Hear This Illustration: Activation and Clonal Selection of a Helper T Cell
This flow diagram depicts the action of helper T cells following the introduction of foreign substances, or antigens, into the body. Recall that there are many functional types of T cells. Most of the time the helper T cells circulate throughout the body in an inactive state, however, the presence of antigen transforms the helper T cells into a cell that can orchestrate the immune response. Antigens are typically processed by antigen-presenting cells, such as macrophages and B cells. Such a cell is shown at the top of the illustration labeled APC. These cells display on their cell surface portions of the antigen bound to the major histocompatibility complex type II proteins, or MHC II. These are unique proteins present on certain cells of an individual. Helper T cells with a receptor that fits the displayed antigen will be able to bind the antigen-presenting cell, as shown in the enlarged view. Note that the recognition of the antigen and binding of the antigen to the T cell receptor is insufficient to activate the helper T cell. Costimulation, as indicated by the arrow from the APC cell, is also required. This involves other surface proteins, or chemicals, released from the cells. One surface protein present on helper T cells is the CD4 protein and, consequently, helper T cells are also called CD4 cells. When the helper T cell receptor binds antigen the CD4 protein can bind to the MHC II protein, which provides a costimulatory signal that activates the helper T cell. Once activated, the helper T cell undergoes a clonal expansion through cell division, resulting in a large increase in the numbers of helper T cells with identical antigen receptors. These activated cells will release chemical signals called cytokines, which in turn will stimulate other immune cells whose primary goal is the destruction of the antigen. It is this signaling role that gives this type of T cell the designation of helper. As shown on the bottom right portion of the diagram, one population of the cloned helper T cells remains in circulation as memory cells. This ensures that future encounters with this specific antigen will result in a much more rapid and intense immune response.