The TH1-TH2 paradigm provides an elegant model of directed response to infectious pathogens. Developed in the mouse, the model has provided a framework for systematic and mechanistic studies of immune regulation, protective immunity, and vaccine development in swine. Interleukin-4 (IL-4) plays a central role in the paradigm as a regulatory molecule directing development of the TH2 phenotype, as a developmental cytokine essential for antibody production, and as a soluble diagnostic marker of the TH2 cell type. In contrast, while characterizing the biological properties of porcine IL-4, we discovered that it was not a stimulatory factor for porcine B cells. Rather, it blocked antibody and IL-6 secretion and suppressed antigen-stimulated proliferation of B cells. Inhibition was not reversed by treatment with IL-2 and IL-6 treatment. IL-4 did not stimulate T lymphocyte proliferation, but induced cell growth in lymphoblasts in a dose-dependent fashion. These results suggest that IL-4 plays a different role in pigs than in mice and humans, in which it stimulates B cells and is essential for antibody production. Furthermore, the functions of IL-4 in swine cannot be inferred from results in model systems such as the mouse. General models of disease resistance show substantial variation between pigs and mice at the cellular and molecular level. Advances in somatic cell technologies and animal engineering to enable gene knockouts in pigs, in combination with a continuously expanding immunological toolkit, promise an exciting future for pig immunology, detailed mechanistic elucidation of the TH1-TH2 paradigm, and an improved understanding of the role of IL-4 in porcine immunity to infectious disease.