The manner in which the human body's defenses identify and reject intruders such as foreign proteins, bacteria or viruses is a fascinating story, not yet fully understood. This film illustrates, with photomicrography and animation sequences, some of the known facts--the action of white blood cells in the first line of defense, and the more intricate, potent response of the lymphatic system in producing antibodies. A lucid exposition for high school and lay audiences, or as a primer for medical and allied courses.