Major histocompatibility complex genes and evidence for their occurrence in the tilapia, Oreochromis niloticus
Cells involved in the immune system
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- 3.1.2.2 T-lymphocytes
| 3.1.2 Cells involved in the immune system
3.1.2.1 B-lymphocytes B-lymphocytes express a unique antigen-binding membrane receptor which is called the antibody molecule. The antibody molecule is composed of two identical heavy polypeptide chains and two identical light polypeptide chains held together by disulfide bonds. The amino-terminal ends of each heavy and light-chain (VH and VL) constitute a variable sequence and form a cleft within which antigen binds. When a B cell encounters the antigen for which its membrane-bound antibody is specific, the cell begins to divide rapidly, its progeny differentiates into memory B cells and plasma cells that secrete soluble antibody proteins. Memory B cells have a longer lifespan and continue to express membrane-bound antibody with the same specificity as the original parent cell. Plasma cells do not express membrane-bound antibody, but secrete enormous amounts of antibody during their short life period. 3.1.2.2 T-lymphocytesT-lymphocytes express a unique membrane receptor for antigen. The receptor is a heterodimer, composed of two protein chains, either or , which are linked by disulfide bonds. The amino-terminal ends of the chains fold together to form the antigen-binding cleft of the T cell receptor. T cells are of two types : i) T helper cells (TH cells) which are restricted for MHC class II molecules and display CD4 membrane glycoproteins. ii) T cytotoxic cells (TC cells) which are restricted for MHC class I molecules and display CD8 membrane glycoproteins. The CD4 and CD8 glycoprotein molecules are invariant in structure and are thought to be accessory molecules which play a role in the interaction of T cells with non-T cells. These glycoproteins appear on the surface of the T-cells just before the appearance of the T-cell receptor during their maturation in the thymus. CD4 acts as a marker of TH cell populations which promote activation and maturation of B-cells and cytotoxic T-cells, and control antigen-specific chronic inflammatory reactions through stimulation of macrophages. These molecules form links with class II MHC on the cell presenting antigen. Similarly, the CD8 molecules on the surface of cytotoxic T-cells associate with MHC class I molecules. An important difference between CD4 and CD8 cells is that CD4 cells are activated by signals provided both by an antigen-MHC class II molecule complex and IL-1 (Interleukin) provided by the antigen presenting cells. On the other hand, the proliferation and differentiation of CD8 cells depend on a signal provided by the antigen-MHC class I complex and an additional signal, IL-2 which is released as a consequence of the activation of CD4 helper T-cells. In response to the recognition of antigen by the receptor in conjunction with the MHC molecule, a TH cell secretes various growth factors, which are collectively known as lymphokines. When a TH cell is activated, it becomes an effector cell secreting various lymphokines which play an important role in activating B cells, TC cells, phagocytic cells, and various other cells that participate in the immune response. Under the influence of TH-derived lymphokines, when the TC cell receptor together with MHC molecules recognises an antigen, it proliferates and differentiates into effector cells called Cytotoxic T Lymphocyte (CTL). In contrast to the TH cell, the CTL does not generally secrete lymphokines and instead acquires cytotoxic activity. The CTLs monitor the cells of the body and eliminate any that display antigen, such as virus-infected cells, tumor cells, and cells of a foreign tissue graft. Such cells displaying foreign antigen complexed to an MHC molecule are called altered self-cells. Download 1.2 Mb. Share with your friends:
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