Nine lives: plasticity among T helper cell subsets

The division of labor among two types of T helper (Th) subsets, first described over 20 yr ago, has been buffeted by the discovery of new subsets and new cytokines that can be coaxed out of T cells with increasing disregard for the subset of origin. Although Th17 cells and regulatory T (T reg) cells are widely accepted subsets, and others are being proposed, their plasticity is difficult to reconcile with the definitions of Th subsets as put forth in the initial description of Th1 and Th2 cells. A deeper molecular context will be required to reconcile the ever-increasing complexity of effector T cells.