CD4+ T cells, via the cytokines that they produce, play a pivotal role in the induction and regulation of cell-mediated and humoral immunity. Recently it has become clear that the CD4+ T-ceil population is heterogeneous and that distinct CD4’T-cell subsets, defined by their cytokine repertoire, regulate cell-mediated and humora~ immune responses. Protective responses to pathogens are dependent on activation of the appropriate T,., subset accompanied by its characteristic set of immure effector functions. Evidence to date suggesfs ihat the cytokines produced by the TH cells themselves are important regulafors of TH subset activation and differentiation. Fiona Powrie and Robert Coffman discuss how manipulation of the levels of these cytokines can be used to alter the balance of T,-cell subsets and illustrate some clinical situations where this may be beneficial.

A successful immune response to an infectious agent is dependent on the activation of an appropriate set of immune effector functions. These can broadly be divided into two types, cell-mediated and humoral. Cell-mediated immune responses involve the activation of macrophages and the induction of cytotoxic CDS’and CD4+ T cells, whereas humoral immunity is characterized by antibody production. Recently it has become clear that these two arms of the immune response are regu-