mucosal immunity

Which organs are part of the mucosal immune system?

GI tract, respiratory tract, urogenital tract, eyes, mouth, salivary glands, mammary glands.

Why is the mucosal immune system unique?

Constant exposure to microbes + huge surface area.

What is mucus made of?

Mucins (glycoproteins) from goblet cells.

Why is mucus important?

Traps microbes, lubricates surfaces, forms protective barrier; varies in properties across tissues.

What are key functions of gut microbiota?

Make vitamins (e.g., K), digest polysaccharides, detoxify substances, outcompete pathogens, help develop MALT/GALT.

What do germ-free mice show?

Underdeveloped mucosal + systemic immunity; demonstrates microbiota is essential.

How can commensals be dangerous?

Overgrowth → infections (E. coli, Salmonella, Shigella, Helicobacter).

Why is the mucosal immune system “proactive”?

Constantly produces IgA, mucus, and effector cells.

Why is it “tolerant”?

Avoids inflammation to prevent epithelial damage.

What happens if tolerance fails?

IBD, celiac disease.

What is the inductive compartment?

Where antigen is presented to activate lymphocytes (e.g., Peyer’s patches, ILFs).

What is the effector compartment?

Contains effector T cells, plasma cells, macrophages, etc., in epithelium + lamina propria.

Examples of inductive tissue?

Peyer’s patches, isolated lymphoid follicles (ILFs).

Examples of effector sites?

Intraepithelial space + lamina propria.

What receptors do gut epithelial cells express?

TLRs (apical/basolateral), NOD proteins (cytoplasm).

What do these receptors activate?

NF-κB → antimicrobial peptides, chemokines, cytokines, defensins.

What type of macrophages dominate in the gut?

M2 (tolerant, non-inflammatory).

Key features of gut macrophages?

Strong phagocytes but no inflammatory cytokines, no respiratory burst, no B7.

How is antigen sampled in the gut?

M cells in Peyer’s patches transcytose lumen contents into the intraepithelial pocket for DCs/T/B cells.

Types of gut DCs?

In Peyer’s patches + lamina propria; some extend dendrites into lumen.

What is oral tolerance?

Food antigens induce tolerance, not inflammation; mediated by CD103⁺ DCs → Tregs.

How do DCs respond to commensals?

Activate Tfh cells → promote secretory IgA production.

Where are lymphocytes found?

Peyer’s patches, ILFs, lamina propria, epithelium.

What do gut-homing lymphocytes express?

CCR9 (binds CCL25) + α4β7 integrin (binds MAdCAM-1).

What are IELs (intraepithelial lymphocytes)?

IELs are a unique population of lymphocytes that reside within the epithelial lining of various tissues, most notably the gastrointestinal tract. They are primarily T cells (both \alpha\beta and \gamma\delta T cells) that play a critical role in immune surveillance, maintaining epithelial barrier

How is sIgA produced?

B cells activated in GALT → IgM → switch to dimeric IgA.

Role of sIgA?

Neutralizes microbes, traps them in mucus, prevents epithelial attachment.

What happens to T/B cells activated in MALT/GALT?

Gain integrin α4β7, lose CCR7 and L-selectin → home to mucosal tissues.

Can activated cells migrate to other mucosal sites?

Yes, activated cells acquire specific homing receptors (e.g., \alpha4\beta7 integrin and CCR9) that direct them to other mucosal sites.