Many books have been written about cancer immunology. However, the subject is still in its infancy regarding full understanding of the complex mechanisms and interactions involved and their relevance to the clinical situation. Exciting developments are being seen in the fields of research, involving, for example, monoclonal antibodies and biological response modifiers. We, therefore, feel fully justified in introducing this new text, which is intended for clinical oncologists wishing to know more about the status of immunology in cancer and as a source of reference for workers, in all branches of oncology research, seeking up-to-date reviews. Contributors have, therefore, given both explanatory and more detailed accounts of developments in their particular fields of expertise. xi IMMUNOLOGICAL ASPECTS OF CANCER 1. BASIC IMMUNOLOGY A. MILFORD WARD INTRODUCTION The immune response represents the normal physiological process by which the body main- tains homeostasis in the response to infection or to introduction of foreign material.The immune system that generates this response is complex in that it exerts its action by means of circulating cellular and humoral components capable of acting at sites far distant from their site of formation and by its interaction with a variety of biological effector systems. CELLS OF THE IMMUNE SYSTEM The major cell types of the immune system are the macrophages and the lymphocytes.