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Questions and Answers
Which of the following best describes the composition of viruses?
Which of the following best describes the composition of viruses?
Viruses have their own metabolism and can replicate independently.
Viruses have their own metabolism and can replicate independently.
False
What is a virion composed of?
What is a virion composed of?
nucleic acid surrounded by a protein coat
Viruses with no lipid bilayer envelope are known as naked __________.
Viruses with no lipid bilayer envelope are known as naked __________.
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What is the term for the process where protein capsid forms and genomes are packaged into the capsid during animal virus replication?
What is the term for the process where protein capsid forms and genomes are packaged into the capsid during animal virus replication?
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Which of the following statements is true regarding RNA viruses replication?
Which of the following statements is true regarding RNA viruses replication?
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Prions consist solely of nucleic acids.
Prions consist solely of nucleic acids.
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___ are proteinaceous infectious agents composed solely of protein and no nucleic acids.
___ are proteinaceous infectious agents composed solely of protein and no nucleic acids.
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Match the following immune cell types with their functions:
Match the following immune cell types with their functions:
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Where are Toll-like receptors (TLRs) anchored?
Where are Toll-like receptors (TLRs) anchored?
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NOD-like receptors (NLRs) are found in the cytoplasm.
NOD-like receptors (NLRs) are found in the cytoplasm.
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What do RIG-like receptors (RLRs) detect?
What do RIG-like receptors (RLRs) detect?
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C proteins become activated in response to signals indicating __________ invasion.
C proteins become activated in response to signals indicating __________ invasion.
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What do TH cells secrete to help in activation of B cells and macrophages?
What do TH cells secrete to help in activation of B cells and macrophages?
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What is the process called when a B cell presents peptide fragments to TH cells at the cell surface?
What is the process called when a B cell presents peptide fragments to TH cells at the cell surface?
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Memory B cells are responsible for the primary immune response.
Memory B cells are responsible for the primary immune response.
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Class switching in plasma cells results in the secretion of antibody class other than IgM, usually __________.
Class switching in plasma cells results in the secretion of antibody class other than IgM, usually __________.
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Which type of antigens can lead to B cell activation without the help of TH cells?
Which type of antigens can lead to B cell activation without the help of TH cells?
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Match the following MHC molecules with the antigens they present:
Match the following MHC molecules with the antigens they present:
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What type of T cells directly interact with target cells?
What type of T cells directly interact with target cells?
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T cells can produce antibodies.
T cells can produce antibodies.
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What are the two general categories of antigens mentioned in the content?
What are the two general categories of antigens mentioned in the content?
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Which type of antigens generally activate B cells without the help of TH cells?
Which type of antigens generally activate B cells without the help of TH cells?
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Proteins generally cause a weak immune response compared to lipids.
Proteins generally cause a weak immune response compared to lipids.
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What are epitopes also known as?
What are epitopes also known as?
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Immunoglobulins are also called ___ antibodies.
Immunoglobulins are also called ___ antibodies.
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Match the following Ig classes with their descriptions:
Match the following Ig classes with their descriptions:
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Study Notes
Viruses, Viroids, and Prions
- Viruses are genetic information composed of DNA or RNA contained within a protective protein coat
- Inert particles, no metabolism, no replication, hijack host cell's replication machinery
- Infectious agents, but not alive
- Viral genome is either DNA or RNA, never both, and affects replication strategy
- Viral protein components are used for attachment, with phages having tail fibers and animal viruses having spikes
- Viruses have three shapes: icosahedral, helical, and complex
- Virus families end in the suffix -viridae, and genus ends in -virus
- Viruses are often categorized informally, such as by route of infection (e.g., oral-fecal, respiratory, zoonotic)
Bacteriophages
- There are three general types of bacteriophages: lytic, temperate, and lysogenic
- Lytic phages exit the host after replicating, causing the host cell to lyse, and involve a five-step process: attachment, genome entry, synthesis, assembly, and release
- Temperate phages have the option of the lytic life cycle or the incorporation of their DNA into the host cell genome
- Lambda phage is a model for the lysogenic life cycle, where the phage DNA circularizes and can direct lytic infection or integrate into the host chromosome
Roles of Bacteriophages in Horizontal Gene Transfer
- Generalized transduction results from packaging errors during phage assembly, and can transfer any gene from the donor cell
- Specialized transduction results from excision mistakes during the transition from lysogenic to lytic cycle, and only transfers genes next to the integrated phage DNA
Bacterial Defenses Against Phages
- Restriction-modification systems protect bacteria by degrading phage DNA using enzymes, and modification enzymes protect host DNA from restriction enzymes
Methods Used to Study Bacteriophages
- Viruses multiply only inside living cells, and must be cultivated in suitable host cells to grow
- Plaque assays are used to quantitate phage particles in samples, and involve counting plaque-forming units (PFU)
Animal Virus Replication
- Animal virus replication involves a five-step infection cycle: attachment, penetration and uncoating, synthesis, assembly, and release
- Attachment involves binding to specific receptors on the host cell surface
- Penetration and uncoating involve the fusion of the virus with the host cell membrane or endocytosis
- Synthesis involves the expression of viral genes and the production of viral structural and catalytic proteins
- Replication strategies vary depending on the type of genome: DNA, RNA, or reverse-transcribing
Categories of Animal Virus Infections
- Acute infections have a rapid onset and short duration
- Persistent infections continue for years or a lifetime, and may be chronic or latent
- Chronic infections involve continuous production of low levels of virus particles, while latent infections involve the integration of the viral genome into the host chromosome
Viruses and Human Tumors
- Viruses can cause tumors by interfering with host cell growth control mechanisms
- Some viruses have oncogenes that are similar to host proto-oncogenes, and can induce tumors
Cultivating and Quantitating Animal Viruses
- Viruses must be grown in an appropriate host, such as cell culture or tissue culture
- Embryonated chicken eggs were historically used, but are now less common
Other Infectious Agents: Prions
- Prions are proteinaceous infectious agents composed solely of protein, with no nucleic acids
- Prions are linked to slow, fatal human diseases and animal diseases, and are usually transmitted within species
- Prion proteins accumulate in neural tissue, causing neurons to die and tissues to develop holes
Immune System: Innate and Adaptive Immunity
- Innate immunity is routine protection, and involves pattern recognition of specific molecules
- Adaptive immunity develops throughout life, and is more specialized, involving antigens and the production of antibodies
Overview of the Innate Defenses
- The first line of defense involves physical barriers, such as skin and mucous membranes
- The second line of defense involves sensor systems, blood cells, and phagocytosis
First-Line Defenses
- Physical barriers, such as skin and mucous membranes, prevent microbe entry into the body
- Antimicrobial substances, such as salt, lysozyme, and defensins, also protect the body
- Normal microbiota, the community of microorganisms that live in and on the body, can also prevent pathogen growth
Second-Line Defenses
- Blood cells, including granulocytes, mononuclear phagocytes, and lymphocytes, are important in host defenses
- Granulocytes, such as neutrophils and basophils, engulf and destroy bacteria and other material
- Mononuclear phagocytes, such as macrophages and dendritic cells, differentiate from monocytes and play a key role in host defenses### The Cells of the Immune System
- Dendritic cells: sentinel cells that function as "scouts" and engulf material in tissues, bringing it to cells of the adaptive immune system for "inspection"
- Lymphocytes: responsible for adaptive immunity, highly specific in recognition of antigen, found in lymph nodes and lymphatic tissues
- B cells: produce antibodies in response to antigen
- T cells: highly specific in recognition of antigen, including natural killer (NK) cells that lack specificity
Cell Communication
- Communication between blood cells allows for a coordinated response to microbial invasion
- Surface receptors: "eyes" and "ears" of cell, proteins that span the membrane, connecting outside to inside
- Binding to specific ligand induces response
- Cytokines: "voices" of cell, produced by one cell, diffuse to others, bind to appropriate cytokine receptors to induce changes
- Adhesion molecules: allow cells to stick to other cells, e.g. endothelial cells adhere to phagocytic cells in the blood, allowing them to exit the bloodstream
Pattern Recognition Receptors (PRRs)
- Allow the body's cells to detect signs of microbial invasion
- Three types: Toll-like receptors, NOD-like receptors, and RIG-like receptors
- Toll-like receptors (TLRs): anchored in membranes of sentinel cells, detect compounds, send signals to nucleus, induce gene expression, and cause a response
- NOD-like receptors (NLRs): found in cytoplasm, detect bacterial components, initiate a series of events to protect the host, sometimes damaging the host cell
- RIG-like receptors (RLRs): found in cytoplasm, detect viral RNA, induce production of interferons, which diffuse to neighboring uninfected cells, causing them to express inactive antiviral proteins
The Complement System
- Assists activities of adaptive immune system
- Complement proteins are inactive proteins circulating in blood and bathing tissues
- Proteins are activated in response to signals indicating microbial invasion
- Activation leads to a cascade of activation, resulting in three major outcomes: opsonization, inflammatory response, and lysis of foreign cells
- Three pathways of activation: alternative, lectin, and classical
Phagocytosis
- Phagocytes engulf and digest material and pathogens
- Chemotaxis: phagocytes are recruited by chemoattractants
- Recognition and attachment: phagocytes bind to pathogen
- Engulfment: pseudopods surround the pathogen, then fuse to form a phagosome
- Phagosome maturation and phagolysosome formation: phagosome fuses with lysosome, pH is lowered, and lysosomes deliver enzymes
- Destruction and digestion: toxic ROS produced, pH decreases, enzymes degrade, defensins damage membrane of invader
- Exocytosis: phagolysosome fuses with plasma membrane, and expels any undigested material
The Inflammatory Response
- Tissue damage results in inflammation
- Inflammation contains the site of damage, localizes the response, eliminates the invader, and restores tissue function
- Triggers cause host cells to release inflammatory mediators
- Inflammation results in swelling, redness, heat, pain, and sometimes loss of function
- Acute inflammation is short-term, macrophages clean up damage by ingesting dead cells and debris
- If acute fails, chronic inflammation results, macrophages and giant cells accumulate, and granulomas form
Fever
- Important host defense mechanism
- Strong indicator of infectious disease, especially bacterial
- Temperature-regulation center in brain normally holds at 37°C, but raises during infection in response to pyrogens
- Increased temperature slows bacterial growth, allows more time for host defenses
Adaptive Immunity
- Develops and matures throughout life
- Takes a week or more to build effective immunity following first exposure to a pathogen
- Adaptive immunity has memory, giving a stronger response to re-exposure to a pathogen
- Adaptive immunity also has molecular specificity, protection against one pathogen does not protect against others
- It is also tolerant, must distinguish between "healthy self" cells and "dangerous" cells such as pathogens and cancer cells
Strategy of the Adaptive Immune Response
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Depends on lymphocytes: B cells and T cells
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First response to an antigen is the primary response
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Adaptive immune system then "remembers" the mechanism that proved effective against that specific antigen
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If encountered again, a stronger secondary response results
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Two basic strategies exist: humoral immunity and cell-mediated immunity
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Humoral immunity: B cells produce antibodies, which eliminate extracellular antigens
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Cell-mediated immunity: T cells eliminate antigens residing within a host cell### T-Cell Function
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T cells secrete cytokines that help in activation of B cells and macrophages
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T cells secrete “death packages” that lead to death of infected host cells and cancer cells
B Lymphocytes and the Antibody Response
- B-cell activation by T-dependent antigens involves:
- B-cell receptor binding to antigen
- Antigen internalization via endocytosis and degradation into peptide fragments
- Fragments delivered to MHC class II molecules at the B cell surface for inspection by TH cells
- Antigen presentation, which leads to activation of B cells by TH cells
- Characteristics of Primary Response:
- Takes 10-14 days for substantial antibody accumulation
- Person may be sick during this time
- Additional exposure to antigen leads to a much faster secondary response
- Characteristics of Primary Response (continued):
- Some B cells differentiate to form plasma cells
- Plasma cells generate antibodies
- Activated B cells continue proliferating and differentiating in the presence of antigen
- Antibody titer steadily increases
- Class Switching:
- All plasma cells initially secrete IgM
- TH cells can induce some activated B cells to become plasma cells that secrete other antibody classes (usually IgG)
- Characteristics of Secondary Response:
- Significantly faster and more effective than primary response
- Pathogens are usually eliminated by this response before causing harm
- Memory B cells are responsible for the secondary response
- Antibodies produced by these cells bind antigen effectively
The Response to T-Independent Antigens
- Some antigens can lead to B cell activation without the help of TH cells
- These antigens have multiple identical epitopes, which are bound by groups of B-cell receptors
- This leads to activation, but the response is not very immunogenic in young children
T Lymphocytes: Antigen Recognition and Response
- T cells play a different role from B cells
- T cells never produce antibodies
- Effector T cells directly interact with target cells
- T cells recognize peptide-MHC complexes
- MHC class I molecules present endogenous antigens
- MHC class II molecules present exogenous antigens
- Cytotoxic T cells recognize antigen presented on MHC class I molecules
- Helper T cells recognize antigen presented on MHC class II molecules
- TH cells have CD4 proteins, which are receptors for HIV
- TC cells have CD8 proteins
Effector Functions of TC (CD8) Cells
- TC cells induce apoptosis in infected “self” cells and cancer cells
- TC cells recognize infected or cancerous cells by the MHCI molecules
- They release proteases and cytotoxins to induce apoptosis
Effector Functions of TH (CD4) Cells
- TH cells coordinate the immune response
- They recognize peptide presented by a B cell or a macrophage and release cytokines to activate those cells
- They control the activities of activated B cells and macrophages
- They also control the activities of other T cells
Role of TH Cells in B Cell Activation
- TH cells recognize antigen presented on MHCII from antigen-presenting cells and activate that cell with cytokines
Role of TH Cells in Macrophage Activation
- TH cells recognize peptides presented by a macrophage on MHCII molecules
- They activate the macrophage by releasing cytokines, which increases its power
- Activated macrophages have increased size and metabolism, and produce toxic compounds
Natural Killer (NK) Cells
- NK cells induce apoptosis in “self” cells
- NK cells recognize host cells with foreign proteins in the membrane bound by antibodies through antibody-dependent cellular cytotoxicity (ADCC)
- NK cells also recognize and destroy stressed host cells lacking MHCI
- Some viruses interfere with antigen presentation and prevent host cells from displaying MHCI, which allows NK cells to recognize and destroy them
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Description
This quiz tests your knowledge of virus composition, replication, and other virology concepts. Answer questions about virions, protein capsids, and RNA viruses.