A lucid, up-to-date account of the body's immune system from a pro who has taught the subject at UCLA for 25 years. Infectious disease is news Big Time these days--it has been ever since AIDS appeared. What is fascinating is the game of catch- up that science has been playing--unraveling the intricacies of the body's immune system and, more recently, its intimate relation to the nervous system. Clark reminds us that just a generation ago the body's defenses were thought to consist only of antibodies: the molecules that destroy foreign invaders. Not only does Clark explain how the body can be stimulated, say, by vaccines, to make millions of antibodies, but he goes into detail about T cells--the other arm of the immune system. T cells (the T is for the thymus, where they are made) come in several varieties, of which the most noted today are the ``killer'' T cells that search and destroy aberrant cells and the CD4 ``helper'' T cells that are killed by the AIDS virus. Sometimes the immune system is overzealous, however, mounting an attack that adds insult and injury: There is a misreading of molecular markers, resulting in attacks on healthy cells, as happens in autoimmune diseases such as multiple sclerosis (the war within), and sometimes there is a need to suppress the system--in the case of organ transplants. Clark's upbeat outlook suggests that we can curb excesses as well as stimulate a weakened immune system with new drugs or gene therapy approaches. He is convinced that ongoing discoveries of the brainimmune system dialogue will lead to new approaches to therapy. He is also aware of the pressing need for organ transplants, raising provocative ethical issues but at the same time providing a valid Uniform Anatomical Gift Act donor card for readers to fill out. A good idea that, given his overall enthusiasm and mastery in explaining complex science, might persuade more than a few readers.