T Lymphocytes Classification, T lymphocytes are classified into two groups on the basis of molecules they express. The expressed proteins may be; CD4 proteins or CD8 proteins. So the T lymphocytes express CD4 proteins as surface cells called CD4 cells and similarly CD8 cells. The T lymphocytes in the peripheral protein express either CD4 or CD8 as surface molecules and not both. Both CD proteins are composed of α and β chains. The surface molecule express on the T cells are called T cell receptors or TCR. TCR are related to antibody molecule but act in different way to the antigen and the gene for which are located on the different chromosomes. Each TCR is composed of two chains; 1 α and 1 β chain. α chain has molecular weight between 40 to 50 kDa while β chain has molecular weight between 35 to 47 kDa. These are different from heavy and light chains of immunoglobulin or antibody only in the CDs. The CD in α chain are Uα and Cα, Cα are located towards the cell membrane and variable are located away from the cell membrane. TCR are never secreted like Ig of β lymphocytes, but are always associated with the cell membrane as integral membrane proteins. TCR are glycolproteins. These proteins (of TCR) are associated with other proteins, known as CD3 proteins, which do not play role in the antigen recognition but play a role in intra cellular signaling.
There are 5 CD3 proteins associated with the TCR. Namely;
1. Gamma (γ) proteins,
2. Delta (δ) proteins,
3. Epsilon (ε) proteins,
4. Zeta (ζ) proteins,
5. Eeta (η) proteins.
The ε chain can form heterodimer with γ and δ. Zeta ζ can form homodiamer with other ζ and heterodiamer with η. ζ-ζ chain are found in 90% and ζ-η chains are found in 10%.
So CD3 proteins play a role in signaling α and β proteins recognize antigen (but unlike Ig), because they do not recognize free antigen and are unable to recognize until the antigen is associated with other molecule or expressed on the surface of other cells or molecules. The major histocompatibility complex bind to same molecule or antigen, then the T cell will recognize them.
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