T lymphocytes are the central regulatory cells of the immune system. Much of their function is mediated by a set of small proteins whose expression, secretion, or both is induced as a result of antigen-stimulated cellular activation. These proteins, designated cytokines, act by binding to high-affinity receptors expressed on target cells and by inducing biochemical signals within those cells that profoundly affect their behavior. In this review, we place the, function of cytokines and cytokine-like cell surface molecules in the context of the function of regulatory T cells.‘We give extended consideration to the process through which naive CD4+ Tcells acquire distinct cytokine-producing patterns, and we discuss the pathophysiologic significance of the function of these cells. The biochemical basis of signaling by cytokines is discussed by Kishimoto (1994 [this issue of Cc//j).

Cytokine Families The cytokines may be subdivided into several groups, including the hematopoietins, the interferons, tumor necrosisfactor (TNF)-related molecules, immunoglobulin superfamily members, and the chemokines. Although members of each of the cytokine classes play important regulatory roles in the immune response, we pay particular attention to the hematopoeitins, to one of the interferons, interferon gamma (IFNy), and to members of the TNF family. In this review, we emphasize the role of these cytokines in the control of immunity, principally in the context of the regulation of immune responsiveness by CD4+ T cells. Figure 1 lists representative cytokines in these three groups. Since the structure of their receptors has, at least in the case of the hematopoietins, been a key to recognizing the relatedness of the molecules, models of the receptors for these classes of cytokines are shown in Figure 1. Hematopoietins