A need for a book on immunology which primarily focuses on the needs of medical and clinical research students was recognized. This book is relatively short and contains topics considered relevant to the understanding of human immune system and its role in health and diseases. Immunology is the study of our protection from foreign macromolecules or invading organisms and our responses to them. These invaders include viruses, bacteria, protozoa or even larger parasites. Certain individuals develop immune responses against their proteins (and other self- molecules) in autoimmunity and against our own aberrant cells as in tumor immunity. .