Q5. The primary immune function of mucus is to:
A. Directly kill microbes by phagocytosis

B. Serve as a physical and chemical barrier that traps microbes and limits contact with epithelium

C. Allow microbes to colonize epithelium safely

D. Replace epithelial cells during damage

B. Serve as a physical and chemical barrier that traps microbes and limits contact with epithelium

Q6. Glycan decoys on mucins:
A. Enhance pathogen invasion

B. Act as binding sites that prevent microbial adhesion to host cells

C. Induce apoptosis in epithelial cells

D. Inhibit mucus secretion

B. Act as binding sites that prevent microbial adhesion to host cells

Q7. Commensal microorganisms:
A. Always cause disease
B. Live on mucosal surfaces without causing harm and often benefit the host
C. Are sterile environments in the body
D. Only exist in germ-free animals

B. Live on mucosal surfaces without causing harm and often benefit the host

Q8. Gnotobiotic mice differ from conventional mice because they:
A. Have no immune system
B. Are exposed to all environmental microbes
C. Lack microbiota-driven immune stimulation
D. Contain more Peyer’s patches

C. Lack microbiota-driven immune stimulation

Q9. Which of the following is an anatomical change in germ-free mice?
A. Thickened mucosal epithelium
B. Larger Peyer’s patches
C. Reduced goblet cells and thinner mucus
D. Increased villus length

C. Reduced goblet cells and thinner mucus

Q10. The microbiome contributes to host health by:
A. Reducing vitamin synthesis
B. Suppressing mucus production
C. Competing with pathogens and supporting epithelial integrity
D. Destroying regulatory T cells

C. Competing with pathogens and supporting epithelial integrity

Q11. Which of the following is not a symbiotic function of the microbiome?
A. Detoxifying xenobiotics
B. Inducing immune tolerance
C. Inhibiting epithelial repair
D. Producing short-chain fatty acids

C. Inhibiting epithelial repair

Q12. Which of the following is an example of GALT?
A. Peyer’s patches
B. Spleen
C. Thymus
D. Bone marrow

A. Peyer’s patches

Q13. The effector compartment of the mucosa primarily contains:
A. Sites for antigen sampling
B. Activated lymphocytes and IgA-producing plasma cells
C. Naïve lymphocytes
D. Bone marrow stem cells

B. Activated lymphocytes and IgA-producing plasma cells

Q14. Which of the following is true of mucosal immunity compared to systemic immunity?
A. Relies primarily on IgG antibodies
B. Promotes tolerance and is dominated by secretory IgA
C. Allows strong inflammatory responses
D. Has unrestricted complement activation

B. Promotes tolerance and is dominated by secretory IgA

Q15. Mucosal immune responses are typically:
A. Highly inflammatory
B. Complement-dependent
C. Anti-inflammatory and tolerance-oriented
D. Systemic in distribution

C. Anti-inflammatory and tolerance-oriented

Q16. Crohn’s disease involves:
A. Autoantibody destruction of red blood cells
B. Dysregulated immune response to gut microbiota
C. Viral infection of Peyer’s patches
D. Deficiency in mucus production only

B. Dysregulated immune response to gut microbiota

Q17. Intestinal epithelial cells (IECs) contribute to immunity by:
A. Blocking cytokine production
B. Serving as a barrier, producing antimicrobial peptides, and communicating with immune cells
C. Producing IgA directly
D. Lacking pattern recognition receptors

B. Serving as a barrier, producing antimicrobial peptides, and communicating with immune cells

Q18. Which of the following is not a PRR found in intestinal epithelial cells?
A. TLR4
B. NOD2
C. RIG-I
D. IgA receptor

D. IgA receptor

Q19. Activation of PRRs on IECs results in:
A. Reduced antimicrobial activity
B. Induction of cytokines, chemokines, and antimicrobial peptides
C. Suppression of epithelial repair
D. Inhibition of tight junction formation

B. Induction of cytokines, chemokines, and antimicrobial peptides

Q20. Overactivation of NFκB in the gut can lead to:
A. Enhanced tolerance
B. Chronic inflammation (as in IBD)
C. Increased IgA secretion
D. Immunodeficiency

B. Chronic inflammation (as in IBD)

Q21. Which epithelial cell type produces mucins?
A. Paneth cells
B. Goblet cells
C. Enterocytes
D. Tuft cells

B. Goblet cells

Q22. Follicle-associated epithelium (FAE) differs from normal epithelium because it:
A. Contains more goblet cells
B. Has a thicker mucus layer
C. Contains M cells that facilitate antigen uptake
D. Lacks immune cells

C. Contains M cells that facilitate antigen uptake

Q23. M cells function by:
A. Producing defensins
B. Transcytosing antigens from the lumen to immune cells
C. Secreting IgA
D. Strengthening tight junctions

B. Transcytosing antigens from the lumen to immune cells

Q24. Oral tolerance prevents:
A. Tolerance to commensals
B. Immune responses to dietary antigens
C. Antibody production
D. Mucus secretion

B. Immune responses to dietary antigens

Q25. CD103⁺ DCs promote gut homeostasis by:
A. Inducing Th1 cells
B. Producing retinoic acid and TGFβ to generate Tregs and IgA
C. Activating NK cells
D. Enhancing inflammation

B. Producing retinoic acid and TGFβ to generate Tregs and IgA

Q26. During infection, CD103⁺ DCs primarily:
A. Induce Tregs
B. Promote effector T cell differentiation and IgG responses
C. Reduce co-stimulation
D. Decrease cytokine production

B. Promote effector T cell differentiation and IgG responses

Q27. Which molecule pair directs lymphocytes back to mucosal tissues?
A. CD28 – CD80
B. α4β7 – MAdCAM-1
C. CD40 – CD40L
D. CCR5 – CCL5

B. α4β7 – MAdCAM-1

Q28. Intraepithelial lymphocytes (IELs) are mainly:
A. Naïve CD4⁺ T cells
B. CD8⁺ or γδ T cells between epithelial cells that provide rapid cytotoxic defense
C. Circulating B cells
D. NK cells only

B. CD8⁺ or γδ T cells between epithelial cells that provide rapid cytotoxic defense

Q29. The first wave of mucosal IgA response is:
A. T-dependent, high-affinity
B. T-independent, lower-affinity and rapid
C. IgG-mediated
D. Complement-dependent

B. T-independent, lower-affinity and rapid

Q30. The poly-Ig receptor (pIgR):
A. Degrades IgA before secretion
B. Transports dimeric IgA across epithelial cells
C. Replaces the Fc receptor
D. Produces cytokines

B. Transports dimeric IgA across epithelial cells

Q31. The secretory component of IgA:
A. Is a microbial enzyme
B. Protects IgA from degradation and anchors it to mucus
C. Signals epithelial apoptosis
D. Recruits macrophages

B. Protects IgA from degradation and anchors it to mucus

Q32. Secretory IgA maintains mucosal tolerance primarily by:
A. Activating complement
B. Inducing inflammation
C. Neutralizing antigens non-inflammatorily and preventing epithelial penetration
D. Enhancing neutrophil recruitment

C. Neutralizing antigens non-inflammatorily and preventing epithelial penetration

Q33. Which statement about IgA1 and IgA2 is correct?
A. IgA1 is resistant to bacterial proteases
B. IgA2 predominates in serum
C. IgA2 is more abundant in the colon and resistant to proteases
D. IgA1 is shorter and found mainly in the colon

C. IgA2 is more abundant in the colon and resistant to proteases

Q34. A major consequence of selective IgA deficiency is:
A. Reduced systemic inflammation
B. Increased risk of mucosal infections
C. Decreased allergy risk
D. Increased complement activation

B. Increased risk of mucosal infections

Q35. Some IgA-deficient patients experience transfusion reactions due to:
A. Formation of anti-IgA antibodies
B. Lack of complement proteins
C. Low IgG
D. Excessive mucus production

A. Formation of anti-IgA antibodies

Q36. A hallmark antibody of mucosal immunity is:
A. IgG
B. IgA
C. IgE
D. IgM

B. IgA

Q37. The mucosal immune system is distinct because it:
A. Encourages strong complement activation
B. Prioritizes tolerance over inflammation
C. Has minimal epithelial participation
D. Lacks antigen sampling

B. Prioritizes tolerance over inflammation

Q38. Which statement best summarizes the mucosal immune strategy?
A. Destroy microbes with inflammation
B. Maintain barrier function while promoting immune tolerance
C. Use complement and neutrophil activation
D. Avoid microbial recognition

B. Maintain barrier function while promoting immune tolerance

Explain why mucosal tissues must maintain a balance between immune defense and tolerance.

Mucosal tissues must balance defense and tolerance because they’re constantly exposed to harmless antigens (food, commensals). Too much activation → chronic inflammation; too little → infection risk.

Describe how the structure of mucosal tissues (large surface area, mucus production, epithelial barriers) supports their immune functions.

Large surface area (villi, microvilli) maximizes absorption and antigen exposure; mucus, tight junctions, and epithelial barriers physically block pathogens while allowing selective permeability.

A patient with a genetic defect that reduces mucin glycosylation suffers frequent gut infections.

Poorly glycosylated mucins can’t form a proper gel layer or trap microbes, weakening the mucus barrier and exposing epithelium to pathogens.

Why is mucus described as both a physical and biochemical barrier to infection?

Mucus is a physical barrier (traps microbes) and a biochemical barrier (contains defensins, IgA, lysozymes, and lactoferrin that neutralize microbes).

Germ-free mice have fewer Peyer’s patches and lower IgA production.
What does this tell us about the role of the microbiome in immune system development?

The microbiome stimulates lymphoid tissue development (e.g., Peyer’s patches) and IgA production; germ-free mice lack this stimulation, showing microbiota’s key role in immune maturation.

If antibiotics drastically reduce gut microbiota, what potential immunological and physiological effects might you expect, and why?

The microbiome stimulates lymphoid tissue development (e.g., Peyer’s patches) and IgA production; germ-free mice lack this stimulation, showing microbiota’s key role in immune maturation

Compare the roles of IgA and IgG in terms of inflammation and tissue protection.

Reduced microbiota → weaker mucosal immunity, less IgA, reduced antimicrobial peptides, poor nutrient absorption, and higher infection/inflammation risk (e.g., C. difficile).

Explain why strong inflammatory responses that are beneficial in systemic tissues can be harmful in mucosal tissues.

In mucosa, inflammation can damage delicate epithelial surfaces and disrupt nutrient absorption; hence mucosal immunity relies on non-inflammatory mechanisms like IgA.

Crohn’s disease involves inappropriate immune activation against commensal microbes.

Defective PRRs or mucus production allow commensals to contact immune cells → excessive cytokine release → chronic inflammation (hallmark of Crohn’s disease).

How do intestinal epithelial cells act as both physical and immunological barriers in the gut?

Epithelial cells form tight junctions to block pathogens and express PRRs to detect microbes and release cytokines/antimicrobial peptides — combining barrier and immune functions.

Describe how PRR activation on intestinal epithelial cells leads to antimicrobial peptide production, and why this response must be short-lived.

PRR binding activates NFκB → induces defensins and cytokines; this must be short-lived to avoid chronic inflammation that damages tissue.

What could happen if NFκB signaling in gut epithelial cells were chronically active?

Chronic NFκB activation → persistent cytokine release → tissue injury and inflammation, contributing to conditions like ulcerative colitis or Crohn’s disease.

Explain the difference between Paneth cells, goblet cells, and M cells in terms of their immune contributions.

• Paneth cells: secrete defensins and lysozyme (antimicrobial).
• Goblet cells: secrete mucins to form mucus barrier.
• M cells: transcytose antigens from lumen to immune cells in Peyer’s patches.

The follicle-associated epithelium has fewer goblet cells.
Why might this structural difference be advantageous for immune surveillance?

Fewer goblet cells → less mucus covering → easier antigen sampling by M cells and dendritic cells.

Summarize how M cells and dendritic cells cooperate to initiate immune responses in Peyer’s patches

M cells capture luminal antigens → deliver to dendritic cells in Peyer’s patches → dendritic cells present antigens to T/B cells, initiating adaptive responses.

In the ovalbumin feeding experiment, animals fed antigen before injection had weaker systemic immune responses.
What principle does this illustrate, and why is it important for dietary tolerance?

Feeding antigen before exposure induces oral tolerance, where the immune system becomes non-responsive to harmless dietary antigens — preventing unnecessary inflammation.

Describe how CD103⁺ DCs maintain mucosal tolerance in the absence of infection.
Which molecules do they use, and what lymphocyte types do they influence?

CD103⁺ DCs release IL-10 and TGF-β, converting naïve T cells into regulatory T cells (Tregs) → promote IgA class switching and tolerance to commensals.

How does the immune function of CD103⁺ DCs change during infection, and why is this switch beneficial?

During infection, these DCs upregulate IL-6 and IL-12, promoting Th1/Th17 responses to fight pathogens — a protective switch from tolerance to defense.

Explain how α4β7 and MAdCAM-1 interactions direct lymphocytes to return to mucosal tissues.
Why is this type of homing important for long-term mucosal immunity?

α4β7 integrin on lymphocytes binds MAdCAM-1 on gut endothelium → directs cells back to mucosal tissues for site-specific immunity.
Ensures IgA-secreting plasma cells return to mucosa.

Differentiate between the first wave and second wave of mucosal IgA responses.
Include whether they are T-dependent or T-independent and the type of IgA they produce.

• First wave: T-independent, polyclonal, mainly IgA1; limited diversity.
• Second wave: T-dependent, antigen-specific, mostly IgA2; provides long-term targeted protection.

Outline the process by which dimeric IgA is transported across epithelial cells to the lumen.
What is the role of the poly-Ig receptor (pIgR) and the secretory component?

Plasma cells secrete dimeric IgA → binds pIgR on epithelial cells → transported to lumen → secretory component (part of pIgR) remains attached, protecting IgA from enzymes.

How does secretory IgA contribute to immune tolerance without causing inflammation?

IgA binds and neutralizes pathogens/toxins without triggering complement, maintaining microbial balance and preventing inflammation — key for mucosal tolerance.

. Predict what clinical problems might occur in a patient with selective IgA deficiency, and explain why compensatory mechanisms may not fully prevent symptoms.

IgA deficiency → recurrent mucosal infections (sinus, gut), allergies, autoimmune diseases; IgM can partially compensate but less stable and less efficient in mucosa.

Imagine a new probiotic therapy designed to enhance mucosal immunity.
What microbial or immunological features should it promote to be effective?

An effective probiotic should:

• Increase beneficial commensals
• Stimulate IgA and defensin production
• Enhance epithelial barrier integrity
• Promote regulatory cytokines (IL-10, TGF-β)

During chronic stress, epithelial barrier integrity is compromised.
Explain how this might alter the balance between defense and tolerance in the mucosa and increase disease risk.

Stress weakens tight junctions → increases permeability (“leaky gut”) → microbes cross barrier → chronic inflammation or autoimmunity due to loss of tolerance.