We recently described a minimal immunologic system containing two vector components whose expression was interrelated, TNF-alpha and interleukin-1b (IL-1b). We demonstrate here that a minimal immunologic system contains a third component, interferon gamma. Interferon gamma was distinguishable from alpha and beta interferons in quality of necessity in an immunologic system. The ligand here likely serves to signal an undesirable entity, infection, danger, or a chronic foreign presence.