Introduction to the Immune System

21 Immunity is defi ned as resistance to disease, specifi cally infectious disease. The collection of cells, tissues, and molecules that mediate resistance to infections is called the immune system, and the coordinated reaction of these cells and molecules to infectious microbes is the immune response. Immunology is the study of the immune system and its responses to invading pathogens. The physiologic function of the immune system is to prevent infections and to eradicate established infections, and this is the principal context in which immune responses are discussed throughout this book. The importance of the immune system for health is dramatically illustrated by the frequent observation that individuals with defective immune responses are susceptible to serious, often life-threatening infections (Fig. 1-1). Conversely, stimulating immune responses against microbes by the process of vaccination is the most effective method for protecting individuals against infections and is, for example, the approach that has led to the worldwide eradication of smallpox (Fig. 1-2). The emergence of the acquired immunodefi ciency syndrome (AIDS) since the 1980s has tragically emphasized the importance of the immune system for defending individuals against infection. The impact of immunology, however, goes beyond infectious disease (see Fig. 1-1). The immune response is the major barrier to successful organ transplantation, an increasingly used therapy for organ failure. Attempts to treat cancers by stimulating immune responses against cancer cells are being tried for many