• T-cells originate in bone marrow and mature and differentiate in thymus. The name T- cells is derived from its site of maturation.
• All T-cells contains T- cell receptor (TCR) on its surface. TCR is specific and recognize MHC bound antigen.
• All T- cells expresses an antigen binding TCR and CD2 and CD3 glycoprotein on their cell membrane.
• Most of the T-cells are distinguished on the basis of CD4 and CD8 glycoprotein receptor on their cell membrane.
• CD4 T- cell: those T- cells which contains CD4 glycoprotein on its surface in addition to TCR, CD2 and CD3. CD4+T-cells recognize antigen bound to MHC-II.
• CD8 T-cell: those T cells which contains CD8 glycoprotein on its surface in addition to TCR, CD2 and CD3. CD8+ T-cellsrecognize antigen bound to MHC-I
• None of the T- cells contains both CD4 and CD8, similarly, non T- cells lack both CD4 and CD8 glycoprotein receptor on its surface.
• Other membrane receptors on surface of mature T- cells are:
• CD28; a receptor for co-stimulatory B7 or for Antigen presenting cells (APC)
• CD45; a signal transduction molecule

Functional sub-population of T-cells:

i. Helper T-cells (TH) or CD4+ T -cells

• TH cells are activated by recognition of antigen on MHC-II on APCs. Ie. MHC-II restricted.
• When antigen binds on TCR of CD+ T –cells, it is activated and divides into memory cell and effector cells. Effector TH cells secretes various cytokines which helps in B-cell activation for production of antibodies.

ii. Cytotoxic T-cells (Tc) or CD8+ T –cells:

• Tc cells are activated by recognition of antigen on MHC-I on altered self cell such as tumor cell or virus infected cells.. Ie. They are MHC-I restricted.
• When antigen binds on TCR of CD8+ cells, it is activated and divides into clones of memory cell and effectors cells. Effector Tc cells is known as cytotoxic T-lymphocytes (CTL) which kills virus infected cell or tumor cells.

iii. Supressor cells (Ts-cells):

• Ts-cells helps to suppress humoral and cell mediated immunity.

Immunological functions of T-lymphocytes:

• Helps B- cell maturation, expression and antibody production
• Helps in recruitment and activation of mononuclear phagocytic cells.
• Helps in recruitment and activation of specialized cytotoxic T- cells (TCL) in antiviral response.
• Secretes cytokines which is responsible for growth and differentiation of T- cells, monocytes, macrophages etc
• Helps in regulation of immune reactions.

T-lymphocyte: types and function

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