Review of Key Concepts

The Lymphatic System (p. 800)

1. The lymphatic system consists of the lymph nodes, spleen, thymus, and tonsils; lymphatic tissue in other organs; a system of lymphatic vessels; and the lymph transported in these vessels. It serves for fluid recovery, immunity, and dietary lipid absorption.

2. Lymph is usually a colorless liquid similar to blood plasma, but is milky when absorbing digested lipids.

3. Lymph originates in blind lymphatic capillaries that pick up tissue fluid throughout the body.

4. Lymphatic capillaries converge to form larger lymphatic vessels with a histology similar to blood vessels. The largest vessels—the right lymphatic duct and thoracic duct— empty lymph into the subclavian veins.

5. There is no heartlike pump to move the lymph; lymph flows under forces similar to those that drive venous return, and like some veins, lymphatic vessels have valves to ensure a one-way flow.

6. The cells of lymphatic tissue are T lymphocytes, B lymphocytes, macrophages, dendritic cells, and reticular cells.

7. Diffuse lymphatic tissue is an aggregation of these cells in the walls of other organs, especially in the respiratory, digestive, urinary, and reproductive tracts. In some places, these cells become especially densely aggregated into lymphatic nodules, such as the Peyer patches of the ileum.

8. Lymphatic organs have well defined anatomical locations and have a fibrous capsule that at least partially separates them from adjacent organs and tissues. They are the lymph nodes, tonsils, thymus, and spleen.

9. Lymph nodes number in the hundreds and are small, encapsulated, elongated or bean-shaped organs found along the course of the lymphatic vessels. They receive afferent lymphatic vessels and give rise to efferent ones.

10. The parenchyma of a lymph node exhibits an outer cortex composed mainly of lymphatic follicles, and a deeper medulla with a network of medullary cords.

11. Lymph nodes filter the lymph, remove impurities before it returns to the bloodstream, contribute lymphocytes to the lymph and blood, and initiate immune responses to foreign antigens in the body fluids.

12. The tonsils encircle the pharynx and include a medial pharyngeal tonsil in the nasopharynx, a pair of palatine tonsils at the rear of the oral cavity, and numerous lingual tonsils clustered in the root of the tongue. Their superficial surface is covered with epithelium and their deep surface with a fibrous partial capsule. The lymphatic follicles are aligned along pits called tonsillar crypts.

13. The thymus is located in the mediastinum above the heart. It is a site of T lymphocyte development and a source of hormones that regulate lymphocyte activity.

14. The spleen lies in the left hypochondriac region between the diaphragm and kidney. Its parenchyma is composed of red pulp containing concentrated RBCs and white pulp composed of lymphocytes and macrophages.

15. The spleen monitors the blood for foreign antigens, activates immune responses to them, disposes of old RBCs, and helps to regulate blood volume.

Nonspecific Resistance (p. 808)

1. Our defenses against pathogens include external barriers to infection; attacks on pathogens by antimicrobial proteins, inflammation, fever, and other means; and the immune system.

2. The first two mechanisms are called nonspecific resistance because they guard equally against a broad range of pathogens and do not require prior exposure to them. Immunity is a specific defense limited to one pathogen or a few closely related ones.

3. The skin acts as a barrier to pathogens because of its tough keratinized surface, its relative dryness, and antimicrobial chemicals such as lactic acid and defensins.

4. Mucous membranes prevent most pathogens from entering the body because of the stickiness of the mucus, the antimicrobial action of lysozyme, and the viscosity of hyaluronic acid.

5. Neutrophils, the most abundant leukocytes, destroy bacteria by phagocytizing and digesting them and by a respiratory burst that produces a chemical killing zone of oxidizing agents.

6. Eosinophils phagocytize antigen-antibody complexes, allergens, and inflammatory chemicals, and produce antiparasitic enzymes.

7. Basophils aid in defense by secreting histamine and heparin.

8. Lymphocytes are of several kinds. Only one type, the natural killer (NK) cells, are involved in nonspecific defense. NK cells secrete perforins that destroy bacteria, transplanted cells, and host cells that are virus-infected or cancerous.

9. Monocytes develop into macrophages, which have voracious phagocytic activity and act as antigen-presenting cells. Macrophages include histiocytes, dendritic cells, microglia, and alveolar and hepatic macrophages.

10. Interferons are polypeptides secreted by cells in response to viral infection. They alert neighboring cells to synthesize antiviral proteins before they become infected, and they activate NK cells and macrophages.

11. The complement system is a group of 20 or more p globulins that are activated by pathogens and combat them by enhancing inflammation, opsonizing bacteria, and causing cytolysis of foreign cells.

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Physiology: The Unity of Immune System Companies, 2003 Form and Function, Third Edition

836 Part Four Regulation and Maintenance

12. Inflammation is a defensive response to infection and trauma, characterized by redness, swelling, heat, and pain (the four cardinal signs).

13. Inflammation begins with a mobilization of defenses by vasoactive inflammatory chemicals such as histamine, bradykinin, and leukotrienes. These chemicals dilate blood vessels, increase blood flow, and make capillary walls more permeable, thus hastening the delivery of defensive cells and chemicals to the site of injury.

14. Leukocytes adhere to the vessel wall (margination), crawl between the endothelial cells into the connective tissues (diapedesis), and migrate toward sources of inflammatory chemicals (chemotaxis).

15. Inflammation continues with containment and destruction of the pathogens. This is achieved by clotting of the tissue fluid and attack by macrophages, leukocytes, and antibodies.

16. Inflammation concludes with tissue cleanup and repair, including phagocytosis of tissue debris and pathogens by macrophages, edema and lymphatic drainage of the inflamed tissue, and tissue repair stimulated by platelet-derived growth factor.

17. Fever (pyrexia) is induced by chemical pyrogens secreted by neutrophils and macrophages. The elevated body temperature inhibits the reproduction of pathogens and the spread of infection.

General Aspects of Specific Immunity

1. The immune system is a group of widely distributed cells that populate most body tissues and help to destroy pathogens.

2. Immunity is characterized by its specificity and memory.

3. The two basic forms of immunity are cellular (cell-mediated) and humoral (antibody-mediated).

4. Immunity can also be characterized as active (production of the body's own antibodies or immune cells) or passive (conferred by antibodies or lymphocytes donated by another individual), and as natural (caused by natural exposure to a pathogen) or artificial (induced by vaccination or injection of immune serum). Only active immunity results in immune memory and lasting protection.

5. Antigens are any molecules that induce immune responses. They are relatively large, complex, genetically unique molecules (proteins, polysaccharides, glycoproteins, and glycolipids).

6. The antigenicity of a molecule is due to a specific region of it called the epitope.

7. Haptens are small molecules that become antigenic by binding to larger host molecules.

8. T cells are lymphocytes that mature in the thymus, survive the process of negative selection, and go on to populate other lymphatic tissues and organs.

9. B cells are lymphocytes that mature in the bone marrow, survive negative selection, and then populate the same organs as T cells.

10. Antigen-presenting cells (APCs) are B cells, macrophages, reticular cells, and dendritic cells that process antigens, display the epitopes on their surface MHC proteins, and alert the immune system to the presence of a pathogen.

11. Interleukins are chemical signals by which immune cells communicate with each other.

Cellular Immunity (p. 818)

1. Cellular immunity employs four classes of T lymphocytes: cytotoxic (TC), helper (TH), suppressor (TS), and memory T cells.

2. Cellular immunity takes place in three stages: recognition, attack, and memory.

3. Recognition: APCs that detect foreign antigens typically migrate to the lymph nodes and display the epitopes there. TH and TC cells respond only to epitopes attached to MHC proteins (MHCPs).

4. MHC-I proteins occur on every nucleated cell of the body and display viral and cancer-related proteins from the host cell. TC cells respond only to antigens bound to MHC-I proteins.

5. MHC-II proteins occur only on APCs and display only foreign antigens. TH cells respond only to antigens bound to MHC-II proteins.

6. When a TC or TH cell recognizes an antigen-MHCP complex, it binds to a second site on the target cell. Costimulation by this site triggers clonal selection, multiplication of the T cell. Some daughter T cells carry out the attack on the invader and some become memory T cells.

7. Attack: Activated TH cells secrete interleukins that attract neutrophils, NK cells, and macrophages and stimulate T and B cell mitosis and maturation. Activated TC cells directly attack and destroy target cells, especially infected host cells, transplanted cells, and cancer cells. They employ a "lethal hit" of cytotoxic chemicals including perforin, lymphotoxins, and tumor necrosis factor. They also secrete interferons and interleukins. TS cells suppress T and B cell activity as the pathogen is defeated and removed from the tissues.

8. Memory: The primary response to first exposure to a pathogen is followed by immune memory. Upon later reexposure, memory T cells respond so quickly (the T cell recall response) that no noticeable illness occurs.

Humoral Immunity (p. 822)

1. Humoral immunity is based on the production of antibodies rather than on lymphocytes directly contacting and attacking enemy cells. It also occurs in recognition, attack, and memory stages.

2. Recognition: An immunocompetent B cell binds and internalizes an antigen, processes it, and displays its epitopes on its surface MHC-II proteins. A TH cell binds to the antigen-MHCP complex and secretes helper factors that activate the B cell.

3. The B cell divides repeatedly. Some daughter cells become memory B cells while others become antibody-synthesizing plasma cells.

4. Attack: Attack is carried out by antibodies (immunoglobulins). The basic antibody monomer is a T- or Y-shaped complex of four polypeptide chains (two heavy and two light chains). Each has a constant (C) region that is identical in all antibodies of a given class, and a variable (V) region that gives each antibody its uniqueness. Each has an antigen-binding site at the tip of each V region and can therefore bind two antigen molecules.

Saladin: Anatomy & I 21. The Lymphatic and I Text I © The McGraw-Hill

Physiology: The Unity of Immune System Companies, 2003 Form and Function, Third Edition

Chapter 21 The Lymphatic and Immune Systems 837

5. There are five classes of antibodies— IgA, IgD, IgE, IgG, and IgM—that differ in the number of antibody monomers (from one to five), structure of the C region, and immune function (table 21.4).

6. Antibodies inactivate antigens by neutralization, complement fixation, agglutination, and precipitation.

7. Memory: Upon reexposure to the same antigen, memory B cells mount a secondary (anamnestic) response so quickly that no illness results.

Immune System Disorders (p. 827)

1. There are three principal dysfunctions of the immune system: too vigorous or too weak a response, or a response that is misdirected against the wrong target.

2. Hypersensitivity is an excessive reaction against antigens that most people tolerate. Allergy is the most common form of hypersensitivity.

3. Type I (acute) hypersensitivity is an IgE-mediated response that begins within seconds of exposure and subsides within about 30 minutes. Examples include asthma, anaphylaxis, and anaphylactic shock.

4. Type II (antibody-dependent cytotoxic) hypersensitivity occurs when IgG or IgM attacks antigens bound to a target cell membrane, as in a transfusion reaction.

5. Type III (immune complex) hypersensitivity results from widespread deposition of antigen-antibody complexes in various tissues, triggering intense inflammation, as in acute glomerulonephritis and systemic lupus erythematosus.

6. Type IV (delayed) hypersensitivity is a cell-mediated reaction (types I—III are antibody-mediated) that appears 12-72 hours after exposure, as in the reaction to poison ivy and the TB skin test.

7. Autoimmune diseases are disorders in which the immune system fails to distinguish self-antigens from foreign antigens and attacks the body's own tissues. They can occur because of cross-reactivity of antibodies, as in rheumatic fever; abnormal exposure of some self-antigens to the blood, as in one form of sterility resulting from sperm destruction; or changes in self-antigen structure, as in type I diabetes mellitus.

8. Immunodeficiency diseases are failures of the immune system to respond strongly enough to defend the body from pathogens. These include severe combined immunodeficiency disease (SCID), present at birth, and acquired immunodeficiency disease (AIDS), resulting from HIV infection.

9. HIV is a retrovirus that destroys TH cells. Since TH cells play a central coordinating role in cellular and humoral immunity and nonspecific defense, HIV knocks out the central control over multiple forms of defense and leaves a person vulnerable to opportunistic infections and certain forms of cancer.

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