Immunology

In TCR genes, combinatorial joining of α and β V gene segments appear to generate segments appear to generate CDR1 and CDR2, whereas junctional flexibility and N-region nucleotide addition generate CD3.

TCR diversity is generated like antibody diversity but without somatic mutation, that is, the functional TCR genes generated by gene rearrangements during T-cell maturation in the thymus have the same sequences as those found in the mature peripheral T-cell population.

Although there are fewer TCR Vαand Vβgene segments than immunoglobulin VH and VL segments, this difference is offset by the greater number of J segments in TCR germ-line DNA.

Such P-region nucleotide addition can occur in the genes encoding all the TCR and Ig chains.

The absence of somatic mutation in T cells ensures that T-cell specificity does not change after thymic selection and therefore reduces the possibility that random mutation might generate a self-reactive T cell.

T-cell receptor Complex: TCR-CD3

Membrane-bound immunoglobulin on B cells associates with another membrane protein, the Ig-α/Ig-β heterodimer, to form the B-cell antigen receptor. Similarly, the T-cell receptor associates with CD3, forming the TCR-CD3 membrane complex. In both cases, the accessory signal transduction after interaction of B or T cell with antigen.

The αβ or γδ TCR heterodimer determines the ligand-binding specificity, whereas the CD3 dimers are required for membrane expression of the T-cell receptor and for signal transduction.

The cytoplasmatic tails of the CD3 chains contain a motif called immunoreceptor tyrosine-based activation motif（ITAM）.

ITAM are found in a number of other receptors, including the Ig-α /Ig-β heterodimer of the B-cells receptor complex and the Fc receptors for IgE and IgG.

The ITAM sites have been shown to interact with tyrosine kinases and to play an important role in signal transduction.