The immune system is a highly evolved security system that protects the body from infection by pathogens such as viruses and bacteria. The immune system also recognizes and rejects a transplanted organ from even the same species. Indeed, the immune system potentially recognizes and eliminates everything that invades the body (nonself). However, it does not normally eliminate self cells or tissues except tumor cells developed from self tissue. Occasionally the immune system breaks down and attacks the body components of the self, manifesting as autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, myasthenia gravis, and insulin-resistant diabetes. This book contains extensive updated reviews describing what kinds of receptors various immune cells use, how they recognize the self and the nonself components (and neoself such as tumors), and how finally the immune system distinguishes the self and nonself - a far more complicated process than a computer security system detecting infected documents. Perfect understanding of this system should make it possible in the future to regulate immunity to transplants, to cure autoimmune disease and allergy, and to facilitate tumor immunity.