Premed · Premed · Immunology
Lecture 4: Innate Immunity I: Physical and Chemical Barriers
Immunology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the role of epithelial barriers as the first line of defense against infection
- Identify the physical, chemical, and microbiological barriers that constitute innate immunity
- Explain the antimicrobial mechanisms of the skin, respiratory tract, gastrointestinal tract, and urogenital tract
- Describe the role of the normal microbiota in immune defense
- Discuss how pathogens breach innate barriers to establish infection
Lecture Content
I. Overview of Innate Barrier Defenses
The body's first line of defense consists of barriers that prevent pathogen entry before infection can become established. These barriers fall into three categories: physical and mechanical barriers such as intact epithelial surfaces, mucus, cilia, and fluid flow; chemical barriers including antimicrobial peptides, enzymes, low pH, and reactive oxygen species; and biological barriers consisting of the commensal microbiota (normal flora) that compete with pathogens for space and nutrients. All of these barriers are constitutive, meaning they are always present and do not require prior exposure to a pathogen to function. If these barriers are breached, the second line of innate defense -- cellular and humoral innate immunity -- becomes activated.
II. The Skin as an Immune Barrier
The skin provides formidable physical protection through its stratified squamous keratinized epithelium, which consists of multiple layers of tightly packed dead cells. The outermost layer, the stratum corneum, is impermeable to most microorganisms, and tight junctions between keratinocytes prevent paracellular passage. The continuous desquamation (shedding) of outer skin cells removes attached microorganisms, and the relatively dry environment of the skin surface inhibits microbial growth.
Chemically, the skin maintains an acidic pH of 5.0-5.5 through lactic acid and fatty acids in sebum, which inhibits bacterial growth. Sebaceous glands produce sebum containing fatty acids such as oleic acid and palmitoleic acid that have antimicrobial properties. Sweat contains lysozyme, which degrades peptidoglycan, and dermcidin, an antimicrobial peptide. Keratinocytes themselves produce defensins (both α-defensins and β-defensins), which are cationic peptides that disrupt microbial membranes, as well as cathelicidin (LL-37), a broad-spectrum antimicrobial peptide, and psoriasin (S100A7), which is particularly effective against E. coli.
The skin also harbors resident immune cells, including Langerhans cells (dendritic cells in the epidermis), dermal dendritic cells, tissue-resident memory T cells, mast cells, and intraepithelial γδ T cells (known as dendritic epidermal T cells in mice).
<image>A detailed cross-sectional diagram of the skin showing immune barrier components. The epidermis is shown with multiple layers: stratum corneum (outermost, dead keratinized cells), stratum granulosum, stratum spinosum, and stratum basale. Langerhans cells (dendritic cells) are positioned in the suprabasal epidermis with dendrites extending between keratinocytes. The dermis below contains dermal dendritic cells, mast cells near blood vessels, and tissue-resident memory T cells. Chemical defenses are annotated: sebum from sebaceous glands (pH 5.0-5.5), defensins and cathelicidins secreted by keratinocytes, lysozyme and dermcidin in sweat from sweat glands. The commensal microbiota (normal flora) is depicted on the skin surface. An inset shows the structure of a defensin peptide disrupting a bacterial membrane.</image>
III. Respiratory Tract Barriers
The upper airways -- the nasal cavity, pharynx, and larynx -- rely on nasal hairs and turbinates to trap large particles, a mucus layer to trap inhaled pathogens, and vibrissae (nose hairs) to filter particles. The lower airways, including the trachea, bronchi, and bronchioles, employ the mucociliary escalator, a system of pseudostratified ciliated columnar epithelium with goblet cells. Goblet cells and submucosal glands secrete mucus containing heavily glycosylated mucins that trap pathogens, while cilia beat in a coordinated wave to move the mucus and trapped particles upward toward the pharynx for swallowing or expulsion. Tight junctions between epithelial cells provide an additional physical barrier, and secretory IgA in airway mucus contributes humoral defense.
The airway surface liquid contains several antimicrobial substances: lysozyme, lactoferrin (which sequesters iron from bacteria), defensins (both α and β types), and surfactant proteins SP-A and SP-D, which are collectins that opsonize pathogens and promote phagocytosis in the alveoli. In the alveoli themselves, alveolar macrophages patrol the surface and surfactant proteins enhance pathogen clearance. The cough and sneeze reflexes provide mechanical expulsion of pathogens.
IV. Gastrointestinal Tract Barriers
The gastrointestinal tract employs a layered defense system that varies along its length. In the oral cavity, saliva contains lysozyme, lactoferrin, histatins (which are antifungal), secretory IgA, and salivary peroxidase, and the continuous flushing action of saliva physically removes microorganisms. The stomach presents a highly hostile environment with gastric acid at pH 1.5-3.5 that kills most ingested microorganisms, along with pepsin, a proteolytic enzyme that degrades microbial proteins, and a mucus layer that protects the gastric epithelium.
In the small intestine, bile salts exert detergent-like antimicrobial activity. Paneth cells at the base of intestinal crypts secrete α-defensins (cryptdins), lysozyme, phospholipase A2, and REGIIIγ (a C-type lectin antimicrobial protein). Peristalsis prevents bacterial stasis and overgrowth, a single mucus layer provides physical protection, and secretory IgA -- the most abundant immunoglobulin in the body -- provides antigen-specific defense. M cells in Peyer's patches sample luminal antigens for immune surveillance. The large intestine features a thick two-layered mucus barrier: an inner layer that is dense, firmly attached, and largely bacteria-free, and an outer layer that is loose and colonized by commensal bacteria. The large intestine harbors a dense commensal microbiota, with 10^11 to 10^12 bacteria per gram of colonic content, along with antimicrobial peptides.
<image>A longitudinal cross-section of the gastrointestinal tract highlighting immune barriers at each level. Panel A: Oral cavity with saliva containing lysozyme, lactoferrin, and sIgA. Panel B: Stomach with gastric acid (pH 1.5-3.5) and pepsin shown destroying bacteria. Panel C: Small intestine showing villi with a single mucus layer, Paneth cells at crypt bases secreting defensins and lysozyme, goblet cells producing mucus, and a Peyer's patch with M cells sampling antigens from the lumen and delivering them to underlying dendritic cells and lymphocytes. Panel D: Large intestine showing the two-layered mucus system -- inner dense layer (bacteria-free) and outer loose layer (colonized by commensals). Secretory IgA molecules are shown throughout, being transported across the epithelium via the polymeric immunoglobulin receptor.</image>
V. Urogenital Tract Barriers
The urinary tract defends against infection through the unidirectional flow of urine, which flushes microorganisms downward, and the acidic pH of urine. Uroepithelial cells produce defensins and cathelicidins, while Tamm-Horsfall protein (uromodulin) binds and traps bacteria. Sphincter mechanisms help prevent ascending infection. In the female reproductive tract, an acidic vaginal pH of 3.8-4.5 is maintained by Lactobacillus species that produce lactic acid. Cervical mucus provides a physical barrier, antimicrobial peptides such as defensins and SLPI provide chemical defense, and the vaginal microbiome -- dominated by Lactobacillus spp. -- provides colonization resistance against pathogens.
VI. Antimicrobial Peptides -- Detailed Mechanisms
Defensins are small cationic peptides of 29-45 amino acids that exist in two main families: α-defensins and β-defensins. They work by electrostatic interaction with negatively charged microbial membranes, leading to pore formation and osmotic lysis, and they also have chemotactic activity for immune cells. Cathelicidins, of which LL-37 is the major human form, have broad-spectrum antimicrobial activity and also promote wound healing, chemotaxis, and inflammation modulation. Lysozyme is an enzyme that cleaves β-1,4-glycosidic bonds between NAM and NAG in peptidoglycan and is most effective against Gram-positive bacteria, which have exposed peptidoglycan. Lactoferrin is an iron-binding glycoprotein that deprives bacteria of the iron they need for growth and also exerts direct antimicrobial and anti-biofilm activity.
VII. The Commensal Microbiota as an Immune Barrier
The human body harbors approximately 38 trillion commensal microorganisms, which contribute to immune defense through several mechanisms. Competitive exclusion means that commensals occupy niches and compete for nutrients, preventing pathogen colonization. Commensals also produce antimicrobial substances such as bacteriocins and short-chain fatty acids (SCFAs) including butyrate, propionate, and acetate. They stimulate innate immune defenses by inducing epithelial production of antimicrobial peptides and strengthening barrier function. Additionally, commensal-derived signals educate the immune system, promoting the development of regulatory T cells and IgA-producing B cells.
Disruption of the microbiota, known as dysbiosis, increases susceptibility to infection. For example, antibiotic use can lead to Clostridioides difficile colitis, and reduced Lactobacillus populations can result in bacterial vaginosis and urinary tract infections.
VIII. How Pathogens Breach Innate Barriers
Pathogens have evolved multiple strategies to overcome innate barriers. Direct penetration occurs through wounds, abrasions, insect bites, or needlesticks. Attachment and invasion employs adhesins that bind host receptors and invasins that promote entry, as seen with Salmonella's type III secretion system. Enzymatic degradation uses hyaluronidase, collagenase, and proteases to break down the extracellular matrix. Some pathogens exploit M cells, such as Shigella and poliovirus, which use M cell transcytosis to cross the mucosal epithelium. Vector-borne transmission by mosquitoes or ticks bypasses the skin barrier entirely through direct injection. Finally, disruption of normal flora by antibiotics removes commensals and creates opportunities for pathogenic colonization.
