Skin and Mucous Membranes: The Body's First Line of Defense

The skin and mucous membranes form the innate immune system's primary physical and chemical barrier against pathogens. Their integrated defenses prevent entry, colonization, and systemic spread of microorganisms.

1. Physical Barriers

• Skin (epidermis): Stratified squamous keratinized epithelium; tight junctions; continuous shedding (desquamation) removes microbes.
• Mucous membranes: Line respiratory, GI, and urogenital tracts. Goblet cells secrete mucus that traps particles and microbes; cilia propel mucus outward (mucociliary escalator).

2. Chemical Barriers

• Skin: Acid mantle (pH 4.5–5.5) inhibits many bacteria; antimicrobial peptides (defensins, cathelicidins); sebum (free fatty acids).
• Mucous membranes: Lysozyme (cleaves bacterial peptidoglycan); secretory IgA (neutralizes viruses/toxins); lactoferrin (sequesters iron); surfactant proteins.

3. Mechanical Defenses

• Coughing, sneezing, vomiting, diarrhea — forceful expulsion.
• Urination and peristalsis — flushing action.
• Ciliary movement — continuous clearance.

4. Cellular Innate Immunity

• Langerhans cells (skin): Dendritic cells that capture antigens, migrate to lymph nodes.
• Macrophages and neutrophils: Phagocytosis, oxidative burst, NETosis.
• Mast cells: Histamine release → vasodilation, recruitment.
• Intraepithelial lymphocytes (IELs): γδ T cells with rapid response.

5. Adaptive Immune Initiation

• Antigen-presenting cells (APCs) process and present antigens to naïve T/B cells in draining lymph nodes.
• IgA class switching in mucosa; homing receptors direct effector cells back to mucosal sites.
• Immunological memory established for faster future responses.

Clinical Relevance

Barrier breaches (burns, wounds, intubation, chemotherapy-induced mucositis) dramatically increase infection risk. Maintaining integrity through hydration, nutrition, hygiene, and avoiding unnecessary antibiotics preserves this critical defense.