Both B-cell and T-cell use surface molecules to recognize antigen, they fulfill this in very different ways. B-cell receptor or antibodies can recognize an antigen alone but T-cell receptor only can recognize pieces of antigen, bound with the surface molecule of other cells. These antigen pieces are attached with the binding groove of a cell surface protein known as Major histocompatibility complex (MHC) molecule. These proteins are encoded by a cluster of gene present on MHC locus. The antigenic fragments are generated inside the cell by the digestion of antigen. Then antigenic peptide and Major histocompatibility complex (MHC) appears on the cell surface.
Structure of MHC: MHC are membrane bound glycoproteins. MHC proteins are highly polymorphic and are encoded by tightly linked cluster of genes present only in vertebrates. There are two main classes of MHC molecules: class-I and class-II. These molecules are very similar in their structure but differ in how the cell express them and in the source of the antigens they to T-cells.
Class-I MHC: It is a transmembrane glycoprotein consisting extracellular domains and can bind protein Ag of 8-10 amino acid residue. Class-I MHC molecule are present on all nucleated cells in the body and specialized in presenting antigens that originates from the cytosol or endogenous protein. These are presented to the cytotoxic T-cell (CD8+), which recognize and kill these cells expressing such intracellular antigens.
Class-II MHC: It is heterodimer of two subunits and consisting four extracellular domains. It can bind protein Ag of 13-18 amino acid residue. Class-II MHC are expressed almost on a subset of leukocytes called antigen-presenting cells (APCs). These are specialized in presenting antigens from extracellular space that have been engulfed by these cells and expressed it on the cell surface. Class-II MHC molecule present the antigenic peptide to helper T-cell (CD4+) which then become activated and go on to stimulate our immune system to destroy the extracellular invaders.
The MHC-II molecules plays a vital role in immune response to a particular antigen by presenting it to the T-cell receptor ant through this it enhances the immune response. MHC-I present on the surface of every normal nucleated cells and protect the from the phagocytic cells and from appoptosis and he;p to keep the healty cells alive.