CHAPTER SUMMARY
43.1 THE INNATE IMMUNE SYSTEM ACTS AGAINST A DIVERSITY OF FOREIGN PATHOGENS.
- The skin is a physical barrier and contains chemicals, cells, and microorganisms that provide protection against infection.
- Phagocytes, including macrophages, dendritic cells, and neutrophils, engulf other cells and debris.
- Toll-like receptors on the surface of phagocytes play a central role in recognition of foreign molecules.
- Eosinophils, basophils, mast cells, and natural killer cells are key players in the innate immune system.
- Inflammation is the response of the body to infection or injury. It entails increases in blood flow and blood vessel permeability and recruitment of phagocytes.
- The complement system consists of circulating proteins that activate one another. Activation results in the formation of a membrane attack complex that lyses bacteria and other cells, production of proteins that enhance phagocytosis, and generation of cytokines.
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43.2 THE ADAPTIVE IMMUNE SYSTEM INCLUDES B CELLS THAT PRODUCE ANTIBODIES AGAINST SPECIFIC PATHOGENS.
- B cells produce antibodies that bind to foreign antigens.
- Antibodies have two light chains and two heavy chains joined by covalent disulfide bonds.
- Light and heavy chains have constant and variable regions.
- Mammals produce five types of antibody: IgG, IgM, IgA, IgD, and IgE. IgG is the most abundant; IgM and IgD are found on the surface of B cells; IgA is secreted; and IgE is involved in allergic reactions.
- Antibody specificity is achieved by clonal selection, in which an antigen binds to one or a few B cells on the basis of the fit with its cell-surface antibody receptor, leading to B cell differentiation into plasma and memory cells.
- 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.
- Antibody diversity is generated by genomic rearrangement, where individual gene segments are joined to make functional genes.
43.3 THE ADAPTIVE IMMUNE SYSTEM INCLUDES HELPER AND CYTOTOXIC T CELLS THAT ATTACK PATHOGENS, FOREIGN CELLS, AND DISEASED CELLS.
- Two types of T cell are helper T cells and cytoxic 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.
- T cells are activated by the binding of a T cell receptor to an antigen in association with an MHC protein.
- As T cells mature in the thymus, they undergo positive and negative selection, so only T cells that recognize self MHC, but not self antigens, survive.
- The ability to distinguish self from non-self antigens is critical for proper immune function; autoimmune diseases result when the immune system attacks cells of the host.
43.4 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 through antigenic drift and antigenic shift.
- The bacterium that causes tuberculosis grows slowly and resides inside cells, particularly macrophages.
- Malaria is caused by a single-celled eukaryote with a complex life cycle having stages spent in mosquitoes and humans. The malaria parasite has evolved several mechanisms, including antigenic variation, that help it reproduce in humans without being eliminated by the immune system.
Self-Assessment Question 1
Compare and contrast features of the innate and adaptive immune systems.
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Model Answer:
Both the innate and adaptive immune systems can target a variety of pathogens while also distinguishing self from non-self. All organisms have an innate immune system, which provides a broad and non-specific 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.
Self-Assessment Question 2
List components of the innate immune system.
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Model 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 leukocytes (e.g., phagocytes).
Self-Assessment Question 3
Describe how cells of the innate immune system recognize pathogens.
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Model Answer:
Leukocytes identify non-self 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.
Self-Assessment Question 4
Explain how the four signs of inflammation relate to the changes that occur in the underlying tissues.
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Model Answer:
Leukocytes 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 to 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) .
Self-Assessment Question 5
Draw a picture of an antibody molecule, labeling each of the parts. Include the light and heavy chains, the variable and constant regions, and the antigen-binding site.
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Model Answer:
Self-Assessment Question 6
Describe how B cells produce a great variety of different antibodies, each specific for a particular antigen.
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Model 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 each B cell produces a unique antibody that can recognize a specific antigen.
Self-Assessment Question 7
Explain how immunological memory is achieved.
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Model 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.
Self-Assessment Question 8
Compare and contrast the activation of T cells and B cells.
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Model 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.
Self-Assessment Question 9
Describe how positive and negative selection leads to a population of T cells that react with foreign antigens but not self antigens, and what happens when tolerance to self antigens is lost.
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Model 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 non-self 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.
Self-Assessment Question 10
Describe three mechanisms that allow pathogens to evade the immune system.
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Model 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.