Interferon ɣ (IFNɣ) is produced mainly by activated lymphocytes and has receptors on virtually all cell types of the body. It thus exerts a multitude of cellular biological effects. Among cytokines, IFNɣ is the main activator of macrophages. In addition it also activates endothelial cells. Together, these two activities are the basis for assigning a proinflammatory role to IFNɣ. The regulatory role of IFNɣ on the antigen-specific phases of the immune response derive from its effects on antigen-presenting cells and on B and T lymphocytes. IFNɣ augments expression of MHC molecules in professional as well as nonprofessional antigen presenting cells; its effects on B and T lymphocyte
proliferation and differentiation are complex. Production of IFNɣ by T helper cells is a hallmark of the TH1-type phenotype. Thus, high-level production of IFNɣ is typically associated with effective host defense against intracellular pathogens, and with immune and autoimmune pathology that depends upon delayed-type hypersensitivity. Genetic defects in the IFNɣ system are rare; genetic polymorphism exists and its impact on disease susceptibility is under study. Despite the powerful immunoregulatory potential of IFNɣ and the availability of preparations for administration to patients, clinical applications are limited.