Immunology Quiz: Complement System and Antibodies

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Questions and Answers

What is the primary function of the Membrane Attack Complex (MAC)?

Enhance antibody production

Increase phagocytosis of pathogens

Stimulate inflammation

Perforate membranes of target cells (correct)

Which antibody class is characterized as pentameric?

IgM (correct)

IgA

IgG

IgE

What role do splenic macrophages play in immune complex (IC) clearance?

Transport ICs to lymph nodes

Produce additional antibodies

Bind to the Fc portion of antibodies in ICs (correct)

Activate the complement system directly

What is the composition of the Fab region of an antibody?

Two heavy chains and two light chains Signup and view all the answers

How are polymeric immunoglobulins such as IgA and IgM held together?

By the J chain Signup and view all the answers

Which type of bacteria is more susceptible to lysis by the Membrane Attack Complex (MAC)?

Gram negative bacteria Signup and view all the answers

What is one of the primary functions of the complement system?

Opsonization and phagocytosis Signup and view all the answers

What structure does the Membrane Attack Complex (MAC) specifically perforate?

Plasma membrane Signup and view all the answers

Which statement accurately describes the relationship between Gram negative bacteria and the Membrane Attack Complex?

Gram negative bacteria are more readily lysed by MAC. Signup and view all the answers

What advantage does opsonization provide to the immune system?

Facilitates pathogen recognition and uptake Signup and view all the answers

Which of the following functions is considered the most important function of the complement system?

Opsonization and phagocytosis Signup and view all the answers

What is the effect of opsonins on the immune response?

They enhance the effectiveness of antibodies and complements. Signup and view all the answers

Which process is NOT directly facilitated by the complement system?

Toxin production Signup and view all the answers

What is the primary role of C3b in complement activation?

To target bacteria for phagocytosis. Signup and view all the answers

Which of the following opsonins directly enhances phagocytosis of pathogens?

All of the above Signup and view all the answers

Which receptor binds specifically to C3b during the opsonization process?

CR1 Signup and view all the answers

Why are encapsulated pathogens typically resistant to phagocytosis?

Their capsules inhibit opsonization. Signup and view all the answers

What does the term 'fixation' of complement refer to?

The activation of complement proteins. Signup and view all the answers

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

1 molecule of C1q and 2 molecules of C1r and C1s Signup and view all the answers

Which immunoglobulin is considered the most efficient at activating the classical complement pathway?

IgM Signup and view all the answers

What is required for C1q to activate the classical pathway?

Binding to two Fc regions Signup and view all the answers

In terms of pathway activation, what is the similarity between the Lectin and Classical pathways?

They follow the same sequence after initial recognition Signup and view all the answers

What initiates the formation of the Membrane Attack Complex (MAC)?

Binding of C5b to C6 Signup and view all the answers

What is the role of anaphylatoxins like C5a and C3a in the immune response?

They activate and attract leukocytes Signup and view all the answers

Which of the following molecules forms a cylindrical pore in membranes during the MAC formation?

C9 Signup and view all the answers

Which event marks the final step in the formation of the Membrane Attack Complex?

C5b678 binding to C9 Signup and view all the answers

What role does C5 convertase play in the complement cascade?

It cleaves C5 into C5a and C5b. Signup and view all the answers

Which component initiates the assembly of the Membrane Attack Complex (MAC)?

C5b Signup and view all the answers

What attracts neutrophils during the complement cascade?

C5a Signup and view all the answers

What does the lectin pathway of complement activation use to detect pathogens?

Soluble receptors Signup and view all the answers

Which serine proteases are involved in the cleavage of C4 and C2 in the lectin pathway?

MASPs Signup and view all the answers

How is C3 convertase generated in the complement system?

Through the deposition of C3b on pathogen surfaces. Signup and view all the answers

Which pathway of complement activation is triggered by the binding of C1 to antibodies?

Classical pathway Signup and view all the answers

Which component of the complement system is more directly involved in the opsonization of pathogens?

C3b Signup and view all the answers

What is the primary outcome of a C3 deficiency in the complement system?

Increased susceptibility to Neisserial infections Signup and view all the answers

Which receptor functions as a B cell coreceptor?

Complement Receptor 2 (CR2) Signup and view all the answers

What is a key function of the C3a receptor in the complement system?

Vascular permeability and mast cell degranulation Signup and view all the answers

Which receptors are involved in phagocytosis and have a role in cell adhesion?

Complement Receptor 1 (CR1) and Complement Receptor 4 (CR4) Signup and view all the answers

What secondary condition can arise from deficiencies in late complement components?

Increased susceptibility to Neisserial infections Signup and view all the answers

What function do complement control proteins serve?

Regulating complement activation pathways Signup and view all the answers

Which of the following is NOT a role of complement split products?

Phagocytosing foreign particles Signup and view all the answers

Which of the following complement receptors is known for its role in leukocyte adhesion?

Complement Receptor 3 (CR3) Signup and view all the answers

Study Notes

Complement System Overview

The complement system comprises over 20 plasma and cell surface proteins interacting with each other.
Complement components are typically denoted by uppercase "C" followed by a number (e.g., C1, C2, C3).
Complement activation is triggered by pattern recognition during inflammation and trauma.
Sequential proteolytic activation of zymogens (inactive precursors) generates active "split products."
Split products are identified by suffixes (e.g., C3a and C3b are split products of C3).

Learning Objectives

Differentiate the three complement activation pathways.
Explain the biological properties of complement activation split products.
Describe the regulation of complement activation and its products in host resistance to infection, inflammation, and immunopathology.
Discuss the effects of deficiencies and defects in complement regulation.

Introduction to the Complement System

The complement system is composed of over 20 proteins interacting; these proteins are crucial for the immune system.
Complement components are numbered sequentially with uppercase "C" as a prefix (e.g., C1, C2, C3).
Complement is activated by pattern recognition during infection or injury.
Proteolytic activation of inactive precursors generates active "split products".
These split products are denoted with suffixes (e.g., C3a and C3b are split products of C3).

Functions of the Complement System

Identifies pathogens as a signal to the immune system.
C3 activation leads to the generation of C3a and the opsonin C3b.
Recruits phagocytes (C5a) and activates mast cells (C3a/C5a).
Kills pathogens by creating pores in their membranes (C5b6789(n)).

Complement Pathways

Alternative pathway—spontaneous activation (continuously active).
Lectin pathway—triggered by plasma lectins binding to microbial surface carbohydrates.
Classical pathway—activated by antibodies (IgG or IgM) bound to antigens.

Overview of Complement Functions

Membrane Attack Complex (MAC): Gram-negative bacteria are more susceptible to lysis by MAC than Gram-positive bacteria.
Opsonization and Phagocytosis: The coating of pathogens with complement factors enhances phagocytosis.
Inflammation: Complement split products promote inflammation/recruitment of immune cells.

Antibody Structure Review

Fab region – binds antigen.
Fc region – binds complement and Fc receptors on cells.
Heavy chains (2) - composed of CH1, hinge, CH2, and CH3 domains.
Light chains (2) - composed of VL and CL domains.
Antigen binding sites contain hypervariable regions.
Hinge regions confer flexibility.

Antibody Classes

Monomeric: IgD, IgE, and IgG.
Dimeric: Secretory IgA (sIgA).
Pentameric: IgM (5 Fc regions).
Polymeric (IgA and IgM) held together by the J chain.

Immune Complex Formation

Immune complexes (Ab-Ag) form when antibodies bind to antigens.
Complement can be activated by immune complexes.
Immune complexes bind to complement receptors on RBCs and are transported to the spleen and liver.
Splenic macrophages and Kupffer cells remove immune complexes from RBCs.

Immune Complex Elimination

Immune Complexes (ICs) bind to complement receptors (CR1) on RBCs and are transported to the spleen and liver.
Splenic macrophages and Kupffer cells bind to Fc receptors (FcRs) of ICs.
ICs are removed from RBC surfaces via phagocytosis.

Regulation of Complement Activation

Properdin (Factor P): stabilizes C3 convertase.
Factor H: inactivates C3b by cleaving it to iC3b.
Decay-accelerating factor (DAF): disrupts C3 convertase (C3bBb).
Properdin (Factor I): inactivates C3b, C4b.
Membrane cofactor protein (MCP): promotes the inactivation of C3b and C4b.
CD59 prevents the formation of the membrane attack complex (MAC).

Pathologic Effects of Complement

Tissue damage resulting from excessive complement activation, in bacterial infections.
Intravascular thrombosis leading to ischemic injury in tissues.
Formation of immune complexes in autoimmune diseases (e.g., systemic lupus erythematosus, vasculitis, glomerulonephritis).
Pathological effects of complement deficiencies or defects in regulation (e.g., increased susceptibility to recurrent infections, hereditary angioedema).

Pathological effects of Complement Deficiencies

C3 deficiency: most severe; leads to increased susceptibility to recurrent bacterial infections.
Deficiencies in terminal complement components (C5-C9) result in insufficient MAC formation and higher risk of infections by Neisseria.
Deficiency in factors D and P (alternative pathways) leads to higher risk of Neisseria infections.

Hereditary Angioedema (HAE)

Deficiency in C1-inhibitor leads to unregulated bradykinin production.
Results in edema and vasodilation/inflammation.

Paroxysmal Nocturnal Hemoglobinuria (PNH)

Genetic defect in the PIGA gene affecting GPI-anchored proteins.
Deficiency in complement regulators (CD55, CD59) on RBCs leads to lysis and hemolytic anemia.

Complement Receptors

Complement receptor 1 (CR1): binds to C3b, C4b and facilitates phagocytosis and clearance of immune complexes.
Complement receptor 2 (CR2): B cell co-receptor; binds iC3b, and is crucial for the immune response.
Complement receptor 3 (CR3: MAC-1): binds iC3b and other ligands promoting phagocytosis and leukocyte adhesion.
Complement receptor 4 (CR4): similar functions to CR3.
C3a receptor: chemotaxis and mast cell degranulation.
C5a receptor: chemotaxis and mast cell degranulation.

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Description

Test your knowledge on the complement system and antibody functions with this quiz. Explore the roles of the Membrane Attack Complex, immunoglobulins, and the immune response dynamics. Ideal for students studying immunology or related fields.