A veritable army of cells is constantly on the alert in our bodies. Its mission is to destroy any foreign cells, viruses, or bacteria that attack the fortress they are protecting, the human body. This program uses stunning imagery to describe the battles our immune system wages inside our bodies, showing how it sets up defenses against viral invasion. It explains how, by means of vaccination, we can use the functions of the immune system to prevent disease. The program also shows how, in the case of organ transplants, the immune system must be suppressed, and shows how the anti-rejection drug, Cyclosporine, works. (23 minutes)