T Lymphocytes and MHC Molecules Quiz
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Questions and Answers

Which cells primarily initiate the clonal expansion of naive T lymphocytes?

  • Macrophages
  • Natural killer cells
  • Dendritic cells (correct)
  • B cells

    • What is the main function of major histocompatibility complex (MHC) molecules?

  • To transport antigens to the nucleus
  • To display peptide antigens for T cell recognition (correct)
  • To initiate T cell apoptosis
  • To bind antibodies for antigen clearance

    • Which type of MHC molecule presents peptides derived from intracellular proteins?

  • Class II MHC
  • Class IV MHC
  • Class III MHC
  • Class I MHC (correct)

    • What mechanism do dendritic cells use to internalize extracellular antigens?

    <p>Receptor-mediated endocytosis</p> Signup and view all the answers

    In which cellular structures are extracellular proteins processed for display by class II MHC molecules?

    <p>Endosomes and lysosomes</p> Signup and view all the answers

    What process occurs after the internalization and digestion of antigens by antigen-presenting cells?

    <p>Binding of peptides to MHC molecules</p> Signup and view all the answers

    Which of the following best describes the property of MHC restriction?

    <p>T cells recognize peptides only when displayed by the same individual's MHC</p> Signup and view all the answers

    Which of the following cells are NOT considered antigen-presenting cells (APCs)?

    <p>Erythrocytes</p> Signup and view all the answers

    What is the primary purpose of CD4+ helper T cells in the immune response?

    <p>To produce antibodies</p> Signup and view all the answers

    What role do dendritic cells play in cross-presentation?

    <p>They ingest infected or dead cells and present their antigens</p> Signup and view all the answers

    What is a key characteristic of naive CD8+ T lymphocytes in relation to infected cells?

    <p>They can respond to antigens presented by dendritic cells without prior activation</p> Signup and view all the answers

    What is the significance of the proteasome in the process of cross-presentation?

    <p>It processes internalized antigens into peptide fragments</p> Signup and view all the answers

    What mechanism allows CD8+ cytotoxic T lymphocytes to eliminate infected cells?

    <p>By directly recognizing and killing infected host cells</p> Signup and view all the answers

    What is the primary role of MHC molecules in antigen display?

    <p>To ensure T cells recognize cell-associated protein antigens</p> Signup and view all the answers

    Which pathway is involved in the processing of cytosolic proteins for display by Class I MHC molecules?

    <p>Proteasome pathway</p> Signup and view all the answers

    What is required for the stabilization and transport of peptide-MHC complexes to the cell surface?

    <p>Correct fit of peptide to MHC molecule</p> Signup and view all the answers

    What type of antigens do B lymphocytes recognize?

    <p>Both proteins and non-protein antigens</p> Signup and view all the answers

    Which mechanism is involved in the immune response to a microbe circulating in the blood?

    <p>Antibodies binding and neutralizing the microbe</p> Signup and view all the answers

    What is an essential factor for T cell activation in response to antigen exposure?

    <p>Costimulators and cytokine signals</p> Signup and view all the answers

    From where can peptides that bind to Class I MHC molecules be derived?

    <p>Cytosolic proteins and phagocytosed microbes</p> Signup and view all the answers

    What is the role of follicular dendritic cells in the immune response?

    <p>To display antigens to B cells</p> Signup and view all the answers

    The activation of CD4+ helper T cells is solely responsible for the production of antibodies.

    <p>False</p> Signup and view all the answers

    Cross-presentation allows dendritic cells to present antigens from infected or dying host cells to CD8+ T lymphocytes.

    <p>True</p> Signup and view all the answers

    CD8+ T lymphocytes require continuous signals from dendritic cells to recognize and eliminate infected cells.

    <p>False</p> Signup and view all the answers

    Cross-presentation occurs exclusively through the major histocompatibility complex (MHC) class II pathway.

    <p>False</p> Signup and view all the answers

    CD8+ T lymphocytes are primarily activated by antigens found inside host cells.

    <p>True</p> Signup and view all the answers

    T lymphocytes can recognize peptide antigens regardless of the specific major histocompatibility complex (MHC) molecules present.

    <p>False</p> Signup and view all the answers

    Proteins in any nucleated cell are processed by class II MHC molecules and displayed for T cell recognition.

    <p>False</p> Signup and view all the answers

    Dendritic cells primarily use phagocytosis to internalize extracellular antigens for class II MHC processing.

    <p>True</p> Signup and view all the answers

    Peptides bound to class I MHC molecules are derived exclusively from extracellular proteins.

    <p>False</p> Signup and view all the answers

    In antigen-presenting cells, microbial proteins are processed in intracellular vesicles called endosomes.

    <p>True</p> Signup and view all the answers

    The end products of antigen processing in antigen-presenting cells are uniform in length and sequence.

    <p>False</p> Signup and view all the answers

    Class II MHC molecules present antigens only to CD8+ T lymphocytes.

    <p>False</p> Signup and view all the answers

    Antigen-preseiting cells must internally process antigens to bind them to MHC molecules for T cell activation.

    <p>True</p> Signup and view all the answers

    Peptide loading destabilizes class II MHC molecules before they are exported to the cell surface.

    <p>False</p> Signup and view all the answers

    Antigenic proteins can originate from non-infected host cells and their mutated genes.

    <p>True</p> Signup and view all the answers

    Binding of peptides to class II MHC molecules occurs in the cytosol.

    <p>False</p> Signup and view all the answers

    The immune system often reacts in the same way to a microbe at different stages of its life cycle.

    <p>False</p> Signup and view all the answers

    Cytokines secreted by activated APCs provide necessary signals for the stimulation of specific T cells.

    <p>True</p> Signup and view all the answers

    The transport of peptide-MHC complexes to the cell surface occurs only if the class I MHC molecule does not find a peptide with an appropriate fit.

    <p>False</p> Signup and view all the answers

    B lymphocytes can only recognize protein antigens and require them to be denatured to bind.

    <p>False</p> Signup and view all the answers

    Follicular dendritic cells play a crucial role in selecting high-affinity B cells during the humoral immune response.

    <p>True</p> Signup and view all the answers

    What is the significance of cross-presentation in the immune response?

    <p>Cross-presentation allows dendritic cells to present antigens from infected or dead cells to naive CD8+ T lymphocytes, facilitating their activation.</p> Signup and view all the answers

    How do CD8+ T lymphocytes differentiate into cytotoxic T lymphocytes (CTLs) without continuous signals from dendritic cells?

    <p>Once activated, CD8+ T lymphocytes differentiate into CTLs and can kill infected cells directly by recognizing specific antigens without needing further signals.</p> Signup and view all the answers

    What role does the proteasome play in the process of cross-presentation?

    <p>The proteasome processes internalized antigens into peptide fragments, which are then transported to the endoplasmic reticulum to bind with class I MHC molecules.</p> Signup and view all the answers

    In what way do tumors present a challenge for immune recognition by CD8+ T lymphocytes?

    <p>Tumors can arise from diverse cell types, making it difficult for naive CD8+ T lymphocytes to recognize and respond to the varied tumor antigens.</p> Signup and view all the answers

    Can naive CD8+ T lymphocytes respond to intracellular antigens if the infected cells do not interact with dendritic cells?

    <p>Yes, naive CD8+ T lymphocytes can respond to intracellular antigens through cross-presentation by dendritic cells that ingest infected or dying cells.</p> Signup and view all the answers

    What differentiates the processing pathways of antigens presented by class I and class II MHC molecules?

    <p>Class I MHC molecules present peptides derived from intracellular proteins processed in proteasomes, while class II MHC molecules present peptides from extracellular proteins processed in endosomes and lysosomes.</p> Signup and view all the answers

    Explain how dendritic cells play a role in the activation of naive T lymphocytes.

    <p>Dendritic cells capture and process antigens, displaying them on class II MHC molecules, which interact with naive T lymphocytes to initiate their clonal expansion.</p> Signup and view all the answers

    Describe the mechanism by which antigen-presenting cells (APCs) internalize extracellular antigens.

    <p>APCs internalize extracellular antigens primarily through phagocytosis and receptor-mediated endocytosis, allowing them to subsequently process these antigens for MHC presentation.</p> Signup and view all the answers

    Discuss the significance of MHC restriction in T cell recognition.

    <p>MHC restriction ensures that T cells can only recognize antigens that are presented by their own individual's MHC molecules, thus contributing to self-tolerance and immune specificity.</p> Signup and view all the answers

    How do proteolytic enzymes affect peptide generation in antigen processing?

    <p>Proteolytic enzymes break down internalized microbial proteins in APCs into peptides of various lengths, which can then bind to class II MHC molecules for T cell recognition.</p> Signup and view all the answers

    What role do intracellular vesicles, such as endosomes and lysosomes, play in antigen processing for class II MHC molecules?

    <p>These vesicles facilitate the digestion and processing of extracellular antigens, allowing for the generation of peptides that can bind to class II MHC molecules for presentation.</p> Signup and view all the answers

    Why is the interaction between T lymphocytes and MHC molecules crucial during the immune response?

    <p>The interaction enables T lymphocytes to recognize and respond to specific antigens presented by MHC molecules, facilitating effective immune activation and response.</p> Signup and view all the answers

    What happens to the peptides generated in the cytosolic processing pathway for class I MHC molecules?

    <p>Peptides generated in the cytosolic pathway are transported to the endoplasmic reticulum, where they bind to class I MHC molecules before being transported to the cell surface.</p> Signup and view all the answers

    What is the primary mechanism by which cytosolic proteins are recognized for antigen presentation by class I MHC molecules?

    <p>Cytosolic proteins are recognized following their proteolytic digestion by the proteasome pathway.</p> Signup and view all the answers

    Explain the role of the endoplasmic reticulum in the transport of peptide-MHC complexes.

    <p>The endoplasmic reticulum is crucial for the binding of peptides to class I MHC molecules before stabilizing these complexes for export to the cell surface.</p> Signup and view all the answers

    Discuss how B lymphocytes differ from T lymphocytes in their recognition of antigens.

    <p>B lymphocytes can recognize both protein and non-protein antigens in their native conformations, while T lymphocytes recognize only processed peptide fragments presented by MHC molecules.</p> Signup and view all the answers

    Describe the significance of second signals provided by APCs in T cell activation.

    <p>Second signals from antigen-presenting cells, such as costimulatory molecules and cytokines, are essential to ensure that T cells respond to microbial antigens and avoid activation by non-harmful substances.</p> Signup and view all the answers

    What distinguishes the responses of the immune system to the same microbe at different stages of its life?

    <p>The immune system tailors its responses based on the microbe's stage, facilitating different mechanisms such as antibody production when the microbe is in circulation versus cellular immunity when it is intracellular.</p> Signup and view all the answers

    What happens if a class I MHC molecule does not find a peptide with the correct fit?

    <p>If a class I MHC molecule does not find a compatible peptide, the peptide-MHC complex is not stabilized and will not be transported to the cell surface.</p> Signup and view all the answers

    What is the role of follicular dendritic cells in B cell activation?

    <p>Follicular dendritic cells present antigens to germinal center B cells, promoting the selection and expansion of high-affinity antibodies during the humoral immune response.</p> Signup and view all the answers

    How can antigens from non-infected host cells contribute to immune responses?

    <p>Antigens from non-infected host cells can originate from mutated genes, resulting in the production of altered proteins that are processed and presented by MHC molecules.</p> Signup and view all the answers

    The activation of CD4+ helper T cells is crucial for the production of potent ______.

    <p>antibodies</p> Signup and view all the answers

    Naive CD8+ T lymphocytes respond to intracellular antigens through a process called ______.

    <p>cross-presentation</p> Signup and view all the answers

    CD8+ cytotoxic T lymphocytes are necessary to kill infected host cells when the antigens are ______ from antibodies.

    <p>safe</p> Signup and view all the answers

    Dendritic cells process ingested antigens in structures called the ______, where they are degraded before presentation.

    <p>proteasome</p> Signup and view all the answers

    Once CD8+ T cells have differentiated into CTLs, they can kill infected or tumor ______ without additional signals.

    <p>cells</p> Signup and view all the answers

    T lymphocytes recognize peptide antigens that are bound to and displayed by major ______ complex (MHC) molecules.

    <p>histocompatibility</p> Signup and view all the answers

    Naive T lymphocytes must see the antigens presented by ______ cells to initiate clonal expansion.

    <p>dendritic</p> Signup and view all the answers

    Extracellular proteins processed by specialized APCs are displayed by class ______ MHC molecules.

    <p>II</p> Signup and view all the answers

    Proteins in the cytosol are processed in proteolytic structures called ______ and displayed by class I MHC molecules.

    <p>proteasomes</p> Signup and view all the answers

    Antigens destined for the class II MHC pathway are usually ______ from the extracellular environment.

    <p>internalized</p> Signup and view all the answers

    After internalization, microbial proteins enter acidic intracellular vesicles called ______ or phagosomes.

    <p>endosomes</p> Signup and view all the answers

    The binding of peptides to class II MHC molecules occurs after the ______ of antigens.

    <p>digestion</p> Signup and view all the answers

    Antigen-presenting cells are responsible for presenting antigens to ______ T lymphocytes.

    <p>naive</p> Signup and view all the answers

    Peptides bind to newly synthesized ______ molecules in specialized vesicles.

    <p>MHC</p> Signup and view all the answers

    The peptides that bind to class I MHC molecules are derived from cytosolic proteins following digestion by the ______ pathway.

    <p>proteasome</p> Signup and view all the answers

    If a class I molecule finds a peptide with the right fit, the complex is stabilized and transported to the cell ______.

    <p>surface</p> Signup and view all the answers

    B lymphocytes recognize proteins as well as ______ antigens, even in their native conformations.

    <p>non-protein</p> Signup and view all the answers

    The immune system reacts in different ways even to the same microbe at different ______ of its life.

    <p>stages</p> Signup and view all the answers

    Antigenic proteins may be produced in the cytoplasm from viruses that are living inside infected ______.

    <p>cells</p> Signup and view all the answers

    Follicular dendritic cells display antigens to germinal center B cells and select high-affinity ______ during humoral immune responses.

    <p>B cells</p> Signup and view all the answers

    Cytokines secreted by activated APCs provide necessary signals for the stimulation of specific ______.

    <p>T cells</p> Signup and view all the answers

    Match the following T cell types with their primary functions:

    <p>CD4+ helper T cells = Activate B cells and facilitate antibody production CD8+ cytotoxic T lymphocytes = Eliminate infected or tumor cells Naive CD8+ T lymphocytes = Respond to intracellular antigens Dendritic cells = Present antigens to T lymphocytes</p> Signup and view all the answers

    Match the following processes to their descriptions in T cell activation:

    <p>Cross-presentation = Dendritic cells display antigens from infected cells to CD8+ T cells Antigen processing = Proteins are broken down into peptide fragments for MHC binding T cell priming = Activation of naive T cells through antigen recognition Cytotoxic T cell differentiation = Naive CD8+ T cells mature into CTLs after activation</p> Signup and view all the answers

    Match the following components involved in immune response with their roles:

    <p>Class I MHC molecules = Present intracellular antigens to CD8+ T lymphocytes Proteasome = Processes intracellular proteins into peptide fragments Class II MHC molecules = Present extracellular antigens to CD4+ T lymphocytes Antigenic peptides = Bind to MHC molecules for T cell recognition</p> Signup and view all the answers

    Match the following immune challenges with their characteristics:

    <p>Infected cells = May not produce all signals for T cell activation Tumor cells = Arise from various cell types presenting diverse antigens Extracellular pathogens = Can be targeted by antibodies outside host cells Intracellular pathogens = Require CD8+ T cell response for elimination</p> Signup and view all the answers

    Match the following terms related to antigen presentation with their definitions:

    <p>Cross-presentation = Process where dendritic cells ingest and present antigens from other cells Antigenic peptide generation = Occurs within the proteasome during antigen processing Naive CD8+ T cell activation = Requires presentation of antigens by dendritic cells Cytosolic processing pathway = Involves breakdown of proteins for class I MHC molecule presentation</p> Signup and view all the answers

    Match the type of antigen-presenting cell (APC) with its primary function:

    <p>Dendritic cells = Initiating T cell activation Macrophages = Phagocytosis of pathogens B cells = Presentation of processed antigens Follicular dendritic cells = Selection of B cells</p> Signup and view all the answers

    Match the major histocompatibility complex (MHC) class with its antigen source:

    <p>Class I MHC = Peptides from proteasomes Class II MHC = Peptides from endosomes</p> Signup and view all the answers

    Match the process with its description in antigen processing:

    <p>Phagocytosis = Internalization of large particles Receptor-mediated endocytosis = Specific uptake via receptors Proteolytic cleavage = Breaking down proteins into peptides Peptide-MHC binding = Stabilization for cell surface transport</p> Signup and view all the answers

    Match the T cell type with its function:

    <p>CD4+ T cells = Helper T cells that activate B cells CD8+ T cells = Cytotoxic T cells that kill infected cells Naive T lymphocytes = Unactivated T cells awaiting signals Memory T cells = Long-lived cells ready for reactivation</p> Signup and view all the answers

    Match the pathway of antigen processing with the involved cell structure:

    <p>Class I pathway = Proteasomes Class II pathway = Endosomes/lysosomes Extracellular antigen processing = Acidic intracellular vesicles Cytosolic antigen processing = Cytosol of nucleated cells</p> Signup and view all the answers

    Match the type of T cell recognition with its characteristic:

    <p>MHC restriction = Peptide recognition specific to MHC type Clonal expansion = Proliferation of activated T cells Cytotoxic action = Destruction of infected cells Memory response = Long-term immunity through quicker activation</p> Signup and view all the answers

    Match the antigen processing step with its associated function:

    <p>Internalization = Uptake of external antigens Digestion = Breakdown of proteins into peptides Binding = Attachment of peptides to MHC molecules Presentation = Display of MHC-bound peptides on cell surface</p> Signup and view all the answers

    Match the antimicrobial response with its corresponding immune mechanism:

    <p>Phagocytosis = Engulfing and digesting pathogens Antibody production = B cell-mediated defense Cytotoxicity = Killing of infected cells by CD8+ T cells Cytokine signaling = Communication between immune cells</p> Signup and view all the answers

    Match the following processes with their associated pathways:

    <p>Proteolysis of cytosolic proteins = Cytosolic Pathway Peptide loading onto MHC molecules = Endocytic Pathway Transport of peptide-MHC complexes = Cytosolic Pathway Binding to class II MHC = Endocytic Pathway</p> Signup and view all the answers

    Match the following types of MHC molecules with their primary antigen sources:

    <p>Class I MHC = Infected host cell proteins Class II MHC = Antigens from endocytic vesicles</p> Signup and view all the answers

    Match the following immune responses with their characteristics:

    <p>Humoral immune response = Involves B lymphocytes Cell-mediated immune response = Involves CD8+ T lymphocytes Naive T cell activation = Requires antigen-presenting cells Antibody-mediated protection = Targets free microbes in circulation</p> Signup and view all the answers

    Match the following roles with the corresponding immune cells:

    <p>Dendritic cells = Activate naive T lymphocytes B lymphocytes = Produce antibodies APCs = Present antigens to T cells CD4+ T cells = Assist other immune cells</p> Signup and view all the answers

    Match the following antigen processing mechanisms with their corresponding pathways:

    <p>Infection of a cell = Cytosolic Pathway Endocytosis of extracellular antigens = Endocytic Pathway Transport to endoplasmic reticulum = Cytosolic Pathway Antigen processing in endosomes = Endocytic Pathway</p> Signup and view all the answers

    Match the following features with the correct MHC class:

    <p>Class I MHC = Associated with CD8+ T cells Class II MHC = Associated with CD4+ T cells</p> Signup and view all the answers

    Match the following cellular processes with their consequences for immune activation:

    <p>Peptide-MHC complex stability = Transport to cell surface Cytokine secretion from APCs = Stimulates T cell activation Binding of peptides to MHC = Formation of antigen-presenting complexes Cross-presentation = Activation of CD8+ T lymphocytes</p> Signup and view all the answers

    Match the following elements of the immune signaling with their respective function:

    <p>Costimulatory signals = Enhance T cell activation Cytokines = Provide specific signals during immune response APCs role in antigen display = Ensure T cells recognize microbial antigens High-affinity B cell selection = Occurs in the germinal center</p> Signup and view all the answers

    Study Notes

    Antigen Recognition by T Lymphocytes

    • T lymphocytes recognize peptide antigens presented by MHC molecules on antigen-presenting cells (APCs).
    • MHC restriction: T cells only recognize peptides when displayed by their own MHC molecules.
    • Naive T cells need to recognize antigens presented by dendritic cells to activate and differentiate into effector and memory cells.

    MHC Molecules in Antigen Presentation

    • MHC molecules are expressed on APCs and display peptides derived from protein antigens.
    • Class II MHC: presents peptides derived from extracellular proteins internalized by APCs (dendritic cells, macrophages, B cells).
    • Class I MHC: presents peptides derived from proteins in the cytosol of any nucleated cell.

    Processing & Presentation of Internalized Antigens (Class II MHC Pathway)

    • Internalization & Digestion: Antigens are internalized by APCs through phagocytosis, receptor-mediated endocytosis, and enter acidic vesicles (endosomes/phagosomes).
    • Peptide Binding: Peptides generated by proteolysis bind to newly synthesized MHC II molecules within specific vesicles.
    • Transport to Cell Surface: Peptide-MHC II complexes are transported to the cell surface.

    Processing & Presentation of Cytosolic Antigens (Class I MHC Pathway)

    • Proteolysis: Cytosolic proteins are degraded by the proteasome pathway.
    • Peptide Transport: Peptides are transported into the endoplasmic reticulum (ER).
    • Binding to Class I MHC: Peptides bind to class I MHC molecules within the ER.
    • Transport to Cell Surface: Stable peptide-MHC I complexes are transported to the cell surface.

    Summary of MHC-Mediated Antigen Presentation

    • MHC molecules ensure T cells only recognize cell-associated protein antigens.
    • MHC class II presents extracellular antigens, while class I presents intracellular antigens.
    • This ensures the appropriate type of T cell (helper or cytotoxic) responds to the specific microbe.

    Induction of Immune Responses

    • APCs express costimulators and secrete cytokines to activate specific T cells.
    • These additional signals ensure T cells respond to microbial antigens and not harmless substances.
    • B lymphocytes recognize diverse antigens, including non-protein antigens, in native conformations.
    • Follicular dendritic cells display antigens to germinal center B cells during humoral immune responses.

    The Need for Different Immune Responses

    • The immune system responds differently to the same microbe depending on its stage of infection.
    • Extracellular microbes: Combatted by antibodies that bind and neutralize the microbe.
    • Intracellular microbes: Activation of CD8+ cytotoxic T lymphocytes (CTLs) is necessary to kill infected cells and eliminate the reservoir.

    Cross-Presentation of Internalized Antigens to CD8+ T Cells

    • Cross-Presentation: Dendritic cells can ingest infected cells, dead tumor cells, and present their antigens to naive CD8+ T cells.
    • Mechanism: Ingested antigens are transported into the cytosol, processed by the proteasome, and presented by MHC class I molecules.
    • This allows CD8+ T cells to respond to intracellular antigens from cells that are not themselves APCs.
    • Once activated, CTLs can kill infected cells or tumor cells directly, without the need for further dendritic cell involvement.

    Antigen Recognition by T Lymphocytes

    • T lymphocytes recognize peptide antigens bound to and displayed by major histocompatibility complex (MHC) molecules on antigen-presenting cells (APCs).
    • T cells are MHC restricted, meaning they can only see peptides displayed by their own MHC molecules.
    • Dendritic cells are the main APCs responsible for initiating T cell activation and differentiation.

    Antigen Processing and Presentation Pathways

    • MHC Class II Pathway:
      • Processes extracellular antigens internalized by APCs like dendritic cells, macrophages, and B cells.
      • Occurs in late endosomes and lysosomes.
      • Peptides bind to newly synthesized MHC class II molecules within specialized vesicles.
      • Peptide-MHC class II complexes are transported to the cell surface for T cell recognition.
    • MHC Class I Pathway:
      • Processes cytosolic proteins, including viral proteins, proteins from phagocytosed microbes that escape into the cytosol, and mutated host proteins.
      • Proteins are digested by proteasomes.
      • Resulting peptides are transported into the endoplasmic reticulum (ER) and bind to class I MHC molecules.
      • Peptide-MHC class I complexes are transported to the cell surface for T cell recognition.

    Summary of Antigen Processing and Presentation

    • MHC molecules are crucial for T cell recognition of cell-associated protein antigens.
    • MHC class II presents extracellular antigens to CD4+ helper T cells.
    • MHC class I presents intracellular antigens to CD8+ cytotoxic T lymphocytes (CTLs).
    • APCs express costimulatory molecules and secrete cytokines to activate T cells in conjunction with antigen presentation.

    Induction of Immune Responses

    • Different immune mechanisms are required to combat microbes at different stages of their life cycle.
    • Antibodies are effective against extracellular microbes, while CTLs are needed to eliminate intracellular microbes.
    • CD4+ helper T cells are essential for antibody production.
    • CD8+ CTLs are essential for killing infected cells and eradicating intracellular microbes.

    Cross-Presentation of Internalized Antigens to CD8+ T Cells

    • Cross-presentation allows dendritic cells to process antigens from infected cells or tumors and present them to naive CD8+ T cells.
    • This process allows for activation of CD8+ T cells against viruses that do not directly infect dendritic cells or against tumor cells that cannot activate T cells directly.
    • Once activated, CTLs can directly kill infected cells or tumor cells without the need for dendritic cells.

    Antigen Recognition by T Lymphocytes

    • T lymphocytes recognize peptide antigens bound to Major Histocompatibility Complex (MHC) molecules on antigen-presenting cells (APCs)
    • Each individual has unique MHC molecules, so T cells can only recognize antigens presented by their own MHC molecules, a property called MHC restriction
    • Dendritic cells are vital for initiating T cell activation and differentiation

    Processing and Presentation of Protein Antigens

    • Two pathways exist for antigen processing: MHC class I and MHC class II
    • MHC class I presents intracellular antigens, while MHC class II presents extracellular antigens

    MHC class II Antigen Presentation Pathway

    • Extracellular antigens, such as microbes or proteins, are internalized by APCs like dendritic cells, macrophages, and B cells
    • Antigens are processed in late endosomes and lysosomes
    • Peptides generated from the processed antigens bind to newly synthesized MHC class II molecules in specialized vesicles
    • Peptide-MHC class II complexes travel to the cell surface for recognition by CD4+ T cells

    MHC class I Antigen Presentation Pathway

    • Cytosolic proteins, derived from viruses, bacteria, or mutated host genes, are processed by proteasomes
    • Peptides generated from proteasomal degradation are transported into the endoplasmic reticulum
    • Peptides bind to newly synthesized MHC class I molecules in the ER
    • Stable peptide-MHC class I complexes move to the cell surface for recognition by CD8+ T cells

    Induction of Immune Responses

    • MHC molecules ensure specific T cell recognition of cell-associated protein antigens
    • APCs express costimulators and secrete cytokines to activate T cells
    • These signals are crucial for a proper immune response to microbial antigens, preventing activation by harmless substances
    • B cells can recognize both proteins and non-protein antigens in their native form

    Cross-Presentation of Internalized Antigens to CD8+ T cells

    • Cross-presentation occurs when dendritic cells internalize infected cells, dead tumor cells, or microbial antigens
    • Internalized antigens are transported into the cytosol, processed by proteasomes, and presented via MHC class I molecules
    • Cross-presentation allows naive CD8+ T cells to recognize antigens from infected cells that may not directly activate APCs
    • This process is crucial for anti-viral and anti-tumor immune responses

    Antigen Recognition by T lymphocytes

    • T lymphocytes recognize peptide antigens that are bound and displayed by major histocompatibility complex (MHC) molecules of antigen-presenting cells (APCs).
    • This property of T cells is called MHC restriction.
    • Cells that capture microbial antigens and display them for recognition by T lymphocytes are called antigen-presenting cells (APCs).
    • Naive T lymphocytes must be exposed to antigens presented by dendritic cells to initiate clonal expansion and differentiation.

    Processing and Presentation of Protein Antigens

    • Extracellular proteins internalized by APCs (dendritic cells, macrophages, B cells) are processed in endosomes and lysosomes and displayed by class II MHC molecules.
    • Intracellular proteins are processed in proteasomes and displayed by class I MHC molecules.
    • Two pathways are designed to sample all proteins present in the extracellular and intracellular environments.

    Processing of Internalized Antigens for Display by Class II MHC Molecules

    • Antigens destined for the class II MHC pathway are usually internalized from the extracellular environment.
    • Dendritic cells and macrophages ingest microbes or microbial proteins by several mechanisms, including phagocytosis and receptor-mediated endocytosis.
    • After internalization, microbial proteins enter acidic vesicles, called endosomes or phagosomes, which fuse with lysosomes.
    • Proteolytic enzymes break down proteins in these vesicles, generating peptides of varying lengths and sequences.
    • Peptides bind to newly synthesized MHC molecules in specialized vesicles.
    • Peptide loading stabilizes class II MHC molecules, which are exported to the cell surface.

    Processing of Cytosolic Antigens for Display by Class I MHC Molecules

    • Peptides that bind to class I MHC molecules are derived from cytosolic proteins following digestion by the proteasome pathway.
    • Antigenic proteins may be produced in the cytoplasm from viruses, from phagocytosed microbes that leak into the cytosol, and from mutated or altered host genes.
    • Peptides must be transported into the endoplasmic reticulum to form peptide-MHC complexes.
    • If a class I molecule finds a peptide with the right fit, the complex is stabilized and transported to the cell surface.

    Cross-Presentation of Internalized Antigens to CD8+ T Cells

    • Some viruses may infect only specific cell types and may not produce the signals needed to initiate T cell activation.
    • How do naive CD8+ T lymphocytes respond to intracellular antigens of infected cells?
    • A subset of classical dendritic cells can ingest infected host cells, dead tumor cells, microbes, and microbial and tumor antigens.
    • These antigens are transported into the cytosol, where they are processed by the proteasome.
    • The generated antigenic peptides enter the ER and bind to class I molecules, which display the antigens for recognition by CD8+ T lymphocytes.
    • This process is called cross-presentation (or cross-priming).
    • Once CD8+ T cells differentiate into CTLs, they kill infected host cells or tumor cells without the need for dendritic cells or signals other than antigen recognition.

    Antigen Recognition by T Lymphocytes

    • T lymphocytes recognize peptide antigens presented by MHC molecules on antigen-presenting cells (APCs).
    • Different T cell clones recognize peptides only when presented by their own individual's MHC molecules, a phenomenon known as MHC restriction.
    • Naive T cells require interaction with dendritic cells to initiate clonal expansion and differentiation into effector and memory cells.

    Antigen Processing and Presentation

    • MHC molecules are expressed on APCs and display peptides derived from protein antigens.
    • Extracellular proteins are processed in late endosomes and lysosomes, presented by class II MHC molecules.
    • Intracellular proteins are processed in proteasomes and presented by class I MHC molecules.

    Processing of Internalized Antigens for Display by Class II MHC Molecules

    • Antigens destined for the class II MHC pathway are internalized from the extracellular environment.
    • Dendritic cells and macrophages internalize antigens via phagocytosis and receptor-mediated endocytosis.
    • Proteins are broken down into peptides in acidic vesicles like endosomes and phagosomes.

    Processing of Cytosolic Antigens for Display by Class I MHC Molecules

    • Peptides presented by class I MHC molecules are derived from cytosolic proteins processed by the proteasome pathway.
    • Antigenic proteins may originate from viruses, phagocytosed microbes, or mutated host genes.
    • Peptides are transported into the endoplasmic reticulum (ER) for binding to class I MHC molecules.

    Role of MHC Molecules in Antigen Display

    • MHC molecules ensure that T cells only recognize cell-associated protein antigens.
    • MHC molecules determine the type of T cell (helper or cytotoxic) that responds to a specific microbe.

    Induction of Immune Responses

    • APCs express costimulatory molecules and secrete cytokines to stimulate specific T cells in concert with antigens.
    • These second signals ensure T cell activation only occurs in response to microbial antigens.
    • B lymphocytes recognize proteins and non-protein antigens in their native conformations.

    Cross-Presentation of Internalized Antigens to CD8+ T Cells

    • Some viruses infect specific cell types and may not travel to lymph nodes or produce signals for naive CD8+ T cell activation.
    • Dendritic cells can ingest infected host cells, dead tumor cells, microbes, and antigens.
    • Cross-presentation occurs when dendritic cells transport ingested antigens into their cytosol, where they are processed by the proteasome.
    • Processed peptides enter the ER and bind to class I molecules, presenting them for CD8+ T cell recognition.
    • This process is known as cross-presentation (or cross-priming) and allows dendritic cells to prime naive CD8+ T cells against antigens from other infected or dying cells.
    • Once activated, CTLs can kill infected host cells or tumor cells without the need for dendritic cells or additional signals.

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    Antigen Processing and Presentation PDF

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    Test your knowledge on how T lymphocytes recognize antigens presented by MHC molecules. This quiz covers MHC restriction, the role of dendritic cells, and the pathways of antigen processing and presentation. Perfect for students studying immunology or related fields.

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