Properties and Overview of Immune Responses

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INNATE AND ADAPTIVE IMMUNITY, 2

TYPES OF ADAPTIVE IMMUNE RESPONSES, 3

CARDINAL FEATURES OF ADAPTIVE IMMUNE RESPONSES, 6

CELLULAR COMPONENTS OF THE ADAPTIVE IMMUNE SYSTEM, 8

CYTOKINES, SOLUBLE MEDIATORS OF THE IMMUNE SYSTEM, 8

OVERVIEW OF IMMUNE RESPONSES TO MICROBES, 10

The Early Innate Immune Response to Microbes, 10 The Adaptive Immune Response, 10

SUMMARY, 13

The term immunity is derived from the Latin word immu-nitas, which referred to the protection from legal prosecution offered to Roman senators during their tenures in office. Historically, immunity meant protection from disease and, more specifically, infectious disease. The cells and molecules responsible for immunity constitute the immune system, and their collective and coordinated response to the introduction of foreign substances is called the immune response.

The physiologic function of the immune system is defense against infectious microbes. However, even noninfectious foreign substances can elicit immune responses. Furthermore, mechanisms that normally protect individuals from infection and eliminate foreign substances are also capable of causing tissue injury and disease in some situations. Therefore, a more inclusive definition of the immune response is a reaction to components of microbes as well as to macromolecules, such as proteins and polysaccharides, and small chemicals that are recognized as foreign, regardless of the physiologic or pathologic consequence of such a reaction. Under some situations, even self molecules can elicit immune responses (so-called autoimmune responses). Immunology is the study of immune responses in this broader sense and of the cellular and molecular events that occur after an organism encounters microbes and other foreign macromolecules.

Historians often credit Thucydides, in the fifth century bc in Athens, as having first mentioned immunity to an infection that he called plague (but that was probably not the bubonic plague we recognize today). The concept of protective immunity may have existed long before, as suggested by the ancient Chinese custom of making children resistant to smallpox by having them inhale powders made from the skin lesions of patients recovering from the disease. Immunology, in its modern form, is an experimental science, in which explanations of immuno-logic phenomena are based on experimental observations and the conclusions drawn from them. The evolution of immunology as an experimental discipline has depended on our ability to manipulate the function of the immune system under controlled conditions. Historically, the first clear example of this manipulation, and one that remains among the most dramatic ever recorded, was Edward Jenner's successful vaccination against smallpox. Jenner, an English physician, noticed that milkmaids who had recovered from cowpox never contracted the more serious smallpox. On the basis of this observation, he injected the material from a cowpox pustule into the arm of an 8-year-old boy. When this boy was later intentionally inoculated with smallpox, the disease did not develop. Jenner's landmark treatise on vaccination (Latin vacci-nus, of or from cows) was published in 1798. It led to the widespread acceptance of this method for inducing immunity to infectious diseases, and vaccination remains the most effective method for preventing infections (Table 1-1). An eloquent testament to the importance of immunology was the announcement by the World Health Organization in 1980 that smallpox was the first disease that had been eradicated worldwide by a program of vaccination.

TABLE 1-1 Effectiveness of Vaccines for Some Common Infectious Diseases

Disease

Maximum Number of Cases (year)

Number of Cases in 2009

Percentage Change

Diphtheria

206,939 (1921)

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