Antibody Functions: Blocking, Fixation, Opsonization, ADCC

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Questions and Answers

Which of the following is the primary mechanism by which antibodies mediate Antibody-Dependent Cellular Cytotoxicity (ADCC)?

Directly inducing apoptosis in the target cell.
Activating the complement cascade, leading to cell lysis.
Recruiting Natural Killer (NK) cells to release cytotoxic granules. (correct)
Blocking the interaction between a pathogen receptor and its ligand.

How does the binding of IgM to a pathogen surface facilitate complement fixation?

IgM undergoes a conformational change to a 'staple form' allowing C1q binding. (correct)
IgM directly activates the serine protease C1s.
IgM recruits two IgG molecules to bind C1q.
IgM blocks the binding of other antibodies, ensuring complement activation.

Which characteristic of IgA contributes to its effectiveness in agglutination?

Its direct cytotoxic effect on microbial cells.
Its high affinity for Fcy receptors on phagocytes.
Its polymeric nature, allowing it to form lattices and trap pathogens. (correct)
Its ability to activate the complement cascade at mucosal surfaces.

What is the direct result of IgE cross-linking on mast cells following exposure to an allergen?

Activation and degranulation, releasing pro-inflammatory molecules. (C) Signup and view all the answers

How does IgG function as an opsonin to enhance phagocytosis?

By coating the pathogen and being recognized by Fcy receptors on phagocytes. (A) Signup and view all the answers

Which part of the antibody molecule is responsible for its specificity in binding to an antigen?

The variable region composed of variable (V), diversity (D), and joining (J) segments. (A) Signup and view all the answers

Why is blocking considered a function of antibodies?

Because antibodies can physically prevent a pathogen from binding to a host cell receptor. (C) Signup and view all the answers

In the context of complement fixation, what is the role of C1q after it binds to an antibody-antigen complex?

It activates the serine protease C1s, initiating the classical complement cascade. (B) Signup and view all the answers

Which of the following is NOT a typical location where polyclonal IgA can be secreted?

Serum. (A) Signup and view all the answers

What transport mechanism does IgA utilize to cross epithelial layers?

Transport via a secretory component that binds to the J chain. (B) Signup and view all the answers

Flashcards

Blocking (Antibodies)

Antibodies bind to a target to prevent it from binding to its receptor, neutralizing its effect.

Complement Fixation

IgG and IgM antibodies activate the complement system by binding to pathogens, leading to the activation of C1q and subsequent cascade.

Opsonisation

IgG acts as an opsonin to enhance phagocytosis by making pathogens more visible to macrophages and neutrophils.

ADCC

Antibody-dependent cellular cytotoxicity (ADCC) occurs when IgG-bound target cells are recognized by natural killer (NK) cells, leading to the release of cytotoxic granules and apoptosis.

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Agglutination

Polymeric antibodies like IgA form a lattice structure, disarming pathogens by clumping them together.

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Mast Cell Degranulation

Mast cell degranulation involves the release of proinflammatory molecules like histamine from mast cells when IgE cross-links on their surface, causing an allergic reaction.

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Antibody Variable Region

The tips of the antibody Y-shape that recognize and bind to antigens. The region is composed of Variable (V), Diversity (D) and Joining (J) segments.

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Study Notes

6 Functions of Antibodies

Blocking

Antibodies bind to a target, preventing it from binding to a receptor.

Complement Fixation

IgG and IgM antibodies can initiate complement fixation when bound to a surface.
Each bound immunoglobulin allows C1q to bind, which then activates the serine protease C1s.
When IgM binds to a pathogen's cell surface, it changes from a flat shape to a "staple form".
Two IgG molecules are needed to bind C1q.
The process initiates the classical complement cascade.

Opsonisation

IgG acts as an opsonin to enhance phagocytosis by macrophages and neutrophils.
IgG bound to the surface of a pathogen makes it more visible to macrophages/neutrophils for phagocytosis.
C3b also acts as an opsonin.
IgG is recognized by Fcγ receptors on phagocytes.

Antibody-Dependent Cellular Cytotoxicity (ADCC)

IgG bound to a target cell is recognized by Natural Killer cells via their Fcγ receptors.
Natural Killer cells secrete cytotoxic granules into the synapse, resulting in cell apoptosis.

Agglutination

Polymeric antibodies can form a lattice, disarming pathogens.
IgA is particularly effective, possessing four binding arms.
Polyclonal IgA can be secreted at mucosal sites like mucus, saliva, eyes, and tears.
IgA is transported across epithelia by a secretory component that binds to the J chain.
IgA can regulate microbial populations by coating and agglutinating bacteria.

Mast Cell Degranulation

Mast cells contain granules with proinflammatory molecules, including histamine.
Mast cells have receptors for IgE (Fcε receptors).
IgE bound to Fcε receptors cross-links upon allergen binding, causing mast cell activation and granule release, resulting in an allergic inflammatory reaction.

Antibody Variable Region

The variable region is located at the tips of the antibody Y-shape.
The variable region of the heavy chain consists of Variable (V), Diversity (D), and Joining (J) segments.