Complement in Immunology

Questions and Answers

What is the primary site of complement synthesis in the body?

Lungs

Liver (correct)

Pancreas

Kidneys

Which of the following statements about the complement system is true?

Complement plays a role in both innate and adaptive immune responses. (correct)

Complement is not involved in inflammatory responses.

Complement proteins are synthesized in the brain.

Complement is only active in the blood.

Which pathway is NOT a method of complement activation?

Classical pathway

Transcription pathway (correct)

Lectin pathway

Alternative pathway

The complement system is primarily located in which of the following?

Serum and tissue fluids Signup and view all the answers

Which of the following is a component of the adaptive immune response that activates complement?

Antigen-antibody complex Signup and view all the answers

Which pathway of complement activation is dependent on antibodies?

Classical Pathway Signup and view all the answers

Which of the following pathways does not rely on antibodies for its initiation?

Alternative Pathway Signup and view all the answers

What is generated as a result of C3 activation in complement pathways?

C5 convertase Signup and view all the answers

Which pathway is directly involved in lytic attacks?

LYTIC ATTACK PATHWAY Signup and view all the answers

Which statement is true regarding the pathways of complement activation?

The Lectin Pathway is antibody independent. Signup and view all the answers

Which of the following is a benefit provided by opsonization?

Enhancement of phagocytosis Signup and view all the answers

What is one of the detrimental effects of an immune response?

Inflammation Signup and view all the answers

Which of the following statements is true regarding phagocyte function?

Phagocyte attraction and activation enhance immune responses Signup and view all the answers

What is the role of lysis in the immune system?

Destroying bacteria and infected cells Signup and view all the answers

Which option does not represent a benefit of the immune response?

Anaphylaxis Signup and view all the answers

What initiates the classical pathway of the immune response?

Formation of antigen-antibody complex (Ag-Ab) Signup and view all the answers

Which components make up the C1 complex in the classical pathway?

C1q, C1s, and C1r Signup and view all the answers

What is the primary product of the classical pathway after the activation of C1?

C4bC2a complex Signup and view all the answers

Which antibodies are mentioned as part of the activation of the classical pathway?

IgM and IgG Signup and view all the answers

What happens when the Ag-Ab complex binds to C1?

C1 activates C4 and C2 Signup and view all the answers

What is the primary function of the complement protein C3?

It plays a central role in all three pathways and is unstable. Signup and view all the answers

How many inactive complement protein molecules are present in the bloodstream?

30 Signup and view all the answers

Which pathways are involved in the activation of the complement system?

Classical pathway, Alternative pathway, and Lectin pathway. Signup and view all the answers

Where does the activation of the complement system primarily occur?

On the surface of microbes. Signup and view all the answers

What characteristic of complement protein C3 contributes to its role in the complement system?

It is rapidly unstable and broken down. Signup and view all the answers

What type of immunity is associated with the classical pathway?

Acquired immunity Signup and view all the answers

Which component is NOT involved in the alternate pathway of complement activation?

C1 Signup and view all the answers

What initiates the alternate pathway in the complement system?

Microbial surface structures Signup and view all the answers

Which complement protein is excluded in the alternate pathway?

C1 Signup and view all the answers

Which statement accurately compares the classical and alternate pathways?

The classical pathway is initiated by antigen-antibody complexes. Signup and view all the answers

What role does C1q play in the classical pathway?

It acts as the recognition element. Signup and view all the answers

Which complement components are involved in the activation stage of the classical pathway?

C4, C2, C3, C5, C6, C7, C8, C9 Signup and view all the answers

What occurs during the membrane attack stage (MAC) of the classical pathway?

C5b, C6, C7, C8, and C9 induce cell lysis. Signup and view all the answers

In which sequence does the activation stage of the classical pathway occur?

C4, C2, C3, C5, C6, C7, C8, C9 Signup and view all the answers

Which of the following is NOT a component of the membrane attack complex?

C5a Signup and view all the answers

What activates the alternate pathway of the immune response?

Lipopolysaccharide (LPS) from bacteria Signup and view all the answers

Which component does NOT bypass the alternate pathway activation?

C3 Signup and view all the answers

What is the role of C3b in the context of the alternate pathway?

Initiates the formation of C3 Convertases Signup and view all the answers

What occurs in healthy conditions concerning C3 cleavage?

C3b recycles to form another C3 Signup and view all the answers

Which of the following describes the nature of immunity provided by the alternate pathway?

Non-specific resistance to various pathogens Signup and view all the answers

What is the composition of the membrane attack complex (MAC)?

C5b, C6, C7, C8, C9 Signup and view all the answers

What is the primary function of the membrane attack complex (MAC) in the immune response?

Directly lyse target cells Signup and view all the answers

In the diagrams of the membrane attack complex (MAC), what does the purple protein tube represent?

The formation of the MAC Signup and view all the answers

How does the MAC initiate damage to the target cell?

By forming pores in the membrane Signup and view all the answers

What role does the fluid in the right diagram, representing the blue fluid, play in relation to the MAC?

It signifies the lytic effect of the MAC Signup and view all the answers

What initiates the lectin pathway of complement activation?

Attachment of mannose-binding lectin to mannose residues Signup and view all the answers

How does mannose-binding lectin (MBL) relate to the classical pathway?

It is structurally similar to a component of C1 in the classical pathway. Signup and view all the answers

What is a characteristic of the lectin pathway in terms of antibody dependence?

It does not depend on antibodies for activation. Signup and view all the answers

What is the outcome after mannose-binding lectin (MBL) binds to mannose on microbes?

Activation of the lytic pathway Signup and view all the answers

Which statement is true regarding the sequence of the lectin pathway?

It involves the same sequence steps as in the classical pathway. Signup and view all the answers

Study Notes

Complement System Overview

Complement consists of proteins essential for host defense.
Found in serum and all tissue fluids except urine and cerebrospinal fluid (CSF).
Synthesized primarily in the liver.
Supports immune and inflammatory responses, involving both innate and adaptive immune pathways.

Activation Pathways

Complement activation is triggered by:
- Antigen-antibody complexes (adaptive immune response).
- Endotoxins or polysaccharide capsules (innate immune response).
Three pathways for activation:
- Classical pathway: antibody-dependent.
- Alternative pathway: antibody-independent.
- Lectin pathway: antibody-independent.

Key Components and Functions

Circulates as 30 inactive protein molecules in the bloodstream.
C3 is the most abundant complement protein, central to all pathways, but is unstable and rapidly breaks down.

Host Benefits of Complement Activation

Enhances phagocytosis via opsonization.
Attracts and activates phagocytes.
Causes lysis of bacteria and infected cells.
Regulates antibody responses.
Clears immune complexes.

Host Detriments of Complement Activation

May lead to inflammation.
Potential for anaphylaxis.

Detailed Pathway Functions

Classical Pathway

Activated through antigen-antibody complexes.
C1 (made of C1q, C1s, and C1r) binds to the Fc region of IgM or IgG.
Sequence includes:
- Recognition Stage: C1q identifies and binds to antibodies.
- Activation Stage: Involves C4, C2, C3, C5, C6, C7, C8, and C9 in order.
- Membrane Attack Stage (MAC): C5b, C6, C7, C8, and C9 lead to cell lysis.

Alternative Pathway

Activated without antibodies, often by lipopolysaccharides (LPS) from Gram-negative bacteria.
Supports non-specific resistance.
C3 cleaves into C3a and C3b; C3b binds to microbial surfaces to create stable C3 convertases (C3bBb).

Lectin Pathway

Activated by mannose-binding lectin (MBL) or mannose-binding protein (MBP), targeting mannose on microbial surfaces.
Activation completes the same sequence as the classical pathway.

Terminal Steps (Lytic Pathway)

All three pathways converge at C5b, C6, C7, C8, and C9 to form the membrane attack complex (MAC).
MAC causes lysis of target cells by forming pores in their membranes.

Comparison of Classical and Alternative Pathways

Classical Pathway:
- Linked to acquired immunity.
- Initiated by antigen-antibody complexes.
- Involves all complement proteins.
Alternative Pathway:
- Part of innate immunity.
- Initiated by microbial surface structures.
- Does not involve C1, C4, or C2, but utilizes Properdin (FP).

Regulation of the Complement System

Regulation is crucial to avoid excessive activation and potential tissue damage.

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Description

This quiz covers essential aspects of the complement system, including its definition, synthesis, site, and function in immune responses. Explore how complement proteins contribute to both innate and adaptive immunity and their role in inflammation. Test your knowledge on these critical components of host defense.