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The immune system protects the body against invasion by microorganisms, removes tissues damaged by trauma, and eliminates malignant growths. Recognizing these problems requires detecting foreign or unexpected molecules, called antigens, against the background of self-molecules. This recognition is accomplished by membrane-bound antigen receptors on immune system cells. When antigen receptors bind an antigen, these cells initiate effector responses to manage the problems.

The immune system provides innate and acquired responses, which differ in several ways but most notably by the nature of the antigen receptors on the two types of cells. Innate responses are present at birth and provide protection from common pathogens. Acquired responses develop over time, which is why many illnesses are common early in life. Once acquired responses develop, however, they provide long-lasting immunity to many diseases. Innate and acquired elements are interdependent and work in tandem.

This chapter briefly reviews features of the innate immune system, which were originally introduced in Chapter 2 and 8, and then describes in more detail the features of the acquired system and lymphoid tissue.


The defense against infection involves all cells in the body. The cells and activities that play major roles in this defense are classified as providing either innate or acquired immune responses. The innate immune responses are generally rapid, stereotyped, and short-term reactions directed against common pathogens and damaged tissue. The acquired immune responses are slower to develop, but are extremely flexible and can provide long-term protection. The activities of innate and acquired elements are highly complex and intricately interwoven.


A normal immune response by innate or acquired cells involves antigen receptors to recognize a problem, such as infection caused by pathogens or tissue damage, inflammatory signals to alert other cells, and effector functions to attack the problem (Figure 9-1A).

Figure 9-1:

Some general features of immune responses. A. Steps from recognition to action. (1) Binding antigens to antigen receptors generates internal signals. (2) Antigen-stimulated cells release inflammatory signals that activate other cells. (3) Effector cells attack the source of antigens with toxic chemicals and engulf debris. B. Toll-like receptors (TLRs). Shown are examples of antigen receptors present on innate immune cells that recognize molecules specific to microbes and viruses. TLR-4 on the outside of the cells binds lipopolysaccharides (LPS) produced by gram-negative bacteria, whereas TLR-9 in endosomes recognizes DNA from bacteria and viruses. Receptor binding results in the activation of genes for inflammatory cytokines. C. Interferons protect against viral infection. Any cell infected with a virus responds by producing interferons. These cytokines bind to neighboring cells and initiate reactions that help resist viral infection. D. Complement proteins identify cells for attack. The attachment of complement protein C3b to the surface of cells induces several ...

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