TCR RECEPTORS Main receptors -αβ(Alpha &beta) - γδ(Gamma& delta) Co-receptors -CD4&CD8 molecules Co-stimulatory receptors -CD28 Signal transduction molecules -e, g, d and z chains Adhesion & co-stimulation -Integrin

αβ(Alpha &beta) and γδ(Gamma& delta) Majority(>95%) of T cell receptors are αβ 1-5% are γδ, role not clearly known Recognize antigens on MHC’S Structure consist of -1 variable (V) domain -1 Ig-like constant (C) domain -A hydrophobic transmembrane region(T m) -A short cytoplasmic region

Genetics Βγ chains-coded for by gene clusters found on chromosome 7 Multiple different V region gene segments are present in each gene cluster One V region gene segment is assembled and is joined to the constant region gene segment. The mRNA) transcript is spliced and a mature mRNA) transcript is produced which is translated into αδ or Βγ protein chains Note The expression of a complete V–C gene product excludes further rearrangement events This phenomenon is referred to as allelic exclusion (allelic exclusion) The T cell becomes committed to the expression of a single V–C α chain, and a single V–C β chain.

Visualization of concepts

The TCR CD3 complex CD3(αβδε) and ζ molecules They exist as dimers i.e. -CD3αβ, -CD3eg dimer -CD3ed dimer -CD3zz dimer They are non covalently associated with TCR Play a role in signal transduction that leads in T cell activation

CD4&CD8 co-receptors Both consist of 1 extracellular Ig domain, 1hydrophobic transmembrane region, and a 1, highly basic cytoplasmic tail about 25 amino acids long.  CD4 has four extracellular Ig-like domains, a hydrophobic transmembrane region, and a highly basic cytoplasmic tail 38 amino acids long Are expressed on the surface of mature T cells Participate with TCR receptors in recognition of antigens on MHC hence the name co-receptors CD4-MHCII CD1-MHCI   CD4 is found in helper T cells and CD8 is found in cytotoxic T cells About 65% of mature a positive T cells in the blood and lymphoid tissues express CD4. and 35% express CD8

The two N-terminal Ig-like domains of the CD4 protein bind to β2 domain of the class II MHC molecule CD8 molecules exist as disulfide-linked heterodimers composed of two related chains called CD8α and CD8β The Ig domain of CD8 binds to the non polymorphic α3 domain of class I MHC molecules.

Co-stimulatory molecules CD28 recognize co stimulatory molecule B7 found APCs, costimulatory molecules are important in B cell activation