The mammalian immune system possesses the intrinsic capacity to evoke a wide variety of functionally distinct effector mechanisms following stimulation by a particular antigenic substance. Such diversity in available responses is absolutely essential to the immunocompetent host, which must continually deal with a diverse set of potential pathogens within its ever-changing environment. The development of appropriate types of immune responses, therefore, represents a highly dynamic process that requires that an equivalent consideration be given to a large array of components, any one of which is capable of modulating the final outcome. While the nature and complexity of the antigen(s), plus the intracellular or extracellular mode of presentation, provide specificity and some selection to the developing process, the route of antigen entry, as well as the physiological status of the host at the time of antigen insult, also contribute significantly to the formation of any immune response. The overall objective of this article is to introduce the concept that platelet-derived growth factor (PDGF) (either preformed or synthesized in response to stimulation), plus a number of steroid hormones (some of which are end-organ metabolized at local tissue sites), can all play significant roles in the genesis of immunologic responses in vivo.