Barriers and epithelial cells

What are the two phases of innate immunity?

Innate phase (acts immediately with preformed mediators) and early-induced phase (sensing, recruitment, inflammation).

What are the main types of anatomical barriers to infection?

Physical, chemical, and microbiological barriers.

What are examples of anatomical barriers in the body?

Skin, mouth and upper alimentary canal, stomach, small intestine, large intestine, airway and lungs, urogenital tract, and glands (salivary, lacrimal, mammary).

What mechanical features protect the skin?

Longitudinal flow of air and fluid, lamellar bodies, and secretory organelles.

What chemical components protect the skin?

Fatty acids, α- and β-defensins, S100 proteins, psoriasin, and calprotectin.

What are the main barrier features of the stomach?

Low pH (~2–3), enzyme digestion (pepsin), muscular contraction, and fluid flow.

What enzymes contribute to small intestine barrier function?

Amylase, lipase, trypsin, and chymotrypsin.

What antimicrobial peptides are present in the small intestine?

Antimicrobial peptides (AMPs).

How does the large intestine act as a barrier?

Contains normal intestinal flora and has fluid/feces movement to expel waste.

What are the main defences of the airway and lungs?

Coughing, sneezing, mucociliary apparatus, macrophages, and surfactant proteins (SP-A, SP-D).

What defences are found in the urogenital tract?

Flushing by urine, urinary mucins, low pH (~6), antimicrobial peptides, and vaginal secretions.

What protect the salivary, lacrimal, and mammary glands?

Flushing secretions, antimicrobial peptides, lysozyme, lactoferrin, and calprotectin.

What are general functions of epithelial cells?

Act as barriers, protect underlying tissues, permit nutrient movement, secrete and absorb substances.

What structural feature keeps epithelial cells tightly connected?

Tight junctions.

What innate immune molecules do epithelial cells produce?

Antimicrobial peptides (AMPs), chemokines (IL-8), cytokines (IL-6), and mucins (MUC1, 4, 16).

What receptors allow epithelial cells to sense microbes?

Pattern recognition receptors (PRRs), including TLRs and NLRs.

What are the main epithelial cell types in the skin?

Stratified squamous epithelial cells from basal stem cells; stratum spinosum produces defensins; lamellar bodies waterproof stratum corneum.

What are the main epithelial cell types in the lungs?

Type I pneumocytes (gas exchange), Type II pneumocytes (secretory), goblet cells (mucus), and ciliated cells (mucus movement).

What are the four main epithelial cell types in the gut?

Epithelial cells, goblet cells, endocrine cells, and Paneth cells.

What do Paneth cells produce?

Antimicrobial peptides (AMPs).

What are antimicrobial peptides (AMPs)?

Small cationic peptides (7–100 aa) with amphipathic α-helical or β-sheet structures that disrupt microbial membranes.

What are the main mammalian AMP families?

Cathelicidins and defensins (α, β, θ).

What is the function of antimicrobial peptides?

Non-oxidative killing of pathogens and immunomodulation.

How do AMPs disrupt microbial membranes?

Insert into lipid bilayers and form pores, causing membrane damage.

What is mucus?

A biological hydrogel coating epithelial surfaces that lubricates and protects against shear and excludes pathogens.

What is the main structural component of mucus?

Mucins.

What are mucins?

High-molecular-weight glycoproteins (200,000 Da), 80% glycosylated, providing hydration, lubrication, and protection.

What are the two types of mucins?

Secreted and membrane-tethered mucins.

How are mucin networks stabilized?

By disulphide bonds, hydrophobic interactions, and electrostatic repulsion.

How do epithelial cells contribute to innate immunity and inflammation?

Through PRRs (e.g., TLRs), NF-κB activation, and secretion of pro-inflammatory cytokines and chemokines.

What cells are recruited during epithelial inflammation?

Neutrophils and macrophages.

What skin diseases are linked to barrier defects?

Ichthyosis and atopic dermatitis (filaggrin gene mutation).

What gut disease is associated with barrier dysfunction?

Inflammatory bowel disease (IBD).

What lung diseases are associated with barrier dysfunction?

Primary ciliary dyskinesia and cystic fibrosis (CFTR mutation).

What are epithelial secretions that contribute to defence?

Cytokines, antimicrobial peptides, and mucins.

What activates epithelial cells during infection?

Pathogen-associated molecular patterns (PAMPs) via TLRs.

What is the overall function of anatomical barriers?

To provide the first line of defence against infection using epithelial cells and mucus.