where are helper t cells found in the body
T helper cell 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.