* T cell responses are initiated by signals that are generated by TCR recognition of peptide-MHC complexes on the surface of an APC and through signals provided at the same time by costimulators expressed on APCs.
* The best-defined costimulators are members of the B7 family, which are recognized by receptors of the CD28 family expressed on T cells. The expression of B7 costimulators on APCs is increased by encounter with microbes, providing a mechanism for generating optimal responses against infectious pathogens. Some members of the CD28 family inhibit T cell responses, and the outcome of T cell antigen recognition is determined by the balance between engagement of activating and inhibitory receptors of this family.
* T cell responses to antigen and costimulators include changes in the expression of surface molecules, synthesis of cytokines and cytokine receptors, cellular proliferation, and differentiation into effector and memory cells.
* The surface molecules whose expression is induced on T cell activation include proteins that are involved in retention of T cells in lymphoid organs, growth factors for cytokines, effector and regulatory molecules, and molecules that influence the migration of the T cells.
* Shortly after activation, T cells produce the cytokine IL-2 and express high levels of the functional IL-2 receptor. IL-2 drives the proliferation of the cells, which can result in marked expansion of antigen-specific clones.
* CD4+ helper T lymphocytes may differentiate into specialized effector TH1 cells that secrete IFN-y, which mediate defense against intracellular microbes, or into TH2 cells that secrete IL-4 and IL-5, which favor IgE- and eosinophil/mast cellmediated immune reactions against helminths, or into TH17 cells, which promote inflammation and mediate defense against extracellular fungi and bacteria.
* The differentiation of naive CD4+ T cells into subsets is induced by cytokines produced by APCs and by the T cells themselves. The differentiation program is governed by transcription factors that promote cytokine gene expression in the T cells and epigenetic changes in cytokine gene loci, which may be associated with stable commitment to a particular subset. Each subset produces cyto-kines that increase its own development and inhibit the development of the other subsets, thus leading to increasing polarization of the response.
* T cells of the CD8+ subset proliferate and differentiate into cytotoxic T lymphocytes (CTLs), which express cytotoxic granules and can kill infected cells.
* Some activated T cells may differentiate into memory cells, which survive for long periods and respond rapidly to antigen challenge. The maintenance of memory cells is dependent on cytokines such as IL-7, which may promote the expression of antiapoptotic proteins and stimulate low-level cycling. Memory T cells are heterogeneous and consist of populations that differ in migration properties and functional responses.
Was this article helpful?
All Natural Immune Boosters Proven To Fight Infection, Disease And More. Discover A Natural, Safe Effective Way To Boost Your Immune System Using Ingredients From Your Kitchen Cupboard. The only common sense, no holds barred guide to hit the market today no gimmicks, no pills, just old fashioned common sense remedies to cure colds, influenza, viral infections and more.