Heterogeneity of CD4(+) and CD8(+) T cells.

There is extensive plasticity in the T-cell response to antigen. Helper CD4(+) T cells, cytotoxic CD8(+) T cells, the progression from naïve to effector and memory T cells, and differentiation into Th1, Tc1, Th2 and Tc2 subsets have long been recognized. More recently it has become apparent that T-cell populations display additional diversity in terms of phenotype, anatomical distribution and effector function.