Autoimmunity - an Introduction
Acquired Immunity - T Cells
The adaptive immune system is composed of highly specialized, systemic cells and processes that eliminate or prevent pathogenic challenges. The adaptive immune response provides the vertebrate immune system with the ability to recognize and remember specific antigens of the pathogens (to generate immunity), and to mount stronger attacks each time the pathogen is encountered. Adaptive immunity is primarily based on two different cell populations.
T cells are intimately involved in cell-mediated immune responses. They either identify and destroy infected cells of the body (cytotoxic T cells) or they secrete molecules that modulate the immune response (T helper cells) and stimulate other immune cells.
Via the specific T cell receptor (TCR), T cells bind to antigen presenting cells, e.g. macrophages, dendritic cells, B cells or infected somatic cells. Every T cell expresses an individual TCR molecule that interacts exclusively with one single antigen. A T cell is activated by binding to a macrophage that presents this special antigen.
An activated cytotoxic T cell recognizes the corresponding antigen displayed on the surface of an infected cell, binds to it and releases the protein perforin. Perforin inserts into the target cell's plasma membrane, forms pores and kills the infected cell.
T helper cells produce and secrete cytokines that modulate the immune response. The interleukins IL-1, IL-2, IL-4, IL-5, IL-10, IL-13, IL-17 or IL-23 belong to this group of biomolecules, as well as interferon-gamma or tumor necrosis factor-beta. With these effector molecules or via direct contact to a target cell, T cells regulate the immune response and modulate the activity of other cells of the immune system. As an example, an activated T helper cell may bind to a B cell. This direct cell-to-cell interaction will induce cell division and maturation of the B cell.