Mhc molecule

MHC molecules are composed of antigen-binding and immunoglobulin-like domains... 

Recognition of antigen is different in MHC molecules compared with immunoglobulins... 

The genes for MHC molecules, unlike immunoglobulin genes, do not undergo rearrangements to create structural diversity. The Pzm light chain is invariant, but the class I MHC heavy chain is the most genetically polymorphic... 

Peptides are bound differently by class I and class II MHC molecules... 

I MHC-peptide complexes and those that recognize class 11 MHC-peptide complexes utilize the same set of Va and Vp genes, and the principal feature that defines the site of class 1 MHC-TCR interaction, the cleft formed by the a and az subunits of the MHC molecule, is shared by both class 1 and class... 

Class 1 and class II MHC molecules bind peptide antigens and present them at the cell surface for interaction with receptors on T cells. The extracellular portion of these molecules consists of a peptide-binding domain formed by two helical regions on top of an eight-stranded antiparallel p sheet, separated from the membrane by two lower domains with immunoglobulin folds. These domains are differently disposed between the two protein subunits in class I and class II molecules. 

Structures of MHC molecules have provided insights into the molecular mechanisms of T-cell activation 312... 

Group of transmembrane proteins engaged in the presentation of small peptide fragments to T-cells. Two classes of Major histocompatibility complex (MHC) molecules exist both of which are encoded by a highly polymorphic gene cluster. MHC class I and class II proteins present peptide fragments to CD8+ and CD4+ T-cells, respectively. The human MHC is also known as HLA, the murine MHC as H-2 complex. 

Bacterial or viral proteins linking T-cell receptors and MHC molecules through simultaneous interaction with the constant domains of all MHC class II molecules and of T-cell receptor (3-chains. Hence, superantigens are polyclonal T-cell activators most likely involved in the development of autoimmune diseases. 

The T cell receptor is a clonotypic, T cell specific surface receptor that mostly recognizes peptides bound to MHC molecules on antigen-presenting cells. 

Class 1 MHC molecules are integral membrane proteins found on the surface of all nucleated cells and platelets. They are the classical antigens involved in graft rejection. 

Class 2 MHC molecules are expressed on the surface of B cells, macrophages, monocytes, various antigen-presenting cells (APCs) and certain cells of the T-cell famify. 

Class 3 MHC molecules consist of several complement components. 

CD4 is a transmemhrane glyeoprotein, approximately 55 kDa in size it serves as a eell-eell adhesion moleeule hy virtue of its speeifie affinity for Class 2 MHC molecules. Likewise, it may transduee signals or faeilitate TCR complex-mediated signal transduetion upon binding Class 2 MHC moleeules. CD4 is also the receptor for HIV (see Chapter 3). 

These T cells recognize peptide antigens bound to Class 1 MHC molecules on the surface of the target cell. During viral infections, viral peptides bind to selfMHCl molecules and are subsequently expressed on the cell surface. The MHCl molecules of transplanted tissues are themselves recognized by the Tc cells. 

Like TH cells, two signals are required to activate the Tc cell. The first is an interaction between the TCRs and the Class 1 MHC molecule/foreign epitope complex on the surface of the target cell. The second signal is that of IL-2 produced by the activated TH cell with the resultant release of cytotoxins which destroy the target cell. 

NK cells are a subset of lymphocytes found in blood and lymphoid tissues, especially the spleen. They are about 15 an in diameter, possess a kidney-shaped nucleus and have two or three large granules in the cytoplasm. They are derived from the bone marrow. NK cells have the ability to kill certain tumour lines and normal cells infected by virus. Killing by NK cells is not specific for viral antigenic epitopes, and is not restricted by MHC molecules. They do not possess CD3 but do express CD2, CD 16 and CD56, together with a low-affinity receptor for the Fc portion of IgG. 

Sprent J.E., Gao E.-K. Webb S.R (1990) T cell reactivity to MHC molecules immunity versus tolerance. Science, 248, 1357-1363. 

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