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To understand antibodies and how they work, the following sections should be read. The section on B cells explains how and when antibodies are produced.
T cells have antigen receptor molecules on the surface, which are composed of two distinct polypeptide chains - alpha and beta chains. A minority of the population of T cells has two other chains, a gamma chain and a delta chain. Little is known of the activities of these gamma-delta T cells. Like antibodies, the TcR has both a variable and a constant region. As described for antibodies, the variable region of the TcR is created by gene rearrangement and selection of minigenes in a random order to create diversity of antigen recognition. The alpha and beta chains are associated with a group of five proteins called CD3. The T cell receptor complex includes the alpha, beta chain TcR and the associated CD3 proteins. The recognition of antigen-MHC by the TcR-CD3 complex does not require any other molecules. However, other proteins on T cells are important in the interaction leading to T cell activation. These "accessory molecules" on T cells (Th or Tc) and their partner ligands on the other cell (APC or target cell) give a second signal to stimulate T cells. T cell receptors recognize antigen, but not the same way that antibody does. Antibodies recognize antigen in its native form, but the T cell antigen recognition by the TcR requires the antigen to be digested, degraded and presented on the surface of another cell (an antigen presenting cell - APC) in the context of MHC. What does that mean? A piece of the antigen is found on the surface groove of the MHC molecule and is on the surface of the APC. Contact results in T cell activation. ANTIGENS An antigen is a substance capable of inducing a specific immune response. The term is derived from the generation of antibodies to such substances. Specific immune responses require recognition molecules like the T cell receptor or antibodies, which recognize the antigen, or parts of it, and stimulate a response by the specific arm of the immune response (from the T or B cells). Often antigens are foreign proteins (or parts of them) which enter the body via an infection. Sometimes, however, the body's own proteins, expressed in an inappropriate manner (where or when they are not usually seen), are treated like antigens by the immune system. It is important to recognize that bacteria or viruses are not themselves antigens but they contain antigens both on their surface and inside them. Such antigens can be isolated and used to safely vaccinate against infection with the whole organism. |
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