IL-4 and IL-13 share a wide range of activities on monocytes, epithelial cells and B cells and thus play an important role in host defense. Many of these activities are not conserved among species as human, but not murine, B cells are thought to be responsive to IL-13. We previously demonstrated that human IL-13 is highly conserved at the nucleic acid level with a candidate bovine IL-13 cDNA homologue. Moreover, recombinant human IL-13 stimulates Ig secretion by appropriately activated bovine B cells. These studies have been extended to examining Ig class switching at both the protein and mRNA levels in addition to examining other markers of cellular activation. Our results suggest that IL-13 influences B cell differentiation by enhancing IgM, IgG1, and IgE production. IL-13 stimulation alone increases MHC class II expression and progression through cell cycle, although at lower levels in comparison to rboIL-4. The biology of the receptors for IL-4 and IL-13 is complex and raises several key questions with regard to IL-4-dependent and -independent mechanisms of host immunomodulation. Recent studies suggest that at least four chains are involved. These include the p140 IL-4 binding chain (IL-4Ralpha), the common gamma chain (gammac chain), IL-13 receptor alpha- chain (IL-13Ralpha-1) and the IL-13 receptor alpha-2 chain (IL-13Ralpha-2). We have recently cloned cDNAs for the bovine homologues of the IL-13Ralpha-1 and IL-4Ralpha chains and evaluated mRNA expression for a variety of cell types following stimulation. The expression patterns and their implications for receptor chain utilization in signaling via these key TH2 signature cytokines will be discussed.