BLOCK 3: IMM: (3.8) B-CELL EFFECTOR FUNCTION

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Questions and Answers

Which Fc receptor specifically binds to IgG?

FcγRI (correct)

Fcε

FcμRI

FcαRI

What role do Fc receptors play in the immune response?

They enable the recognition of antibodies on leukocytes. (correct)

They neutralize toxins directly.

They trigger inflammation independently from antibodies.

They facilitate the secretion of antibodies.

What is the impact of antibody binding through Fc receptors on phagocytosis?

It enhances the ability of phagocytes to engulf pathogens. (correct)

It solely activates T cells.

It inhibits the activation of phagocytes.

It has no effect on phagocytosis.

Which immune cells use Fc receptors to interact with infected cells?

Natural Killer cells Signup and view all the answers

How are Fc receptors named in relation to the immunoglobulin molecules they bind?

They are named using Greek letters that correspond to each immunoglobulin type. Signup and view all the answers

What is the primary function of IgG antibodies in relation to bacterial toxins?

Neutralizing microbial toxins Signup and view all the answers

What happens when antibodies are absent in a strep naïve child?

Bacterial population increases uncontrollably Signup and view all the answers

What is the role of opsonization in the immune response?

To coat bacteria for enhanced phagocytosis Signup and view all the answers

How do high-affinity neutralizing antibodies prevent toxins from affecting human cells?

By covering binding sites on the toxins Signup and view all the answers

What part of an antibody is recognized by Fc receptors?

Fc fragment Signup and view all the answers

Which mechanism is NOT associated with the function of IgA antibodies?

Triggering cell lysis Signup and view all the answers

What is the effect of protein F on bacterial attachment in the absence of antibodies?

It enables effective attachment to host cells Signup and view all the answers

In what way do neutralizing IgG antibodies function to protect cells?

By blocking the entry of toxins into cells Signup and view all the answers

What is the role of Fcγ receptors in the immune response?

Facilitating the binding of IgG to pathogens for uptake. Signup and view all the answers

Which statement accurately describes the binding preferences of FcγRI?

It binds to IgG in the absence of antigen, especially IgG3. Signup and view all the answers

What triggers the uptake and degradation of antibody-coated pathogens by phagocytes?

Signals from Fc receptors coupled with antigen recognition. Signup and view all the answers

Which of the following receptors is involved in NK cell-mediated immunity?

FcγRIII, recognizing IgG-coated tumor cells. Signup and view all the answers

What is the main effect of signaling from Fc receptors on phagocytes?

Enhancement of phagocytosis and bacterial destruction. Signup and view all the answers

What mechanism does the Fc receptor-mediated action work alongside?

Complement receptors recognizing C3 fragments. Signup and view all the answers

What characteristic differentiates the Fc receptor for monomeric IgA from Fc receptors for IgG and IgE?

It belongs to a different family despite structural similarities. Signup and view all the answers

How do Nk cells ultimately eliminate tumor cells?

Through the secretion of lytic granules inducing apoptosis. Signup and view all the answers

What establishes stable binding between an NK cell and its target tumor cell?

Interactions between IgG and FcγRIII molecules. Signup and view all the answers

Which receptor is responsible for high-affinity binding of IgG1 and IgG3?

FcγRI Signup and view all the answers

Study Notes

Fc Receptors

Fc receptors are proteins found on the surface of leukocytes that recognize antibodies.
They contribute to the immune system's protection by binding to antibodies attached to infected cells or pathogens.
Different Fc receptors bind to different immunoglobulin isotypes and subtypes, e.g., FcγRI binds IgG, and Fcε binds IgE.
Fc receptors for IgG and IgE are structurally related and encoded by closely linked genes.
The Fc receptor for monomeric IgA belongs to a different family than Fc receptors for IgG and IgE, despite having a similar structure.

Fc Receptors and Antibody Effector Functions

Fcγ receptors enable effector cells to bind to IgG and be activated by IgG bound to pathogens.
The IgG-binding function of FcγRI is a property of its three extracellular immunoglobulin-like domains of the receptor's α chain.
The signaling function of FcγRI is a property of its γ chain, which forms a homodimer.
This enables IgG bound to FcγRI at the surfaces of macrophages, dendritic cells, and neutrophils to trap pathogens and target them for uptake and degradation.
Of the Fc receptors for IgG, only FcγRI binds to IgG in the absence of antigen.

Phagocytosis and Fc Receptors

Phagocytic cells have Fc receptors that bind to the Fc regions of antibodies that are attached to pathogens.
This tethers the bacterium to the phagocyte's surface and enhances phagocytosis and subsequent destruction of the bacterium in lysosomes.
This provides a mechanism for eliminating pathogens that have been opsonized with a coating of specific antibody.
This mechanism works together with complement receptors (CR1 & CR2) which recognize C3 fragments attached to pathogens after activation of the classical pathway of complement activation.

Fc Receptors on NK Cells

Fc receptors act as antigen receptors on Natural Killer (NK) cells.
When therapeutic antibodies bind to the CD20 antigen on tumor cells, FcγRIII receptors on NK cells bind to the Fc regions of the bound IgG.
This establishes stable binding between the NK cell and its target, which activates the NK cell to form a conjugate pair and synapse with the tumor cell.
The NK cell then secretes the contents of its lytic granules on the tumor cell surface, condemning the tumor cell to die by apoptosis.

Neutralization of Microbial Toxins and Animal Venoms

High-affinity IgG and IgA antibodies neutralize microbial toxins and animal venoms.
Many common diseases are caused by bacterial toxins; neutralization of toxins by IgG antibodies protects cells from toxin action.
Protein toxins produced by many bacteria have two functional models:
- One binds to a component at the surface of a human cell, allowing it to be internalized.
- The other is a poison that interferes with a vital function of the cell.
High-affinity neutralizing IgG antibodies cover up the binding site in the toxin's first module, preventing its attachment to human cells.

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Description

This quiz covers the role and structure of Fc receptors in the immune system, focusing on their interaction with antibodies. Learn about the various types of Fc receptors, their specific immunoglobulin targets, and their functions in immune responses against pathogens.