The immune system is self-restricted and the self-restriction is seen with immunological responses that involve both the MHC class I

class 1 MHC genes express glycoproteins and MHC molecules, sometimes referred to as MHC antigens on nearly all nucleated cells. each MHC class 1 gene codes for a transmembrane glycoprotein of approximate molecular weight 43 k da, which is referred to as the alpha or heavy chain. it comprises three extracellular domains alpha 1 alpha 2 alpha 3. the alpha 3 domain is highly conserved and interacts with CD8 membrane molecule present on Tc cells. the highest levels of class 1 molecules are expressed by lymphocytes whereas fibroblasts, liver hepatocytes and neural cells expresses very low levels of class 1 MHC molecules.

the T cell molecule CD4 binds to the invariant portion of all MHC class 2 molecules.

Class 2 MHC gene express MHC molecule constitrutively only on antigen presenting cells.APCs are professional and non professional APCs. professional APCs are Dendritic cells, macrophages, and B lymphocytes. non professional antigen presenting cells are like fibroblasts, thymic epithelial cells, glial cells can be induced to express class 2 MHC molecules or a co stimulatory signal.