Lymphocyte antigen-specific receptors

In the defense mechanisms of the immnne system, helper T cell activation is essential for the initiation of a protective immnne response to pathogens and tumors. HLA-DR, the predommant isotype of the human class II major histocompatibility complex (MHC), plays a central role in helper T cell selection and activation. HLA-DR proteins bind peptide fragments derived from protein antigens and display them on the surface of antigen-presenting cells (APC) for interaction with antigen-specific receptors of T lymphocytes. 

An antigen is a molecule recognised by specific receptors on cells of the immune system such as B lymphocytes. 

Immunologically competent cells, whether they are T or B lymphocytes, have membrane receptors that are specific for an antigen. It is basically the binding of the antigen to the specific receptor on the appropriate lymphocyte which initiates the whole process, stimulating the cell to proliferate and producing a clone of identical cells, a process known as clonal selection . The nature of the secondary response is due in the main to this large number of cells now available. 

There is also considered to be a population of T suppressor cells which acts to down-regulate both T helper cells and B cells, whether through antigen-specific or idiotype-specific mechanisms. The epitopes on the lymphocyte receptor (id-iotype) recognized by the receptor on another lymphocyte (anti-idotype) can form a network of interactions through which suppression may be mediated. 

At the surface of T and B lymphocytes, specific receptors are found that bind antigens and set intracellular signal chains in motion (review Weiss and Littman, 1994 Paul and Seder, 1994 Qian and Weiss, 1997). These may lead to increased cell division, programmed cell death or a functional recoining of lymphocytes. 

There are three different kinds of lymphocytes that have specific functions T cells, B cells and NK cells. T cells develop in the thymus while B cells develop in the adult bone marrow. The thymus and the bone marrow are the primary lymphoid organs where lymphocytes acquire specific cell surface receptors that give them the ability to recognize antigens. NK cells are cytotoxic lymphocytes that develop in the bone marrow. The phagocytes are made up of either monocytes (macrophages) or polymorphonuclear granulocytes, which include neutrophils, eosinophils and basophils. 

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