Jean Hamburger is a distinguished French surgeon and pioneer in the field of kidney transplants. A major reason for the impressive success rate of this form of organ transplant has been the phenomenal increase in understanding of the body's immune defense system. This makes it possible to type the tissue of patient and donor to achieve the best possible match. In this little volume Hamburger reviews the many intricacies of the immune system. He describes the various types of lymphocytes. Some ""recognize"" intruders, others can change their shape or direct the manufacture of antibodies; all work in a carefully orchestrated program of defense of self and attack on that which is not self. These scientific facts become the springboard for the second half of the book--a philosophical statement about the importance of the uniqueness of the individual in the evolution and preservation of the species. For example, mutations, which occur at random, may be neutral insofar as survival is concerned, but may nonetheless, under certain environmental circumstances, prove the difference between life and death. Finally, Hamburger discourses on human intelligence, thought, and language as the instruments by which man says ""no"" to the often blind or cruel injustices of nature. Instead, guided by spiritual or ethical dictates, man seeks to preserve the weak and heal the ill. It is this struggle against nature which Hamburger finds ennobling. He asks the reader to consider the delicate equilibrium: how much human intervention can we support without undoing nature, and ultimately ourselves? The mode of thought, opposing reason to spirit, seems typically French in the existentialist tradition. It establishes Hamburger, the man of science, as a man of reason, but also ""irrational"" in the best sense. Would that more surgeons were so inclined!