T- Cells

T- cells are among the two important cells of the adaptive immune system (others are B- Cells). T-cells arise in the bone marrow and mature in the thymus.

T- cells constitute 60-70% of the circulating peripheral lymphocytes.

T- cells categories:-

1.) Helper or Inducer T-cells:

60% of T- cells are helper T- cells. They have CD4 surface marker and are MHC II restricted. Types of CD4 helper cells are:

A) Effector cells- TH1, TH2 and TH17 cells. The TH1 cells are activated by IFN- gamma and themselves produce IL-2, IFN-gamma and IL-12. They are the primary cells involved in delayed hypersensitivity, cell mediated immunity, macrophage activation and killing of intracellular microbes. TH2 cells are activated by IL4 and themselves produce IL4, IL5, IL6 and IL13. They produce all antibodies except for IgG2b and also provide defence against helminthic parasites. TH17 cells are powerful recruiters of neutrophils and monocytes to play a role in severe inflammatory diseases.

B) Memory cells- They retain the antigenic affinity of previously activated T-cells and are used in a second immune response.

2.) Cytotoxic T-cells:

30% of the total T-cells. They have CD8 surface marker and MHC I restricted. They kill amd lyse target cells which include tumour cells, virus infected cells and allograft.

THE RATIO OF CD4:CD8 T-cell is normally 2:1.

3.) Suppressor T-cells:

They have CD8 surface marker and are MHC I restricted. They are responsible for down regulation of the immune response.

TCELL RECEPTOR (TCR)

TCR is a molecule found on the T-cell which is responsible for recognising the antigen bound to MHC molecule. It is a heterodomer made up of an alpha and a beta chain. The presence of TCR gene rearrangements demonstrated by molecular analysis is a marker of T-cell lineage.

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