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The Immune System ukocyte infiltration ukocyte infiltration

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Functions of B Lymphocytes

B lymphocytes secrete antibodies that can bind to antigens in a specific fashion.This bonding stimulates a cascade of reactions whereby a system of plasma proteins called complement is activated. -m Some of the activated complement proteins kill the cells containing the antigen; others promote phagocytosis, resulting in a more effective

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■ Figure 15.6 Infiltration of an inflamed site by leukocytes.

Different types of leukocytes infiltrate the site of a local inflammation. Neutrophils arrive first, followed by monocytes and then lymphocytes.

anticoagulant ability (chapter 13). However, mast cells produce a variety of other molecules that play important roles in inflammation (and in allergy, discussed in a later section).

Mast cells release histamine which is stored in intracellu-lar granules and secreted during inflammation and allergy. Hista-mine binds to its H1 histamine receptors in the smooth muscle of bronchioles to stimulate bronchiolar constriction (as in asthma), but produces relaxation of the smooth muscles in blood vessels (causing vasodilation). Histamine also promotes increased capillary permeability, bringing more leukocytes to the infected area.

With a time delay, mast cells release inflammatory prostaglandins and leukotrienes (chapter 11), as well as a variety of cytokines that promote inflammation. In addition, mast cells secrete tumor necrosis factora (TNFa), which acts as a chemokine to recruit neutrophils to the infected site.

These effects produce the characteristic symptoms of a local inflammation: redness and warmth (due to histamine-stimulated vasodilation); swelling (edema) and pus (the accumulation of dead leukocytes); and pain. If the infection continues, the release of endogenous pyrogen from leukocytes and macrophages may also produce a fever, as previously discussed.

Test Yourself Before You Continue

1. List the phagocytic cells found in blood and lymph, and indicate which organs contain fixed phagocytes.

2. Describe the actions of interferons.

3. Distinguish between innate and adaptive immunity, and describe the properties of antigens.

4. Distinguish between B and T lymphocytes in terms of their origins and immune functions.

Exposure of a B lymphocyte to the appropriate antigen results in cell growth followed by many cell divisions. Some of the progeny become memory cells; these are visually indistinguishable from the original cell and are important in active immunity. Others are transformed into plasma cells (fig. 15.7). Plasma cells are protein factories that produce about 2,000 antibody proteins per second.

The antibodies that are produced by plasma cells when B lymphocytes are exposed to a particular antigen react specifically with that antigen. Such antigens may be isolated molecules, as illustrated in figure 15.7, or they may be molecules at the surface of an invading foreign cell (fig. 15.8). The specific bonding of antibodies to antigens serves to identify the enemy and to activate defense mechanisms that lead to the invader's destruction.

Antibodies

Antibody proteins aie also known as immunoglobulins. They aie found in the gamma globulin class of plasma proteins, as identified by a technique called electrophoresis in which different types of plasma proteins aie separated by their movement in an electric field (fig. 15.9). The five distinct bands of proteins that appear are albumin, alpha-1 globulin, alpha-2 globulin, beta globulin, and gamma globulin.

The gamma globulin band is wide and diffuse because it represents a heterogeneous class of molecules. Since antibodies are specific in their actions, it follows that different types of antibodies should have different structures. An antibody against smallpox, for example, does not confer immunity to poliomyelitis and, therefore, must have a slightly different structure than an antibody against polio. Despite these differences, antibodies are structurally related and form only a few classes.

There are five immunoglobulin (abbreviated Ig) subclasses: IgG, IgA, IgM, IgD, and IgE. Most of the antibodies in serum are in the IgG subclass, whereas most of the antibodies in external secretions (saliva and milk) are IgA (table 15.6). Antibodies in the IgE subclass are involved in certain allergic reactions.

List the phagocytic cells found in blood and lymph, and indicate"" which organs contain fixed phagocytes.

2. Describe the actions of interferons.

3. Distinguish between innate and adaptive immunity, and describe the properties of antigens.

4. Distinguish between B and T lymphocytes in terms of their origins and immune functions.

5. Identify the primary and secondary lymphoid organs and describe their functions.

6. Describe the events that occur during a local inflammation.

Perspectives

Immediately following each major section heading is a concise statement of the section's central concepts, or organizing themes, that will be illustrated in detail in the text that follows. These brief introductions are designed to help students place the sections in perspective, before getting involved with the specifics.

Test Yourself Before You Continue

Each major chapter section ends with a set of learning activities and essay questions that relate only to the material presented in the section. Students are encouraged to answer the essay questions, draw the outlines and flowcharts requested, and otherwise actively participate in their learning of this material. Thus, these sections serve as both a "reality check" for the student and as a mechanism for active learning.

Fox: Human Physiology, I Front Matter I Preface I I © The McGraw-Hill

Eighth Edition Companies, 2003

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