CH13 - Barrier Immunity: The Immunology of Mucosa and Skin

What is the difference between microbiota and microbiome?

Microbiota = actual microbes; microbiome = microbes + their genes and functions.

What is the role of epithelial cells in barrier immunity?

Physical barrier + active immune role (detect pathogens via PRRs and secrete cytokines/chemokines).

What are the two parts of the immune system?

Systemic immune system and mucosal (barrier) immune system.

What do all barrier organs have in common structurally?

Layers of epithelial cells interacting with immune cells.

List 3 innate immune cells in barrier immunity.

Epithelial cells, dendritic cells, macrophages, ILCs.

List 3 adaptive immune cells in barrier immunity.

B cells (IgA), T cells (CD8, Treg, Th17), IELs, MAIT cells.

What are innate lymphoid cells (ILCs)?

Innate cells without antigen-specific receptors that respond to cytokines.

How do ILCs help barrier immunity?

Produce cytokines (Type 1, 2, or 3 responses) to regulate immune defense.

What makes the skin immune system unique?

Multilayered epithelium, Langerhans cells, high CD8+ T cells, no mucus.

What is iSALT?

Induced skin-associated lymphoid tissue formed during inflammation.

What are the 3 divisions of the GI tract?

Stomach, small intestine, large intestine.

Rank GI regions by microbial load.

Large intestine > Small intestine > Stomach.

Where does most digestion occur?

Small intestine.

What is the lamina propria?

Connective tissue under epithelium where immune cells are active.

How do epithelial cells defend mucosal tissue?

Detect pathogens, release cytokines, recruit immune cells, produce mucus/AMPs.

What do goblet cells do?

Secrete mucus + antimicrobial peptides.

What do Paneth cells do?

Secrete defensins, lysozyme, maintain stem cells.

What do M cells do?

Transport antigen to immune cells.

What is the role of commensal bacteria?

Maintain tolerance and promote Treg + IgA responses.

What happens in germ-free mice?

↓ immune cells, ↓ IgA, ↓ Th17, need more calories, weaker immunity.

How does the gut communicate with the brain?

Enteric nervous system (vagus nerve) + cytokines/neuropeptides.

What are psychobiotics?

Microbes that influence mood/behavior.

How do microbial communities differ in the respiratory tract?

Upper = microbes present; Lower = mostly sterile.

What immune responses occur in the respiratory tract?

Type 1, 2, and 3 responses.

Describe the epithelial immune activation process.

PRRs detect pathogen; Cytokines/chemokines released; Recruit neutrophils/monocytes; Activate ILCs + adaptive cells; Pathogen cleared.

Describe tolerogenic response in barrier tissues.

Commensals detected; IL-10 + TGF-β released; Treg + IgA B cells activated; Immune tolerance maintained.

Describe Type 1 immune response (e.g. bacteria like Salmonella).

PRRs detect pathogen; IL-23 + IL-17 released; Activate Th1 + ILC1/3; Recruit neutrophils; Kill pathogen.

Describe Type 2 immune response (worms).

Tuft cells detect worms; IL-25 released; Activate ILC2 + Th2; IL-4, IL-5, IL-13 produced; Eosinophils + IgE kill worms.

How is IgA used in barrier immunity?

Produced by B cells; Transported into lumen; Binds microbes; Prevents epithelial contact; Anti-inflammatory effect.

How are antigens transported across the intestinal barrier?

M cells → transcytosis; Goblet cells → antigen passage; APCs → extend into lumen; IgA complexes → transported.

What are the 2 main barrier immune responses?

Tolerogenic (homeostasis) vs inflammatory (infection).

What cytokines dominate tolerance?

IL-10, TGF-β.

What happens during dysbiosis?

Imbalance → inflammation → diseases (IBD, Crohn's, UC).

What is the role of IL-22?

Stimulates epithelial cells to produce antimicrobial peptides.