Forming cellular conjugates with T cells, macrophages can help their targets to mount an immune response or they can destroy the targeted T cell. The two functions are performed by two distinct macrophage subsets that can be distinguished by cell surface marker phenotypes, B7+ CD16- and B7- CD16+. Interferon-gamma (IFN-gamma) induces the former, interleukin-10 (IL-10) induces the latter phenotype. The two macrophage differentiation pathways are mutually exclusive; each cytokine inhibits the effect of the other cytokine. The second messenger cAMP enhances the macrophage B7 expression and suppresses the macrophage CD16 expression. However, together with IL-10, cAMP blocks the generation of both macrophage phenotypes. In the chimpanzee we noted deviations from this differentiation pattern that are suggestive of an enhanced IL-10 presence in the primate environment.