The immune system is the umbrella term for an extremely complex process which involves specialized cells, several major organs and a complicated set of biological procedures designed to rid us of toxins, waste, and to defend us from microscopic biological threats.
Below are definitions from two different health resources: KidsHealth.org, which is a part of The Nemours Foundation; a nonprofit organization devoted to improving the health of children, and the National Institute of Allergy and Infectious Diseases, which is a associated with the National Institute of Health. Both of these organizations have extensive online information and are worth exploring.