Immunology Quiz on Complement System Functions
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Questions and Answers

What is the primary function of C3b in the opsonization process?

  • It neutralizes toxins produced by pathogens.
  • It triggers the inflammation response.
  • It directly kills pathogens by lysis.
  • It enhances the binding of pathogens to phagocytes. (correct)

Which complement component is the most potent chemoattractant that attracts neutrophils to the site of infection?

  • C2b
  • C3a
  • C5a (correct)
  • C4a

What occurs during class switching of B cells?

  • B cells enhance opsonization ability.
  • B cells change from producing IgG to IgM.
  • B cells change the type of antibody produced to meet immune needs. (correct)
  • B cells stop producing antibodies.

How does the binding of C3a, C4a, and C5a to mast cells contribute to inflammation?

<p>By causing mast cell degranulation and histamine release. (C)</p> Signup and view all the answers

What happens if excess immune complexes are not cleared from circulation?

<p>They deposit in blood vessel walls and cause inflammation. (A)</p> Signup and view all the answers

What is the main role of the membrane attack complex (MAC)?

<p>Mediate lysis of pathogens (C)</p> Signup and view all the answers

Which pathway does not require antibodies for activation?

<p>Both B and C (B)</p> Signup and view all the answers

Which protein is involved in the lectin pathway that binds to mannose on the surface of pathogens?

<p>MBL (C)</p> Signup and view all the answers

Which of the following is a biological effect of complement activation?

<p>Opsonization of pathogen (D)</p> Signup and view all the answers

What components are unique to the classical pathway compared to the other pathways?

<p>C4 and C2 (A)</p> Signup and view all the answers

What triggers the cleavage of MASP in the lectin pathway?

<p>Binding of mannose to MBL (C)</p> Signup and view all the answers

Which factor is needed for the activation of the alternative pathway?

<p>Both A and B (A)</p> Signup and view all the answers

Which component is not required for the lectin pathway to function?

<p>C1 (A)</p> Signup and view all the answers

What is the primary function of complement proteins?

<p>To act as a cascade of enzymes for lysis of antigens (C)</p> Signup and view all the answers

What are the main proteins in the classical complement pathway?

<p>C1 to C9 (C)</p> Signup and view all the answers

What is unique about IgM in relation to complement fixation?

<p>IgM can fix complement more efficiently than IgG due to its pentamer structure (A)</p> Signup and view all the answers

How is complement inactivated?

<p>By incubating it at 56°C for 30 minutes (D)</p> Signup and view all the answers

What distinguishes the larger and smaller fragments of complement proteins after activation?

<p>The smaller fragment typically diffuses into circulation (A)</p> Signup and view all the answers

Which of the following is NOT a characteristic of complement proteins?

<p>Produced only in the spleen (A)</p> Signup and view all the answers

Which statement correctly describes the assembly of the complement pathway's C1 protein?

<p>C1 consists of three proteins: C1q, C1r, and C1s (C)</p> Signup and view all the answers

Which ion is necessary for the activation of C1 subunits in the classic pathway?

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

Which microbe-associated molecular pattern (MAMP) is specifically NOT found in the host?

<p>IgM antibodies (D)</p> Signup and view all the answers

What complex is formed by the cleavage of C4 and C2 in the classic pathway?

<p>C3 convertase (B)</p> Signup and view all the answers

Which component stabilizes the C3 convertase in the alternative pathway?

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

What triggers the activation of the alternative pathway?

<p>Presence of endotoxins (A)</p> Signup and view all the answers

What is the primary role of C5b in the classic pathway?

<p>Activate membrane attack complex (MAC) (A)</p> Signup and view all the answers

Which of the following statements about C3b is correct?

<p>C3b is involved in both classic and alternative pathways. (B)</p> Signup and view all the answers

What best describes the function of C5a in the context of inflammation?

<p>Acts as an inflammation mediator (D)</p> Signup and view all the answers

Which factor cleaves B factor into Ba and Bb in the alternative pathway?

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

Flashcards

Complement

A series of proteins present in serum that act as an enzyme cascade, produced mainly by the liver.

Complement in inactive form

The inactive form of complement proteins found in blood, lymph, and other body fluids.

Complement activation

The activation of complement proteins triggers a chain reaction that ultimately results in the lysis (destruction) of the antigen.

C1 to C9 proteins

The most important proteins of the complement system, designated by C1 to C9, contributing significantly to immune defense.

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C1 protein

The first protein in the classical pathway, consisting of three subunits: C1q, C1r, and C1s.

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C1q

The recognition subunit of the classic pathway that binds to the Fc region of antibodies (IgM and IgG) to trigger the complement cascade.

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Classic pathway activation

The classic pathway is initiated when C1 binds to the Fc portion of antibodies, particularly IgM and IgG, forming the first step in the complement cascade.

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IgM

A pentamer antibody with five Fc portions, making it more efficient than IgG in activating complement because a single IgM molecule can bind to C1q.

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What is C1s?

A key protein in the classical pathway that activates C1r, which in turn activates C1s, leading to the cleavage of C4 and C2, initiating the cascade.

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What is C3 convertase?

The complex formed when C4b and C2a bind together; it plays a crucial role in activating C3, a vital step in the complement cascade.

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What is C5 convertase?

The complex formed when C3b binds to C4b2a, creating a potent enzyme that activates C5, another crucial step for the complement cascade.

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What is the Membrane Attack Complex (MAC)?

The end product of the complement cascade, formed by the assembly of proteins C5b, C6, C7, C8, and C9 on the surface of a target cell.

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What is the alternative pathway?

A pathway of complement activation triggered directly by certain components of pathogens, like LPS, without the need for antibodies.

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What is Properdin?

A protein in the alternative pathway that binds to C3b, stabilizing the C3 convertase complex.

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What is Factor B?

A crucial protein in the alternative pathway that is cleaved by factor D, leading to the formation of the C3 convertase complex.

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What is Factor D?

A protein that cleaves factor B in the alternative pathway, initiating the formation of the C3 convertase complex.

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Mannose-Binding Protein (MBP)

A serum protein that binds to mannose, a sugar commonly found on bacterial and fungal surfaces. When MBP binds to these surfaces, it activates the cleavage of MASP proteins, initiating the lectin pathway.

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Mannose-Binding Protein Associated Serine Proteases (MASP)

A group of serine proteases associated with MBP. They become activated when MBP binds to mannose on pathogen surfaces, initiating the lectin pathway by cleaving C4 and C2.

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Lectin Pathway

A pathway of complement activation triggered by the binding of MBP to mannose on pathogen surfaces. Unlike the classical pathway, it doesn't require antibodies.

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Membrane Attack Complex (MAC)

A complex of proteins that forms on the surface of pathogens and ultimately punches holes in their membranes, leading to lysis.

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Common Final Pathway

The final part of the complement pathway, shared by all three pathways (classical, alternative, and lectin), where the formation of MAC occurs.

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Opsonization

The process by which complement proteins, particularly C3b and C4b, coat the surface of pathogens, making them more easily recognizable and phagocytized by immune cells.

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C3a and C5a

Key complement components that contribute to inflammation by attracting immune cells to the site of infection. These components can also trigger other inflammatory responses, such as vasodilation and increased vascular permeability.

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Lysis of pathogen

A process where complement proteins create pores in the membrane of pathogens, leading to their rupture and death.

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Chemotaxis by complement

Complement proteins, especially C5a, attract neutrophils to the site of infection to fight off pathogens.

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Neutralization of virus and toxin by C3b

A process where C3b blocks the binding site of a virus or toxin, neutralizing its harmful effects.

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Enhancement of antibody production

Complement proteins enhance antibody production by activating B cells. This involves C3b binding to B cell receptors, triggering their proliferation and the ability to switch to different antibody types.

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Study Notes

Complement System

  • The complement system is a series of serum proteins that act as an enzyme cascade, produced by the liver.
  • It's present in all body fluids except cerebrospinal fluid (CSF) and urine.
  • It exists in an inactive form, becoming active upon activation, then leading to lysis of antigens (Ag).
  • The complement system has more than 23 proteins, ranging from C1 to C9.
  • A high concentration of complement proteins is found in guinea pig serum, making it suitable for in vitro experiments related to complement activation.
  • Complement proteins are thermolabile, meaning they are inactivated at 56°C for 30 minutes or even fewer hours at room temperature.
  • Complement proteins are designated by capital letter C followed by Arabic numbers (e.g., C1, C2).
  • Each protein, after activation, has two subunits (a and b).
  • The smaller fragment is designated 'a'.
  • The larger fragment is designated 'b', except for C2a, which is larger than C2b.
  • Larger fragments bind to functional complexes while smaller fragments diffuse into the circulatory system.

Complement Pathways

  • Complement activation occurs through three pathways: the classical pathway, the lectin pathway, and the alternative pathway.
  • Classical pathway: Triggered by antigen-antibody complexes (IgM and IgG).
    • C1 binds to the Fc portion of the antibody.
    • C1 activates C1r and C1s (esterase activity).
    • C1s cleaves C4 and C2.
    • C4b and C2a form C3 convertase (C4b2a).
    • C3 convertase cleaves C3 into C3a and C3b.
    • C3b joins C4b2a to form C5 convertase (C4b2a3b).
    • C5 convertase cleaves C5 into C5a and C5b.
    • C5b initiates the formation of the membrane attack complex (MAC).
  • Alternative pathway: Activated by microbial components like lipopolysaccharide (LPS) and endotoxins, and is part of the innate immune system.
    • No antibody (Ab) is required.
    • C3 undergoes spontaneous hydrolysis to form C3(H2O).
    • Factor B binds to C3(H2O).
    • Factor D cleaves Factor B into Ba and Bb.
    • C3b, Bb, and properdin (factor P) form C3 convertase (C3bBb).
    • C3 convertase further cleaves C3 and forms C5 convertase (C3bBb3b) which cleaves C5 into C5a and C5b.
    • C5b initiates the formation of the MAC.
  • Lectin pathway: Activated by mannose-binding lectin (MBL) binding to mannose residues on microbial surfaces.
    • MBL binds to pathogen surfaces.
    • MBL-associated serine proteases (MASPs) cleave C4 and C2.
    • This forms the C3 convertase, similar to the classical pathway.

Complement Activity Regulation

  • Complement activation is tightly regulated by proteins.
    • Proteins on cell surfaces, such as sialic acid, facilitate the inactivation of complement components.
    • Control proteins such as CD59, inhibits MAC formation.
    • Serum proteins like C1 inhibitor, Factor H, and Factor I regulate complement activation.

Consequences of Complement Activation

  • Lysis of pathogens: MAC formation causes lysis of cells by forming pores in their membranes.
  • Opsonization: C3b and C4b enhance phagocytosis by binding to pathogens.
  • Inflammation: C3a, C4a, and C5a act as chemoattractants, attracting neutrophils.
  • Neutralization of viruses and toxins: C3b contributes to this process.
  • Clearance of immune complexes: Immune complexes are removed from the circulation.
  • Enhancement of antibody production: C3b binds to complement receptors on B cells leading to B cell proliferation and class switching.

Complement Deficiencies

  • Deficiencies in complement components can lead to recurrent infections, immune complex diseases, and other issues.

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Description

Test your knowledge on the complement system, including the functions of various complement components and their roles in opsonization, inflammation, and B cell class switching. This quiz covers key processes and pathways within the immune response.

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