The immune processes are probably ongoing and, in most cases, lead to the elimination of antigens without producing clinically detectable inflammation. The development of clinically apparent inflammation indicates that the immune system has encountered either an unusually large amount of antigen, antigen in an unusual location, or antigen that is dificult to digest. In some diseases, such as rhematoid arthritis, the iniciating agent is unknown or may be normal host tissue components. In others (e.g. systemic lupus erythematosus), inherent or acquired immunoregulatory abnormalities may contribute to the sustained nature of the inflammatory process.
Coombs and Gell divided inflammatory responses mediated by the immune system into four categories, called I, II, III, and IV, which represent four distinct immune mechanisms that result in tissue injury. These same four processes represent mechanisms of immune protection from infectious agents: