Mature T cells can be divided into two types. This is based on the expression of two accessory molecules CD4 and CD8 on their plasma membrane.
• The T cells expressing CD4 molecules on the plasma membrane are designated as CD4 positive T cells.
• The T cells expressing CD8 molecules on the plasma membrane are designated as CD8 positive T cells.
CD4 and CD8 molecules are known as Co-receptors. The T cells recognise and bind peptide antigens only when they are presented in combination (complex) with MHC molecule.
T cells not only recognise the peptide antigen but also recognise the class of MHC molecule. The recognition of MHC molecule is mediated by co-receptorreceptor.
• The CD4 receptor bind to the class-II MHC molecule of MHC 2 peptide(antigen) complex.
• CD8 receptor binds to the class-I MHC molecule of the MHC 1 peptide(antigen) complex.
CD4 Co-receptor
• CD4 is expressed by Helper T-cells.
• CD4 co-receptor is expressed as a single polypeptide chain which has a molecular weight of 55 kilo daltons.
• CD4 co-receptor has four extracellular domains designated as D1 D2 D3 and D4.
• CD4 co-receptor also has a hydrophobic transmembrane region in alongside a plasmic tail.
• CD4 molecule binds to MHC 2 molecule. CD4 binds to the beta 2 domain of this MHC 2 molecule.
CD8 Co-recreceptor
• CD8 is expressed by Cytotoxic T-cells.
• CD8 Co-receptor is expressed as a dimer consisting of two polypeptide chains, Alpha chain and Beta chain.
• Each chain has a molecular weight of 30 to 39 kilo daltons.
• Each chain has a single extracellular domain and a hydrophobic transmembrane region in a cytoplasmic tail.
• CD8 molecule binds to MHC 1 molecule. CD8 is binds to the Alpha 3 domain of this MHC 1 molecule.
Functions of Co-receptor CD4 and CD8
• They act as recognition in adhesion molecules.• They recognise and bind MHC molecule and enhance intracellular adhesion between T-cells and the target or antigen presenting cells
• They enhance the signaling process for T-cell activation.
