors specific to a given antigen; these receptors are called TCRs (T-cell receptors) and allow the body to recognize the antigen, which it considers a foreign element. Antigen recognition by T4 and T8 lymphocytes involves the intervention of an antigen-presenting cell (APC), such as a dendritic cell or a macrophage that has performed phagocytosis. This first phase of adaptive immunity corresponds to the clonal selection phase, where all clones of immunocompetent lymphocytes—that is, those possessing the correct specific receptor—are sensitized to the antigen. The T4 lymphocytes then differentiate into helper T cells that secrete interleukin. Interleukin triggers the proliferation of lymphocytes sensitized to the antigen: proliferation of helper T cells as well as T8 lymphocytes. In the latter case, a clone of antigen-specific T8 lymphocytes is obtained, ready to act, along with a clone of memory T8 lymphocytes. This second step corresponds to the clonal proliferation phase, which increases the pool of immune cells ready to act. Interleukin also differentiates T8 lymphocytes into cytotoxic T cells. CTLs are rich in granules containing perforin. This third step corresponds to the clonal differentiation phase. In a final step, CTLs recognize infected cells because these cells possess antigenic fragments on their surface, presented by MHC molecules. Contact between the cytotoxic T lymphocyte (CTL) and the infected cell triggers the migration of granules and the exocytosis of perforin, leading to the destruction of the infected cell in a mechanism known as the "kiss of death." This final step corresponds to the effector phase.
The production of killer cells, or cytotoxic T lymphocytes, requires the involvement of several categories of immune cells, and in particular, cell-cell cooperation between T4 and T8 lymphocytes.