Lecture 04: The Complement System and Pathways in Immunology {MCQ 1]
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Questions and Answers

What is the primary role of C5a in the complement activation process?

  • Inhibits the complement cascade
  • Tags microbes for opsonization
  • Forms cylindrical pores in bacterial membranes
  • Acts as a powerful anaphylatoxin that recruits immune cells (correct)

    • How does C3b contribute to microbial cell targeting?

  • Covalently attaches to microbial cells via exposed -OH/NH2 groups (correct)
  • Inhibits the recruitment of C6, C7, and C8
  • Forms a cylindrical pore to release cytosolic contents
  • Binds to the inner membrane of bacterial cells to induce lysis

    • What prevents the complement system from attacking host cells?

  • The formation of C5 convertase exclusively in pathogens
  • A higher concentration of anaphylatoxins in self-cells
  • Presence of specific receptors that inhibit the complement cascade (correct)
  • The intrinsic activation of C3 convertase

    • Which statement describes the effect of the membrane attack complex (MAC) on bacteria?

    <p>It creates pores that lead to cytosol leakage and cell death. (C)</p> Signup and view all the answers

    What is the role of cleaved ‘a’ products in the complement system?

    <p>They bind to specific receptors on immune cells to attract and activate leukocytes. (A)</p> Signup and view all the answers

    What is the function of the C1 complex in the classical pathway of complement activation?

    <p>The C1 complex acts as a protease that cleaves C1s and C4, initiating the formation of the classical C3 convertase. (B)</p> Signup and view all the answers

    Which of the following statements accurately describes the role of mannose-binding lectin (MBL) in the lectin pathway of complement activation?

    <p>MBL acts as a receptor that recognizes mannose residues on microbial surfaces, initiating the pathway. (C)</p> Signup and view all the answers

    How does the alternative pathway of complement activation differ from the classical and lectin pathways?

    <p>Unlike the other pathways, the alternative pathway relies on the spontaneous hydrolysis of C3 to initiate activation. (D)</p> Signup and view all the answers

    What is the primary function of the C5 convertase in the terminal complement pathway?

    <p>It cleaves C5, generating C5a and C5b, which initiates the formation of the membrane attack complex (MAC). (A)</p> Signup and view all the answers

    Which of these components is NOT directly involved in the formation of the classical C3 convertase?

    <p>Factor B (D)</p> Signup and view all the answers

    Which of the following statements correctly describes the role of C3b in the complement system?

    <p>C3b acts as an opsonin that enhances phagocytosis of microbes by immune cells. (B)</p> Signup and view all the answers

    How does the C3 convertase function in the amplification loop of the alternative pathway?

    <p>It continuously cleaves C3 into C3a and C3b, generating more C3 convertase, amplifying the cascade. (A)</p> Signup and view all the answers

    What is the role of the membrane attack complex (MAC) in the complement system?

    <p>The MAC directly lyses microbial cells by creating pores in their membranes. (B)</p> Signup and view all the answers

    Flashcards

    Complement System

    A part of the innate immune system, found in blood plasma and produced by the liver.

    Functions of Complement

    Killing foreign cells, tagging materials, and proinflammatory signaling.

    C3 Convertase

    An enzyme complex formed from all three complement pathways, crucial for activating C3.

    Classical Pathway

    Starts with antibody binding, involving C1 complex activating C3 convertase via C4 and C2.

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    Lectin Pathway

    Initiated by mannose-binding lectin during inflammation, leading to C3 convertase formation.

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    Alternative Pathway

    Activated without antibodies, begins with hydrolyzed C3 and forms initial C3 convertase rapidly.

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    C5 Convertase

    A complex formed to cleave C5, crucial for terminal complement activity, generated from alternative pathway.

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    Membrane Attack Complex (MAC)

    A structure formed from complement components that leads to lysis of the target cell.

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    Anaphylatoxin C5a

    A fragment that recruits and activates immune cells during inflammation.

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    CD59 Function

    A receptor on mammalian cells that inhibits MAC formation to protect against self-damage.

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    Study Notes

    The Complement System

    • Part of the innate immune system, found in blood plasma, produced in the liver.
    • Three key roles: killing (lysis) of foreign cells, tagging foreign material (opsonization), and initiating pro-inflammatory signaling and chemotaxis.

    Complement Pathways

    • All pathways converge to create C3 convertase and the membrane attack complex (MAC).
    • Classical Pathway:
      • Begins with antibodies binding to antigens on a pathogen's surface.
      • C1 complex (C1q, C1r, C1s) is activated when two or more antibodies bind.
      • C1r activates C1s, and C1s cleaves C4 and C2.
      • C4b and C2a form the classical pathway C3 convertase (C4b2a).
      • Alternatively can bind directly on a bacteria surface (e.g., lipoteichoic acids).
    • Lectin Pathway:
      • Initiated by mannose-binding lectin (MBL) binding to repeating mannose molecules common on bacterial and fungal surfaces.
      • MBL is produced by the liver, especially during inflammation.
      • MBL-associated serine proteases (MASPs) cleave C4 and C2 to form the lectin pathway C3 convertase (C4b2a).
    • Alternative Pathway:
      • Does not require antibody or antigen.
      • C3 is spontaneously hydrolyzed to a reactive form, C3(H2O).
      • Factor B binds to C3(H2O), and Factor D cleaves Factor B, forming C3bBb (initial C3 convertase).
      • C3b binds to a microbial surface, further forming a C3 convertase which creates an amplification loop.

    Terminal Complement Pathway

    • C5 convertase (formed from the alternative, lectin, or classical pathways) cleaves C5 into C5a and C5b.

      • C5a is a powerful anaphylatoxin, recruiting and activating immune cells.
      • C5b initiates the formation of the membrane attack complex (MAC).

    • MAC formation:

      • C5b recruits C6, C7, C8, then C9 molecules.
      • C9 monomers polymerize to form a transmembrane pore in the microbial cell membrane.
      • This pore disrupts the microbial cell integrity, leading to lysis (bacterial death).

    • Complement Function:

      • Opsonization: C3b and C4b tag microbes, marking them for phagocytosis by immune cells.
      • Inflammation: Cleaved complement products (e.g., C3a, C5a) act as anaphylatoxins, increasing vascular permeability and chemoattracting immune cells.
      • Lysis: The MAC forms pores in bacterial cell membranes, leading to lysis and death.

    Complement Regulation

    • Complement inhibitors prevent complement activation on host cells.
    • Examples: CD59 prevents C9 polymerization on mammalian cells.
    • Deficiency in terminal complement components can lead to recurrent infections.

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    Description

    Explore the roles and mechanisms of the complement system, a vital part of the innate immune response. This quiz covers key pathways, including the classical and lectin pathways, detailing how they function to protect the body from pathogens. Test your understanding of these essential immunological processes.

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