Immunology is the study of the physiological mechanisms that humans and other animals use to defend their bodies from invasion by other organisms. The origins of the subject lie in the practice of medicine and in historical observations that people who survived the ravages of epidemic disease were untouched when faced with that same disease again-they had become immune to infection. Infectious diseases are caused by microorganisms, which have the advantage of reproducing and evolving much more rapidly than their human hosts. During the course of an infection, the microorganism can pit enormous populations of its species against an individual Homo sapiens. In response, the human body invests heavily in cells dedicated to defense, which collectively form the immune system.
The immune system is crucial to human survival. In the absence of a working immune system, even minor infections can take hold and prove fatal. Without intensive treatment, children born without a functional immune system die in early childhood from the effects of common infections. However, in spite of their immune systems, all humans suffer from infectious diseases, especially when young. This is because the immune system takes time to build up its strongest response to an invading microorganism, time during which the invader can multiply and cause disease. To provide immunity that will pro vide protection from the disease in the future, the immune system must first do battle with the microorganism. This places people at highest risk during their first infection with a microorganism and, in the absence of modern medicine, leads to substantial child mortality, as witnessed in the developing world. When entire populations face a completely new infection, the outcome can be catastrophic, as experienced by indigenous Americans who were killed in large numbers by European diseases to which they were suddenly exposed after 1 492 . Today, infection with human immunodeficiency virus (HIV) and the acquired immune deficiency syndrome (AIDS) it causes are having a similarly tragic impact on the populations of several African countries.
In medicine the greatest triumph of immunology has been vaccination, or immunization, a procedure whereby severe disease is prevented by prior exposure to the infectious agent in a form that cannot cause disease. Vaccination provides the opportunity for the immune system to gain the experience needed to make a protective response with little risk to health or life. Vaccination was first used against smallpox, a viral scourge that once ravaged populations and disfigured the survivors. In Asia, small amounts of smallpox virus had been used to induce protective immunity for hundreds of years before 1 72 1 , when Lady Mary Wortley Montagu introduced the method into Western Europe. Subsequently, in 1 796, Edward Jenner, a doctor in rural England, showed how inoculation with cowpox virus offered protection against the related smallpox virus with less risk than the earlier methods. Jenner called his procedure vaccination, after vaccinia, the name given to the mild disease produced by cowpox, and he is generally credited with its invention. Since his time, vaccination has dramatically reduced the incidence of smallpox worldwide, with the last cases being seen by physicians in the 1 970s (Figure 1) .
Effective vaccines have been made from only a fraction of the agents that cause disease and some are of limited availability because of their cost. Most of the widely used vaccines were first developed many years ago by a process of trial and error, before very much was known about the workings of the immune system. That approach is no longer so successful for developing new vaccines, perhaps because all the easily won vaccines have been made. But deeper understanding of the mechanisms of immunity is spawning new ideas for vaccines against infectious diseases and even against other types of dis ease such as cancer. Much is now known about the molecular and cellular components of the immune system and what they can do in the laboratory. Current research seeks to understand the contributions of these immune components to fighting infections in the world at large. The new knowledge is also being used to find better ways of manipulating the immune system to prevent the unwanted immune responses that cause allergies, autoimmune diseases, and rejection of organ transplants.
In this chapter we first consider the microorganisms that infect human beings and then the defenses they must overcome to start and propagate an infection. The individual cells and tissues of the immune system will be described, and how they integrate their functions with the rest of the human body. The first line of defense is innate immunity, which includes physical and chemical barriers to infection, and responses that are ready and waiting to halt infections before they can barely start. Most infections are stopped by these mechanisms, but when they fail, the more flexible and forceful defenses of the adaptive immune response are brought into play. The adaptive immune response is always targeted to the specific problem at hand and is made and refined during the course of the infection. When successful, it clears the infection and provides long-lasting immunity that prevents its recurrence.
Figure 1 The eradication of small pox by vaccination .