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The immune system is the complex system in the body responsible for fighting disease. Its primary function is to identify foreign substances in the body ([[bacteria]], [[viruses]], [[fungi]] or [[parasites]]) and develop a defense against them. This defense is known as the immune response.  

The immune system is the complex system in the body responsible for fighting disease. Its primary function is to identify foreign substances in the body ([[bacteria]], [[viruses]], [[fungi]] or [[parasites]]) and develop a defense against them. This defense is known as the immune response.  

It involves production of [[protein]] molecules called [[antibodies]] to eliminate foreign organisms that invade the body.

It involves production of [[protein]] molecules called [[antibodies]] to eliminate foreign organisms that invade the body.<ref>http://www.cdc.gov/vaccines/about/terms/glossary.htm#i</ref>