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Questions and Answers
What are the processes by which antigens are presented to T cells?
Phagocytosis and macropinocytosis.
What is the purpose of antigen processing?
To present protein antigens to lymphocytes in the form of short peptide fragments.
What is the role of dendritic cells in antigen presentation?
Dendritic cells take up antigens and carry them to lymph nodes for presentation to naive T cells.
Which type of MHC molecule is found in all nucleated cells?
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What is the significance of the invariant chain?
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In the process of cross-presentation, antigen presenting cells typically present extracellular antigens with MHC class ______ molecules.
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MHC class II makes a distinction between self and non-self peptides.
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What happens to dendritic cells when they sense a threat?
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What are the main components of the peptide-loading complex for MHC class I?
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Where does peptide loading onto MHC class I occur?
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What cellular structures engulf microbes, particles, or molecules during macropinocytosis?
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What occurs after a cytoplasmic vesicle fuses with a lysosome during macropinocytosis?
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How are empty phagolysosomes managed after their contents have been degraded?
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MHC class II molecules bind peptides that are produced where in the cell?
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What is the main function of the MHC class I pathway?
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Which of the following describes the fate of MHC class I molecules after peptide loading?
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In which compartment does the peptide loading onto MHC class II occur?
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What differentiates MHC class I from MHC class II in terms of peptide source?
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Which statement correctly describes the roles of phagocytosis and macropinocytosis in antigen presentation?
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What is the primary function of MHC class I molecules in antigen presentation?
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Which of the following correctly describes the role of dendritic cell maturation in antigen presentation?
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What is the significance of the invariant chain in MHC class II processing?
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How are proteins from intracellular pathogens presented by MHC class II molecules?
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What is a key step in the process of phagocytosis?
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Which component is essential for the assembly and peptide loading of MHC class I?
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What distinguishes MHC class I from MHC class II in the context of dendritic cells?
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What activity is increased in activated dendritic cells to facilitate T cell activation?
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How do naive T cells first encounter their specific antigens?
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Which cells primarily orchestrate the division and differentiation of naive T cells?
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What is the fate of some helper T cells after activation in lymph nodes?
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What type of MHC molecule do intracellular pathogens primarily load their peptide fragments into?
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What happens to plasma cells after they are activated within the lymph node?
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What is one method by which some pathogens avoid detection by the adaptive immune system?
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What is a characteristic of MHC class II molecules in terms of peptide origin?
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What is the primary role of dendritic cells in the context of antigen presentation?
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Which mechanism is most likely involved in the cross-presentation of extracellular antigens?
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What happens to MHC class I molecules if a suitable peptide is not loaded?
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What chaperone molecule replaces calnexin during the MHC class I loading process?
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Which cellular component assists in transporting peptide fragments to the endoplasmic reticulum for MHC class I loading?
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In the context of cross-presentation, what is primarily presented to CD8 T cells?
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What is one significant pathway suggested for the process of cross-presentation?
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What is the main function of tapasin during the MHC class I loading process?
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What is the role of transporter associated with antigen processing (TAP) in the endoplasmic reticulum?
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Which of the following components is NOT part of the MHC class I peptide-loading complex?
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What is the function of ER aminopeptidase (ERAP) in antigen processing?
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How are peptides generated for binding to MHC class II molecules?
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What prevents MHC class II from binding peptides in the endoplasmic reticulum?
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Which type of receptor is primarily responsible for the detection of pathogen-associated molecular patterns (PAMPs)?
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What is the relationship between TAP and the peptide-loading complex in the endoplasmic reticulum?
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What is formed by the disulfide bond between tapasin and ERp57 in the peptide-loading complex?
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What type of chain structure binds with the MHC class I heavy chain at asparagine 86?
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What cellular event triggers dendritic cell maturation in response to a threat?
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Study Notes
Antigen Presentation Overview
- T cells only recognize antigen presented by MHC molecules
- Antigen presenting cells (APCs) process and present antigens to T cells
- Antigen presentation involves enzymatic degradation of ingested proteins into peptide fragments
- These peptide fragments are loaded into MHC class I or II molecules
MHC Class I & MHC Class II Pathways
- MHC class I presents peptides derived from intracellular proteins
- MHC class II presents peptides derived from extracellular proteins
- Both MHC I and MHC II are loaded with peptides in distinct intracellular compartments
MHC Class I Pathway
- Proteins are degraded in the cytosol by proteasomes
- Peptides are transported from the cytosol into the ER lumen by the transporter associated with antigen processing (TAP)
- Peptide-loading complex facilitates assembly and peptide loading of MHC class I
- The peptide-loading complex comprises chaperones calreticulin, ERp57, and tapasin
MHC Class II Pathway
- Pathogens are taken up by endocytosis and transported to lysosomes
- Invariant chain prevents MHC class II from binding peptides in the ER
- The invariant chain is degraded in the acidic environment of the phagolysosome, allowing peptides to bind to MHC class II
Dendritic Cell Maturation
- Immature dendritic cells sense invasive threats through PRRs
- Threat sensing causes dendritic cells to mature, migrate to lymph nodes, and increase MHC II synthesis
- Mature dendritic cells present antigens to T cells in the lymph nodes
Cross-Presentation
- Extracellular antigens can be presented by MHC class I to CD8+ T cells
- Cross-presentation enables the immune system to respond to intracellular pathogens that circumvent phagolysosome vesicles
- The exact molecular pathway of cross-presentation is unclear, but it may involve transport of antigens directly from phagolysosomes to MHC class I
Summary of Antigen Processing and Presentation
- MHC class I pathways are responsible for presenting intracellular antigens to CD8+ T cells.
- MHC class II pathways are responsible for presenting extracellular antigens to CD4+ T cells.
- Dendritic cells are critical for antigen presentation to naive T cells in lymph nodes.
- Cross-presentation allows dendritic cells to present extracellular antigens to CD8+ T cells and expand the range of immune responses against intracellular pathogens
Antigen Presentation Overview
- T cells can only recognize antigens that are presented by MHC molecules.
- MHC restriction refers to the requirement for antigens to be processed and presented to T cells by antigen-presenting cells (APCs).
- Antigen presentation is the process by which proteins are broken down into peptide fragments and loaded onto MHC molecules.
Phagocytosis & Macropinocytosis
- Phagocytosis is the process by which cells internalize solid particles and engulf them in a phagosome.
- Macropinocytosis is the process by which cells internalize large amounts of extracellular fluid and form a macropinosome.
- Both phagocytosis and macropinocytosis are important for antigen presentation.
MHC Class I and Class II:
- MHC Class I is expressed by all nucleated cells, and presents peptides from the cytosol.
- MHC Class II is expressed by antigen presenting cells (APCs), and presents peptides from the extracellular environment.
MHC Class I Antigen Processing and Presentation
- Peptides are produced in the cytosol and transported to the ER via the transporter associated with antigen processing (TAP) protein.
- The peptide-loading complex (PLC) facilitates the binding of peptides to MHC Class I molecules.
- The PLC includes the chaperones calreticulin, ERp57, and tapasin.
- Tapasin stabilizes the empty MHC Class I conformation and interacts with TAP to increase peptide affinity.
MHC Class II Antigen Processing and Presentation
- The invariant chain prevents peptides from binding to MHC Class II molecules in the ER.
- The invariant chain is degraded in endocytic vesicles, allowing peptides from exogenous sources to bind to MHC Class II molecules.
Dendritic Cell Maturation
- Dendritic cells are important APCs and undergo maturation when they encounter pathogens.
- Maturation is triggered by pathogen-associated molecular patterns (PAMPs) or indirect signals like antibodies or complement molecules.
- Maturation includes migration to lymph nodes, decreased phagocytic and macropinocytic activity, and increased MHC Class II synthesis.
- Most self-reactive CD4+ T cells have been eliminated during development, so self-antigens on dendritic cells are usually not recognized.
Cross-Presentation
- Cross-presentation is the process by which extracellular antigens can be presented on MHC Class I molecules.
- This allows CD8+ T cells to respond to antigens that are not directly produced by the cell.
- Cross-presentation is important for the immune response to viruses, bacteria, and other pathogens.
Intracellular Pathogens and Antigen Presentation
- Some pathogens can escape phagolysosome vesicles and enter the cytoplasm.
- This makes it difficult for the immune system to detect them.
- However, dendritic cells can take up dead cells and debris from infected cells, allowing them to present peptides from intracellular pathogens on MHC Class II.
MHC Class I and Class II Summary
- MHC Class I: presents peptides from the cytosol, generated by proteasomes.
- MHC Class II: presents peptides from the extracellular environment, generated by endocytic vesicles.
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Description
This quiz explores the mechanisms of antigen presentation and the roles of MHC class I and II. Understand how T cells recognize antigens and the distinct pathways for intracellular and extracellular peptide processing. Test your knowledge of the key concepts related to antigen presenting cells (APCs) and MHC molecules.
