1. 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

2. 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

3. 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.

4. Visualization of concepts

6. 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

7. 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

8. 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.

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