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The Immune System in General.

The human body is under near constant attack from pathogens, or disease carrying parasites, of the type discussed in Infection, Infectious Diseases, and Parasites and Parasitology. No human would live very long without the immune system, which includes two levels or layers of protection, the nonspecific and the specific defenses. The nonspecific defenses, including the skin and mucous membranes, serve as a first defensive line for preventing pathogens from entering the body. The specific defenses are activated when these microorganisms get past the nonspecific defenses and invade the body.

For the immune system to work properly, two things must happen: first, the body must recognize that it has been invaded, either by pathogens or toxins or by some other outside threat. Second, the immune response must be activated quickly, before the invaders destroy many body tissue cells. For the immune system to respond effectively, several conditions must be in place, including the proper interaction of nonspecific and specific defenses. The nonspecific defenses on the skin do not identify the antigen (a substance capable of stimulating an immune response or reaction) that is attacking or potentially attacking the body; instead, these defenses simply react to the presence of what it identifies as something foreign. Often, the nonspecific defenses effectively destroy microorganisms, but if these defenses prove ineffective and the microorganisms manage to infect tissues, the specific defenses go into action. The specific defenses function by detecting the antigen in question and mounting a response that targets it for destruction.​