The immune system of a vertebrate organism is designed to respond specifically to an indefinite number of foreign molecules. The ability to recognize and eliminate nonself material is of vast importance for the survival of the organism, as it provides the principal defense mechanism against parasites such as bacteria or viruses. Though the ability of the immune system to eliminate foreign material is crucial for the well-being of the host, “pathological” immune responses such as severe allergic reactions or autoimmune responses threaten the very existence of the afflicted host.