Immune Response to Bacterial Infections
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Questions and Answers

What is one key function of lipopolysaccharide (LPS) in gram negative bacteria?

  • It serves as a component of the bacterial capsule.
  • It is a primary component of the teichoic acid in gram positive bacteria.
  • It helps the bacteria replicate within host cells.
  • It triggers inflammation and fever by activating phagocytic cells. (correct)

    • Which of the following best describes bacterial superantigens?

  • They are involved in the replication of bacterial DNA within host cells.
  • They are toxins that only affect gram negative bacteria.
  • They activate T cells and cause an exaggerated immune response. (correct)
  • They specifically bind to B cells and inhibit antibody production.

    • What is a major distinguishing feature of gram positive bacteria compared to gram negative bacteria?

  • Gram positive bacteria lack a cell wall.
  • Gram positive bacteria have an outer lipid membrane.
  • Gram positive bacteria have peptidoglycan cell walls with teichoic acids. (correct)
  • Gram positive bacteria do not produce endotoxins.

    • Which immune response is primarily triggered by the activation of B1 and mantle zone B cells?

    <p>IgM predominant response.</p> Signup and view all the answers

    How do bacterial capsules contribute to immune evasion?

    <p>By masking underlying antigenic structures.</p> Signup and view all the answers

    What role do MHC Class II proteins play in the immune response to Listeria monocytogenes?

    <p>They present antigens to Th1 cells.</p> Signup and view all the answers

    What is the primary consequence of bacterial superantigen activity?

    <p>Systemic immune dysregulation due to cytokine overproduction.</p> Signup and view all the answers

    During which phase does Listeria monocytogenes present antigens to Tc cells?

    <p>Cytosolic phase with MHC Class I.</p> Signup and view all the answers

    Which type of T cells can be activated by bacterial superantigens?

    <p>A broad range of CD4+ T cells, up to 20%.</p> Signup and view all the answers

    What is a potential severe outcome of extensive T cell activation by bacterial superantigens?

    <p>Toxic shock syndrome and systemic toxicity.</p> Signup and view all the answers

    What is the primary function of commensal organisms in the body?

    <p>To prevent colonization by pathogenic bacteria</p> Signup and view all the answers

    Which type of bacteria are resistant to lysis by the Membrane Attack Complex (MAC)?

    <p>Gram positive bacteria</p> Signup and view all the answers

    How do encapsulated bacteria evade phagocytosis?

    <p>By having a protective saccharide coating</p> Signup and view all the answers

    What characterizes a T-cell independent (TI) response?

    <p>It can occur with some bacterial polysaccharides</p> Signup and view all the answers

    Which immune mechanism is essential for the uptake of encapsulated bacteria?

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

    What is a characteristic action of Th1 cells in the immune response?

    <p>They mediate the inflammatory response</p> Signup and view all the answers

    Which immune response is primarily associated with antibody-mediated immunity to Listeria monocytogenes?

    <p>Antibody-mediated opsonization</p> Signup and view all the answers

    What distinguishes TI-2 antigens from TI-1 antigens?

    <p>TI-2 antigens do not have mitogenic activity</p> Signup and view all the answers

    Study Notes

    Immune Response to Infectious Diseases - Bacteria

    • The immune system uses a multifaceted approach to recognize and fight bacterial pathogens.
    • Bacterial superantigens are toxins that activate a large number of T cells, which is not specific to one type of bacteria
    • These superantigens work by simultaneously binding to the T cell receptor (TCR) and the major histocompatibility complex (MHC) class II molecules
    • Massive T cell activation causes the release of pro-inflammatory cytokines
    • These cytokines lead to systemic immune dysregulation and potentially severe conditions like toxic shock syndrome.

    Learning Objectives

    • Describe how the immune system recognizes and responds to bacterial pathogens.
    • Identify strategies bacterial pathogens employ to evade the immune system's detection and elimination.
    • Understand bacterial superantigens and the immunological consequences of their activation.

    Learning Aims and Objectives

    • The different immune responses are tailored to various pathogens (bacteria, viruses, parasites).
    • The body's response to pathogens and how pathogens exploit their abilities.

    Introduction

    • Innate and adaptive immunity (humoral and cell-mediated) are involved in the response to infection.
    • The extent of involvement depends on the infectious agent.
    • Three types of infectious agents: bacteria, viruses, and eukaryotic parasites.

    Bacteria

    • Prokaryotic cells that do not need a host's machinery to replicate.
    • Gram-positive and gram-negative bacteria.
    • All bacteria have a cell wall surrounding the plasma membrane, containing peptidoglycan.
    • Gram-negative bacteria have an additional outer membrane with lipopolysaccharide (LPS), anchored to the wall by lipoproteins.
    • LPS is an endotoxin, causing antigenic variation.

    Bacteria (cont'd)

    • LPS triggers inflammation and fever by activating phagocytic cells, also activating B1 and mantle zone B cells.
    • Gram-positive cells have a cell wall with peptidoglycan combined with teichoic acids, which are antigenic structures
    • Capsules are saccharide material, masking underlying antigenic structures.

    Immune Response to Bacterial Infection

    • Physical barriers (skin, mucosa)
    • Commensal organisms prevent colonization by pathogens.
    • Lysozyme, found in secretions, digests peptidoglycan (especially in Gram-positive bacteria).
    • Activated complement via classical and alternative pathways.
    • Phagocytosis (direct or mediated by opsonization with antibody or complement).
    • Antibodies (neutralization, opsonization, complement activation).
    • Th1 cells activate phagocytes and mediate the inflammatory response.

    Immune Response to Bacterial Infection (cont'd)

    • Immune responses depend on the specific bacterium.
    • Gram-positive bacteria resist lysis by the MAC.
    • Complement activation leads to destruction of Gram-positive bacteria through opsonization.
    • Encapsulated bacteria are resistant to direct phagocytosis, needing opsonization for uptake.

    Mechanisms of Evading Immune Detection

    • Capsule (e.g., Streptococcus pneumoniae): a saccharide coating protecting from phagocytosis and complement activation (MAC). Associated with virulence.
    • Alteration of antigenic structures (e.g., LPS in Gram-negative bacteria).
    • Localization within cells (e.g., Listeria monocytogenes).

    Antibody Responses to Bacterial Antigens

    • Most protein antigens elicit T-dependent responses.
    • Some bacterial polysaccharides, nucleic acids, and polymeric proteins, and LPS can stimulate B cells without T cell help (TI responses).
    • Two classes of TI antigens:
      • TI-1: Stimulate B cell division through TLR binding, leading to specific antibody response, predominantly IgM, early response, poor inducer of class switching.
      • TI-2: Do not have mitogenic activity but stimulate B1 and marginal zone B cells, cross-linking BCR, producing specific IgM response early in anti-bacterial response, important in response to encapsulated bacteria, triggering complement activation, opsonization, and eventual uptake and destruction.

    Intracellular Bacteria

    • Bacteria can be taken up by phagocytes, remaining within a phagosome or escaping into the cytosol.
    • Listeria monocytogenes: produces a pore-forming toxin to disintegrate the phagosome membrane allowing it to replicate within the cytosol.
    • During the initial extracellular phase, antibodies mediate immunity.
    • Inside the phagosome, Listeria antigens are presented to Th1 cells via MHC Class II proteins.
    • In the cytosolic phase, Listeria antigens are presented to Tc cells via MHC Class I proteins.

    Bacterial Superantigens

    • Stimulate T cell responses similar to allogeneic MHC response.
    • Recognised by T cells without processing into peptides and binding to MHC.
    • Bind to outer surface of Class II proteins and the Vβ region of many TCRs.
    • A superantigen can activate 2-20% of all CD4+ T cells, resulting in massive cytokine production and systemic toxicity.
    • Examples: Staphylococcal enterotoxins (food poisoning), S. aureus toxic shock syndrome toxin-1 (TSST-1).

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    Immune Response to Infectious Diseases - Bacteria PDF

    Description

    This quiz explores how the immune system recognizes and battles bacterial pathogens, focusing on the role of superantigens. Participants will learn about the multifaceted immune response, T cell activation, and the implications of cytokine release. Prepare to deepen your understanding of immunology and the challenges posed by bacterial infections.

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