<P> CD154, also called CD40 ligand or CD40L, is a cell surface protein that mediates T cell helper function in a contact - dependent process and is a member of the TNF superfamily of molecules . It binds to CD40 on antigen - presenting cells (APC), which leads to many effects depending on the target cell type . CD154 acts as a costimulatory molecule and is particularly important on a subset of T cells called T follicular helper cells (T cells). On T cells, CD154 promotes B cell maturation and function by engaging CD40 on the B cell surface and therefore facilitating cell - cell communication . A defect in this gene results in an inability to undergo immunoglobulin class switching and is associated with hyper IgM syndrome . Absence of CD154 also stops the formation of germinal centers and therefore prohibiting antibody affinity maturation, an important process in the adaptive immune system . </P> <P> The importance of helper T cells can be seen from HIV, a virus that primarily infects CD4 T cells . In the advanced stages of HIV infection, loss of functional CD4 T cells leads to the symptomatic stage of infection known as the acquired immunodeficiency syndrome (AIDS). When the HIV virus is detected early in blood or other bodily fluids, continuous therapy can delay the time at which this fall happens . Therapy can also better manage the course of AIDS if and when it occurs . There are other rare disorders such as lymphocytopenia which result in the absence or dysfunction of CD4 T cells . These disorders produce similar symptoms, many of which are fatal . </P> <P> Following T cell development, matured, naïve T cells leave the thymus and begin to spread throughout the body, including the lymph nodes . (Naïve T cells are those T cells that have never been exposed to the antigen that they are programmed to respond to). Like all T cells, they express the T cell receptor - CD3 complex . The T cell receptor (TCR) consists of both constant and variable regions . The variable region determines what antigen the T cell can respond to . CD4 T cells have TCRs with an affinity for Class II MHC, and CD4 is involved in determining MHC affinity during maturation in the thymus . Class II MHC proteins are generally only found on the surface of specialised antigen - presenting cells (APCs). Specialised antigen presenting cells are primarily dendritic cells, macrophages and B cells, although dendritic cells are the only cell group that expresses MHC Class II constitutively (at all times). Some APCs also bind native (or unprocessed) antigens to their surface, such as follicular dendritic cells, but unprocessed antigens do not interact with T cells and are not involved in their activation . The antigens that bind to MHC proteins are always short peptides, 8 - 10 amino acids long for MHC Class I, and up to 25 or so for MHC Class II . </P> <P> During an immune response, professional antigen - presenting cells (APCs) endocytose foreign material (typically bacteria or viruses), which undergoes processing, then travel from the infection site to the lymph nodes . Once at the lymph nodes, the APC begin to present antigen peptides that are bound to Class II MHC, allowing CD4 T cells that express the specific TCRs against the peptide / MHC complex to activate . </P>

Where are helper t cells found in the body
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