T cell Heterogeneity

T cells are a subset of lymphocytes that play a large role in the primary immune response. These cells exhibit phenotypic and/or functional heterogeneity [1].

Phenotypic heterogeneity arises from the expression of different markers on the cell surface. There are two well-defined subpopulations of T cells, namely T helper (Th) and T cytotoxic (Tc) cells, both of which express one or the other of two membrane glycoprotein molecules CD4 and CD8. T cells that express CD4 i.e. Th can only recognise antigen bound to class II MHC molecules, whereas T cells expressing CD8 (Tc) are restricted to recognition of antigen bound to class I MHC molecule.

The differences in phenotypes of the T cell results in differences in function. Within the T helper cells are two distinct kinds, Th1 and Th2 cells. Just like all T cells, these mature in the thymus but are only produced when T cells encounter dendritic cells, which present antige...