Understanding the control exerted by cytokines on T helper cell subsets 1 and 2 (T H 1-T H 2) development has progressed to a fairly satisfying knowledge of intracellular signals and transcription factors. Less is understood about the molecular basis of T H 1-T H 2 development exerted by other parameters, such as how the antigen presenting cell can influence this process. Recent work suggests that dendritic cell subsets contribute significant polarizing influences on T helper differentiation, but how this comes about is less clear. In some cases known pathways may be used, as in the dendritic cell subset 1 exerting T H 1 polarization by interleukin 12 (IL-12) production and STAT4 activation. In others, the effects are still in need of explanation.