Cytokines are proteins produced by one cell that are capable of altering the behavior of another cell. Their biologic effects are both pleomorphic and redundant. A number of cytokines have been implicated in the genesis of allergic reactions, including IL-4, IL-5, IL-6, and tumor necrosis factor (TNF). In contrast, other cytokines such as gamma interferon (gamma-IFN) may have biologic actions that act to limit allergic responses. In studies on patients with severe food allergy, we have found cytokine production from peripheral T cells to be abnormal following mitogenic stimulation. These lymphokine abnormalities may contribute to the elevation in IgE and the eosinophilia seen in severe food allergy.