The immune system is a network of cells and organs that work together to defend the body against attacks by "foreign" invaders. The foreign invaders are germs. The body provides an excellent environment for germs and microbes. When they do break into a system it is the immune system's job to keep them out or to seek and destroy them. When the immune system is malfunctioning it can cause a variety of diseases, such as allergy, arthritis, cancer or AIDS.