Chapter 17 Glossary, Summary, and Practice Questions

KEY TERMS

acellular lacking cells

active immunity an immunity that occurs as a result of the activity of the body’s own cells rather than from antibodies acquired from an external source

adaptive immunity a specific immune response that occurs after exposure to an antigen either from a pathogen or a vaccination

allergy an immune reaction that results from immediate hypersensitivities in which an antibody-mediated immune response occurs within minutes of exposure to a harmless antigen

antibody a protein that is produced by plasma cells after stimulation by an antigen; also known as an immunoglobulin antigen a macromolecule that reacts with cells of the immune system and which may or may not have a stimulatory effect antigen-presenting cell (APC) an immune cell that detects, engulfs, and informs the adaptive immune response about

an infection by presenting the processed antigen on its cell surface

apoptosis the cell death caused by induction of a cell’s own internal mechanisms either as a natural step in the development of a multicellular organism or by other environmental factors such as signals from cells of the immune system

attenuation the weakening of a virus during vaccine development

autoantibody an antibody that incorrectly marks “self” components as foreign and stimulates the immune response

autoimmunity a type of hypersensitivity to self-antigens B cell a lymphocyte that matures in the bone marrow capsid the protein coating of the viral core

cell-mediated immune response an adaptive immune response that is controlled by T cells

complement system an array of approximately 20 soluble proteins of the innate immune system that enhance phagocytosis, bore holes in pathogens, and recruit lymphocytes

cytokine a chemical messenger that regulates cell differentiation, proliferation, and gene expression to effect immune responses

cytopathic causing cell damage

cytotoxic T lymphocyte (TC) an adaptive immune cell that directly kills infected cells via enzymes, and that releases cytokines to enhance the immune response

dendritic cell an immune cell that processes antigen material and presents it on the surface of its cell in MHC class II molecules and induces an immune response in other cells

effector cell a lymphocyte that has differentiated, such as a B cell, plasma cell, or cytotoxic T cell

glycoprotein a protein molecule with attached carbohydrate molecules

helper T lymphocyte (TH) a cell of the adaptive immune system that binds APCs via MHC class II molecules and stimulates B cells or secretes cytokines to initiate the immune response

humoral immune response the adaptive immune response that is controlled by activated B cells and antibodies

hypersensitivity a spectrum of inappropriate immune responses toward harmless foreign particles or self-antigens; occurs after tissue sensitization and includes immediate-type (allergy), delayed-type, and autoimmunity

immune tolerance an acquired ability to prevent an unnecessary or harmful immune response to a detected foreign body known not to cause disease

immunodeficiency a failure, insufficiency, or delay at any level of the immune system, which may be acquired or inherited

inflammation the localized redness, swelling, heat, and pain that results from the movement of leukocytes through opened capillaries to a site of infection

innate immunity an immunity that occurs naturally because of genetic factors or physiology, and is not caused by infection or vaccination

interferon a cytokine that inhibits viral replication

lymph the watery fluid present in the lymphatic circulatory system that bathes tissues and organs with protective white blood cells and does not contain erythrocytes

lymphocyte a type of white blood cell that includes natural killer cells of the innate immune system and B and T cells of the adaptive immune system

macrophage a large phagocytic cell that engulfs foreign particles and pathogens

major histocompatibility class (MHC) I a group of proteins found on the surface of all nucleated cells that signals to

immune cells whether the cell is normal or is infected or cancerous; it also provides the appropriate sites into which antigens can be loaded for recognition by lymphocytes

major histocompatibility class (MHC) II molecule a protein found on the surface of antigen-presenting cells that

signals to immune cells whether the cell is normal or is infected or cancerous; it provides the appropriate template into which antigens can be loaded for recognition by lymphocytes

mast cell a leukocyte that produces inflammatory molecules, such as histamine, in response to large pathogens

memory cell an antigen-specific B or T lymphocyte that does not differentiate into an effector cell during the primary immune response but that can immediately become an effector cell on reexposure to the same pathogen

monocyte a type of white blood cell that circulates in the blood and lymph and differentiates into a macrophage after it moves into infected tissue

natural killer (NK) cell a lymphocyte that can kill cells infected with viruses or tumor cells

neutrophil a phagocytic leukocyte that engulfs and digests pathogens

passive immunity an immunity that does not result from the activity of the body’s own immune cells but by transfer of antibodies from one individual to another

primary immune response the response of the adaptive immune system to the first exposure to an antigen

secondary immune response the response of the adaptive immune system to a second or later exposure to an antigen mediated by memory cells

T cell a lymphocyte that matures in the thymus gland

vaccine a weakened solution of virus components, viruses, or other agents that produce an immune response

viral envelope a lipid bilayer that envelops some viruses

virion an individual virus particle outside a host cell

white blood cell a nucleated cell found in the blood that is a part of the immune system; also called leukocytes

CHAPTER SUMMARY

Viruses

Viruses are acellular entities that can usually only be seen with an electron microscope. Their genomes contain either DNA or RNA, and they replicate using the replication proteins of a host cell. Viruses are diverse, infecting archaea, bacteria, fungi, plants, and animals. Viruses consist of a nucleic-acid core surrounded by a protein capsid with or without an outer lipid envelope.

Viral replication within a living cell always produces changes in the cell, sometimes resulting in cell death and sometimes slowly killing the infected cells. There are six basic stages in the virus replication cycle: attachment, penetration, uncoating, replication, assembly, and release. A viral infection may be productive, resulting in new virions, or nonproductive, meaning the virus remains inside the cell without producing new virions.

Viruses cause a variety of diseases in humans. Many of these diseases can be prevented by the use of viral vaccines, which stimulate protective immunity against the virus without causing major disease. Viral vaccines may also be used in active viral infections, boosting the ability of the immune system to control or destroy the virus. Antiviral drugs that target enzymes and other protein products of viral genes have been developed and used with mixed success. Combinations of anti-HIV drugs have been used to effectively control the virus, extending the lifespan of infected individuals.

Innate Immunity

The innate immune system consists first of physical and chemical barriers to infection including the skin and mucous membranes and their secretions, ciliated surfaces, and body hairs. The second line of defense is an internal defense system designed to counter pathogenic threats that bypass the physical and chemical barriers of the body. Using a combination of cellular and molecular responses, the innate immune system identifies the nature of a pathogen and responds with inflammation, phagocytosis, cytokine release, destruction by NK cells, or the complement system.

Adaptive Immunity

The adaptive immune response is a slower-acting, longer-lasting, and more specific response than the innate response. However, the adaptive response requires information from the innate immune system to function. APCs display antigens on MHC molecules to naïve T cells. T cells with cell-surface receptors that bind a specific antigen will bind to that APC. In response, the T cells differentiate and proliferate, becoming TH cells or TC cells. TH cells stimulate B cells that have engulfed and presented pathogen-derived antigens. B cells differentiate into plasma cells that secrete antibodies, whereas TC cells destroy infected or cancerous cells. Memory cells are produced by activated and proliferating B and T cells and persist after a primary exposure to a pathogen. If re-exposure occurs, memory cells differentiate into effector cells without input from the innate immune system. The mucosal immune system is largely independent of the systemic immune system but functions in parallel to protect the extensive mucosal surfaces of the body. Immune tolerance is brought about by Treg cells to limit reactions to harmless antigens and the body’s own molecules.

Disruptions in the Immune System

Immune disruptions may involve insufficient immune responses or inappropriate immune responses. Immunodeficiency increases an individual’s susceptibility to infections and cancers. Hypersensitivities are misdirected responses either to harmless foreign particles, as in the case of allergies, or to the individual’s own tissues, as in the case of autoimmunity. Reactions to self-components may be the result of molecular mimicry.

ART CONNECTION QUESTIONS

• Figure 17.5 Which of the following statements about virus structure is true?
• All viruses are encased in a viral membrane.
• The capsomere is made up of small protein subunits called capsids.
• DNA is the genetic material in all viruses.
• Glycoproteins help the virus attach to the host cell.
• Figure 17.6 Influenza virus is packaged in a viral envelope, which fuses with the plasma membrane. This

way, the virus can exit the host cell without killing it. What advantage does the virus gain by keeping the host cell alive?

• Figure 17.17 The Rh antigen is found on Rh-positive red blood cells. An Rh-negative female can usually carry an Rh-positive fetus to term without difficulty. However, if she has a second Rh-positive fetus, her body may launch an immune attack that causes hemolytic disease of the newborn. Why do you think hemolytic disease is only a problem during the second or subsequent pregnancies?

REVIEW QUESTIONS

• Which statement is true?
• A virion contains DNA and RNA.
• Viruses are acellular.
• Viruses replicate outside of the cell.
• Most viruses are easily visualized with a light microscope.
• The viral plays a role in attaching a virion to the host cell.
• core
• capsid
• envelope
• both b and c
• Which statement is true of viral replication?
• In the process of apoptosis, the cell survives.
• During attachment, the virus attaches at specific sites on the cell surface.
• The viral capsid helps the host cell produce more copies of the viral genome.
• mRNA works outside of the host cell to produce enzymes and proteins.
• Which of the following is a barrier against pathogens provided by the skin?
• low pH
• mucus
• tears
• cilia
• Although interferons have several effects, they are particularly useful against infections with which type of pathogen?
• bacteria
• viruses
• fungi
• helminths
• Which innate immune system component uses MHC class I molecules directly in its defense strategy?

• macrophages
• neutrophils
• NK cells

CRITICAL THINKING QUESTIONS

• Why can’t dogs catch the measles?
• Why is immunization after being bitten by a rabid animal so effective?
• Different MHC class I molecules between donor and recipient cells can lead to rejection of a transplanted organ or tissue. Suggest a reason for this.
• If a series of genetic mutations prevented some, but not all, of the complement proteins from binding antibodies or pathogens, would the entire complement system be compromised?

• interferon
• The humoral immune response depends on which cells?
• TC cells
• B cells
• B and TH cells
• TC and TH cells
• The fact that the body does not normally mount an immune response to the molecules in food is an example of .
• secondary immune response
• immunological memory
• immune tolerance
• passive immunity
• Foreign particles circulating in the blood are filtered by the .
• spleen
• lymph nodes
• MALT
• lymph
• Allergy to pollen is classified as .
• an autoimmune reaction
• immunodeficiency
• delayed hypersensitivity
• immediate hypersensitivity
• A potential cause of acquired autoimmunity is

.

• tissue hypersensitivity
• molecular mimicry
• histamine release
• radiation exposure
• Autoantibodies are probably involved in .

• reactions to poison ivy
• pollen allergies
• systemic lupus erythematosus
• HIV/AIDS

• How do B and T cells differ with respect to antigens that they bind?
• Why is the immune response after reinfection much faster than the adaptive immune response after the initial infection?
• Some photographers develop a sensitivity to certain film developing chemicals leading to severe rashes on their hands such that they are unable to work with them. Explain what is probably happening.