Chapter 43 Summary

Core Concepts Summary

43.1 The immune system protects organisms from pathogens.

Pathogens are agents or organisms that cause disease. page 922

The immune system is able to distinguish host molecules and cells, called self, from foreign molecules and cells, called nonself. page 922

The immune system consists of two parts: The innate immune system acts nonspecifically and the adaptive immune system acts specifically against pathogens. page 923

43.2 The innate immune system acts generally against a diversity of pathogens.

The skin is a physical barrier and contains chemicals, cells, and microorganisms that provide protection against pathogens. page 924

Phagocytes engulf other cells and debris. page 925

Eosinophils, basophils, mast cells, and natural killer cells participate in the innate immune system. page 925

Toll-like receptors on the surface of phagocytes play a key role in recognition of foreign molecules. page 926

Inflammation is the body’s response to infection or injury. It involves an increase in blood flow and blood vessel permeability, recruitment of phagocytes, and production of cytokines. page 926

The complement system consists of circulating proteins that activate one another. Activation results in the formation of a membrane attack complex that lyses bacteria, production of proteins that enhance phagocytosis, and generation of cytokines. page 927

43.3 The adaptive immune system includes B cells that produce antibodies against specific pathogens that reside outside cells in tissues.

B cells produce antibodies that bind to antigens. page 929

Antibodies have two light chains and two heavy chains joined by covalent disulfide bonds. page 929

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Light and heavy chains have constant and variable regions. page 929

Mammals produce five types of antibody: IgG, IgM, IgA, IgD, and IgE. page 929

Antibody specificity is achieved by clonal selection, in which an antigen binds to antibodies on the surface of one or a few B cells, leading to B cell differentiation into plasma and memory cells. page 930

The primary immune response is characterized by a long lag period and a short response. The secondary immune response is quicker and more robust, and provides the basis for immunity following infection or vaccination. page 931

Antibody diversity is generated by genomic rearrangement, in which individual gene segments are joined to make a functional gene. page 931

43.4 The adaptive immune system includes helper and cytotoxic T cells that attack infected, diseased, and foreign cells.

Two types of T cell are helper T cells and cytotoxic T cells. Helper T cells activate other cells of the immune system, including macrophages, B cells, and cytotoxic T cells. Cytotoxic T cells kill altered host cells. page 934

T cells are activated by the binding of a T cell receptor to an antigen in association with an MHC protein. page 934

As T cells mature in the thymus, they undergo positive and negative selection, so only T cells that recognize MHC, but not self antigens, survive. page 936

The ability to distinguish self from nonself antigens is critical for proper immune function; autoimmune diseases result when the immune system attacks cells of the host. page 936

43.5 Some pathogens have evolved mechanisms that enable them to evade the immune system.

The flu is caused by an RNA virus that can change its surface proteins by antigenic drift and antigenic shift. page 937

The bacterium that causes tuberculosis grows slowly and resides inside cells, particularly macrophages. page 937

Malaria is caused by a single-celled eukaryote with a complex life cycle having stages in mosquitoes and in humans. The malaria parasite has evolved several mechanisms, including antigenic variation, that help it reproduce in humans without being eliminated by the immune system. page 938

Self-Assessment

  1. Compare and contrast the innate and adaptive immune systems.

    Self-Assessment 1 Answer

    Both the innate and adaptive immune systems can target a variety of pathogens while also distinguishing self from nonself. All organisms have an innate immune system, which provides a broad and nonspecific defense against pathogens. Adaptive immune systems are only found in vertebrates, allowing these organisms to have immunological memory of previous exposures to pathogens and the ability to mount a specific response to pathogens.

  2. List components of the innate immune system.

    Self-Assessment 2 Answer

    The innate immune system is composed of physical barriers (e.g., skin, mucous membranes); chemical barriers (e.g., low pH of the stomach, enzymes); mutualistic microorganisms that compete with pathogenic organisms for resources; and white blood cells (e.g., phagocytes).

  3. Describe how cells of the innate immune system recognize pathogens.

    Self-Assessment 3 Answer

    White blood cells identify nonself invaders using cell-surface receptors, known as toll-like receptors. These receptors recognize conserved surface markers present on a variety of pathogens but not on host cells.

  4. Explain how the four signs of inflammation relate to the changes that occur in the underlying tissues.

    Self-Assessment 4 Answer

    White blood cells present at the site of an infection release chemical messengers that cause blood vessels to dilate, increasing blood flow to the area and leading to the redness (rubor) and warmth (calor) that are characteristic of inflammation. These chemicals can also increase the permeability of blood vessels. As a result, cells and fluid leak out into the interstitial space, causing swelling of the tissue (tumor). The chemical messengers can also target nerve cells in the infected area, causing pain (dolor).

  5. Draw a picture of an antibody molecule, labeling each of the components and the places where antigens bind.

    Self-Assessment 5 Answer

  6. Describe how B cells produce a great variety of different antibodies, each specific for a particular antigen.

    Self-Assessment 6 Answer

    The heavy and light chains of an antibody are encoded by multiple gene segments, and each segment is present in multiple copies within the genome. As a B cell differentiates, genomic rearrangements occur such that only one copy of each segment is used to produce the resulting antibody. These rearrangements occur independently in each B cell, so it produces a unique antibody that can recognize a specific antigen.

  7. Explain how immunological memory is achieved.

    Self-Assessment 7 Answer

    After the first exposure to a given pathogen, clonal selection of B cells with antibodies that recognize antigens on that pathogen leads to the production of memory B cells that are specific for that particular pathogen. These memory B cells secrete pathogen-specific antibodies in response to re-exposure to the same antigen. They are long-lived and respond more quickly to a pathogen than naïve B cells. Moreover, the pool of memory B cells is larger than the pool of naïve B cells for the same antigen. For these reasons, the response to subsequent exposures to the same pathogen is faster, stronger, and more sustained.

  8. Compare and contrast the activation of B cells and of T cells.

    Self-Assessment 8 Answer

    Both B cells and T cells are activated by the binding of an antigen to their cell-surface receptors (antibodies or T-cell receptors, respectively). Both B cells and cytotoxic T cells are activated by cytokines released from helper T cells. B cells can recognize free or pathogen-bound antigens, while T cells require the antigen to be presented in association with major histocompatability complex proteins expressed on the surface of host cells.

  9. Describe how positive and negative selection leads to a population of T cells that react to MHC molecules in association with nonself antigens, but not self antigens, and what happens when tolerance to self antigens is lost.

    Self-Assessment 9 Answer

    T cells expressing T-cell receptors that efficiently recognize host MHC proteins undergo positive selection during maturation in the thymus. These selected cells continue the maturation process, whereas those that do not recognize MHC proteins are destroyed. In addition, T cells with receptors that react with self antigens presented by MHC proteins are eliminated by negative selection during maturation. As a result, the selected T-cell population will only effectively recognize nonself antigens presented by MHC proteins. If tolerance to self antigens is lost, autoimmune diseases can arise as the immune system attacks and destroys the host’s own body cells.

  10. Describe three mechanisms that allow pathogens to evade the immune system.

    Self-Assessment 10 Answer

    Pathogens that have the ability to change their surface antigens over time can evade the immune system because the antibodies and memory cells generated after the first encounter do not recognize the new antigens presented by the pathogen. Some pathogens are able to change their surface antigens through antigenic drift, where random mutations acquired during replication lead to gradual antigenic variation over time. Other pathogens can undergo sudden changes in surface antigens (antigenic shift) as a result of genetic reassortment. Still other pathogens have multiple, slightly different copies of a particular antigenic protein and can change which one is expressed. In this way, they can alter the antigens presented to the immune system. Pathogens that grow within a host cell can also escape the immune response because they are shielded by the host cell from the circulating immune cells.