In At War Within, I explain how we discovered that we have immune systems, and how the various components of the immune system function. I then explore some of the ways in which the immune system works to defend us, as in the case of infectious diseases, but also how it sometimes fails (as in the case of cancer or AIDS), gets in the way (as in the case of organ transplants) or actually harms us (allergy and autoimmune disease, among other examples.) I think it is important for people to know that although we couldn't possibly survive without our immune system, like any other weapon it is dangerous and can cause harm under the wrong circumstances.