The immune system is a network of organs, glands, and tissues that protects the body from foreign substances. These substances include bacteria, viruses, and other infection-causing parasites and pathogens. Usually, the immune system is extremely effective in performing its work of defending the body, but sometimes an error occurs in this highly complex system, and it can lead to terrible mistakes. The result can be an allergic reaction, which can be as simple as a case of the sniffles and as serious as a fatal condition. Or the error can manifest as an autoimmune disorder, such as lupus, in which the body rejects its own constituents as foreign invaders.