T-cell differentiation: Commitment factors for T helper cells

The immune system is designed to eradicate a large variety of invading pathogens with minimum immunopathology. T helper cells, also known as CD4+ T cells, may be subdivided into the T helper 1 (Th1) and the T helper 2 (Th2) subsets, which produce distinct profiles of cytokines [1,2]. These Th1 and Th2 cells have opposing roles, regulating immune responses to intracellular and extracellular pathogens, respectively [1,2]. Th1 cells produce interferon-g (IFN-g), interleukin-2 (IL-2) and tumor necrosis factor b (TNF-b), and give rise to a cell-mediated immune response protective against intracellular pathogens and viruses, but these cells have also been implicated in organ-specific autoimmune diseases (reviewed in [3]). Th2 cells produce IL-4, IL-5 and IL-13 and activate mast cells and eosinophils, thereby eradicating helminths and other extracellular parasites, but in addition these cells are mediators of allergic and atopic manifestations [2,4].