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l^k i ear the end of the nineteenth century, diphtheria was I a terrifying disease that killed many infants and small

JL. if children. The first symptom of the disease was a sore throat, often followed by the development of a gray membrane in the throat that made breathing impossible. Death occurred rapidly, even in the absence of a membrane. Frederick Loeffler, working in Robert Koch's laboratory in Berlin, found club-shaped bacteria growing in the throat of people with the disease but not elsewhere in their bodies. He guessed that the organisms were making a poison that spread through the bloodstream. In Paris, at the Pasteur Institute, Emile Roux and Alexandre Yersin followed up by growing the bacteria in quantity and extracting the poison, or toxin, from strained culture fluids. When injected into guinea pigs, the toxin killed the animals.

Back in Berlin, Emil von Behring injected the diphtheria toxin into guinea pigs that had been previously inoculated with the bacteria and had recovered from diphtheria. These guinea pigs did not become ill from the toxin, suggesting to von Behring that something in their blood, which he called antitoxin, protected against the toxin. To test this theory, he mixed toxin with serum from a guinea pig that had recovered from diphtheria and injected this mixture into an animal that had not had the disease. The guinea pig remained well. In further experiments, he cured animals with diphtheria by giving them antitoxin.

The results of these experiments in animals were put to the test in people in late 1891, when an epidemic of diphtheria occurred in Berlin. On Christmas night of that year, antitoxin was first given to an infected child, who then recovered from the dreaded diphtheria. The substances in blood with antitoxin properties soon were given the more general name of antibodies, and materials that generated antibody production were called antigens.

Emil von Behring received the first Nobel Prize in Medicine in 1901 for this work on antibody therapy. It took many more decades of investigation before the biochemical nature of antibodies was elucidated. In 1972, Rodney Porter and Gerald Edelman were awarded the Nobel Prize for their part in determining the chemical structure of antibodies.

—A Glimpse of History

IN CONTRAST TO THE INNATE IMMUNE RESPONSE, which is always ready to respond to patterns that signify damage or invasion, the adaptive immune response matures throughout life, developing from the immune system arsenal the most effec-

Immune cells tive response against specific invaders as each is encountered. An important hallmark of the adaptive immune response is that it has memory, a greatly enhanced response to re-exposure. Individuals who survived diseases such as measles, mumps, or diphtheria generally never developed the acute disease again. Vaccination now prevents these diseases by exposing a person's immune system to harmless forms of the causative microbe or its products. While it is true that some diseases can be contracted repeatedly, that phenomenon is generally due to the causative agent's ability to evade the host defenses, a topic we will discuss in chapter 19. The adaptive immune response also has molecular specificity. The response that protects an individual from developing symptoms of measles does not prevent the person from contracting a different disease, for example, chickenpox. The immune system can also discriminate healthy "self," your own normal cells, from "dangerous," such as invading bacteria. If this were not the case, the immune system would routinely turn against the body's own cells, attacking them just as it does an invading microbe. This is not a fail-safe system, however, which is why autoimmune diseases can occur. ■ acute disease p. 463 ■ vaccination, p. 421 ■ autoimmune disease, p. 452

The adaptive immune system is extraordinarily complex, involving an intricate network of cells, cytokines, and other compounds. In fact, immunologists are still working out many of its secrets. The recently discovered toll-like receptors, for

394 Chapter 16 The Adaptive Immune Response example, provide insight into how the body learns to distinguish substances that merit an adaptive response from those that do not. Scientists now recognize that the innate immune response, which for many years was viewed as a non-specific and relatively static participant in the host defenses, alerts critical cells of the adaptive response when generic patterns associated with microbes are found. ■ toll-like receptors, p. 381

In this chapter, we will first cover the general strategies the body uses to eliminate invading microbes and develop the memory that characterizes adaptive immunity. This will then lead to a more detailed description of the various cells and molecules involved. At the end of the chapter, we will focus on the development of the immune system, concentrating on how the cells involved in adaptive immunity develop the specificity required to respond to an incredibly diverse and ever-changing assortment of microbes. Throughout the chapter, we will describe some of the mechanisms used by the adaptive immune system to develop tolerance, which is the ability to ignore any given molecule, particularly those that characterize healthy "self," such as the proteins that make up your tissues. This process occurs both during the development of the lymphocytes and as a consequence of certain types of exposures to antigen.

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