IMM: BLOCK 2: WEEK 6 REVIEW

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Questions and Answers

What is the primary role of cross-presentation by antigen-presenting cells?

To process and present extracellular antigens with MHC class I molecules to CD8 T cells. (correct)
To present extracellular antigens to CD8 T cells using MHC class II molecules.
To recognize and eliminate infected host cells.
To enhance phagocytosis of extracellular pathogens.

Which of the following best describes the antigen receptors of the adaptive immune system?

BCRs are membrane-bound monomeric immunoglobulins, while TCRs are heterodimers. (correct)
B-cell receptors (BCRs) are dimers, while T-cell receptors (TCRs) are monomers.
TCRs consist solely of Igα and Igβ heterodimers present on B cells.
Both BCRs and TCRs are secreted proteins found in the bloodstream.

What are the components of the T-cell receptor (TCR) complex primarily composed of?

One α chain and one β chain, with associated Igα and Igβ molecules. (correct)
A single polypeptide chain combined with multiple immunoglobulin domains.
Two heterodimers formed by α and β chains.
Two identical α chains and one β chain.

What distinguishes αβ T-cell receptors from γδ T-cell receptors?

αβ TCRs are formed from one α chain and one β chain, while γδ TCRs consist of one γ chain and one δ chain. (D) Signup and view all the answers

What is the significance of dendritic cell maturation in the context of the adaptive immune response?

It enhances the ability of dendritic cells to trigger a response in naïve T cells. (B) Signup and view all the answers

Which of the following best distinguishes phagocytosis from macropinocytosis?

Phagocytosis is selective, while macropinocytosis is non-selective. (B) Signup and view all the answers

What is the primary purpose of antigen processing and presentation?

To present processed peptide fragments to T cells through MHC molecules. (C) Signup and view all the answers

Which statement accurately describes the steps of MHC I processing and presentation?

Peptide fragments from cytosolic proteins are transported into the endoplasmic reticulum for loading onto MHC I. (A) Signup and view all the answers

How do MHC II molecules primarily acquire peptide fragments for presentation?

Through the uptake and degradation of extracellular proteins via phagocytosis or macropinocytosis. (D) Signup and view all the answers

What role does the invariant chain play in MHC II processing?

It helps in the proper folding and stability of MHC II molecules until peptide loading. (B) Signup and view all the answers

What is a key feature of dendritic cell maturation for effective antigen presentation?

Mature dendritic cells increase their surface expression of co-stimulatory molecules. (C) Signup and view all the answers

What is the significance of cross-presentation in antigen processing?

It enables dendritic cells to present extracellular antigens on MHC I molecules. (D) Signup and view all the answers

In which situation would a peptide derived from an intracellular pathogen be loaded onto MHC II molecules?

If the dendritic cell utilizes the cross-presentation pathway. (D) Signup and view all the answers

What is the primary role of plasma cells that migrate to the bone marrow?

To secrete pathogen-specific antibodies in larger quantities (B) Signup and view all the answers

How do MHC class I molecules differ from MHC class II molecules in their peptide transport mechanisms?

MHC class I molecules are loaded with peptides in the cytosol, while MHC class II molecules receive peptides from endocytic vesicles. (A) Signup and view all the answers

Which of the following correctly describes the peptide-binding process for MHC class II?

Peptides from extracellular fluids are processed in lysosomes and then loaded onto MHC class II. (C) Signup and view all the answers

Which of the following proteins is primarily involved in the assembly and peptide loading onto MHC class I molecules?

Calnexin (B) Signup and view all the answers

What is the importance of cross-presentation in the immune system?

It enhances the presentation of extracellular antigens by MHC class I molecules. (B) Signup and view all the answers

Which statement best compares the structures of MHC class I and MHC class II molecules?

MHC class I has a β2 microglobulin component, whereas MHC class II has both α and β chains. (C) Signup and view all the answers

What is a distinguishing feature of dendritic cell maturation?

Dendritic cells undergo maturation after encountering pathogens, enhancing their antigen-presenting capabilities. (B) Signup and view all the answers

In which compartment are proteins degraded into peptides for loading onto MHC class I molecules?

Cytosol (A) Signup and view all the answers

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Study Notes

Adaptive Immune Receptors

B cell receptors (BCRs) are monomeric immunoglobulins associated with disulfide-linked heterodimers called Igα and Igβ; located on the surface of B cells
T cell receptors (TCRs) are heterodimers composed of either an αβ or a γδ chain pair; located on the surface of T cells

Immunoglobulin Structure

Immunoglobulins (antibodies, BCRs, TCRs) have a basic structure comprising two heavy chains and two light chains linked by disulfide bonds.

T Cell Receptor Complex

The TCR complex consists of the TCR, CD3 complex (CD3γ, CD3δ, CD3ε, and CD3ζ chains), and the ζζ homodimer
TCR complex facilitates signaling upon antigen recognition
TCR recognizes and binds to antigen presented by MHC molecules

T Cell Receptor Gene Rearrangement

TCR genes undergo variable, diversity, and joining (VDJ) recombination to generate diverse TCR repertoires
Rearrangement of gene segments leads to the formation of unique α and β chains, contributing to antigen specificity

αβ vs. γδ TCRs

αβ TCRs recognize peptide antigens presented by MHC molecules
γδ TCRs recognize a wider range of antigens, including non-peptide antigens and microbial stress signals

Antigen Processing and Presentation

Antigen processing and presentation is the mechanism by which cells present fragments of antigens to T cells
MHC class I presents intracellular antigens to CD8+ T cells, while MHC class II presents extracellular antigens to CD4+ T cells

MHC Class I Processing and Presentation

MHC I molecules are loaded with peptides derived from intracellular proteins in the endoplasmic reticulum (ER)
Peptides are transported into the ER lumen via the transporter associated with antigen processing (TAP)
Peptide loading complex facilitates peptide binding to MHC I molecules

MHC Class II Processing and Presentation

MHC II molecules are loaded with peptides from exogenous antigens captured via endocytosis
Invariant chain blocks peptide binding within the ER
Invariant chain is degraded within vesicles, leaving a fragment called CLIP, which is replaced by peptides

MHC Class I and Class II Differences

MHC I is expressed on almost all nucleated cells, while MHC II is mainly expressed on antigen presenting cells (APCs)
MHC I presents peptides from intracellular proteins, while MHC II presents peptides from extracellular proteins

Cross-Presentation

Cross-presentation: the ability of certain cells, primarily dendritic cells, to present extracellular antigens on MHC class I molecules to CD8 T cells
This enables the activation of cytotoxic T cells against extracellular pathogens that would not normally access the MHC class I processing pathway

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