2A. Barriers to Infection
Innate Immunity: Physical and Chemical Barriers
Innate immunity provides immediate, non-specific protection against pathogens. The first line of defence consists of physical and chemical barriers that prevent microorganisms from entering and establishing infection.
Chemical Barriers
Antimicrobial Peptides (AMPs)
- Produced by skin and epithelial cells.
- Examples include defensins.
- Disrupt microbial cell membranes.
- Effective against bacteria, fungi, and some viruses.
Antimicrobial Enzymes
Lysozyme
- Found in tears, saliva, and mucus.
- Breaks down bacterial cell walls.
- Kills bacteria by destroying peptidoglycan.
Stomach Acid
- Hydrochloric acid (HCl) maintains a pH of approximately 1–3.
- Destroys most ingested pathogens.
- One of the body's most powerful chemical defenses.
pH Barriers
Different body sites maintain characteristic pH levels that inhibit microbial growth:
| Site | pH Characteristic | Protective Effect |
|---|---|---|
| Skin | Slightly acidic | Inhibits many microbes |
| Stomach | Highly acidic | Destroys pathogens |
| Vagina | Acidic | Maintained by normal microbiota |
Normal Microbiota
The body's commensal microorganisms provide protection by:
- Competing with pathogens for nutrients and space.
- Producing antimicrobial substances.
- Maintaining appropriate pH levels.
- Preventing colonisation by harmful microbes (colonisation resistance).
How Physical and Chemical Barriers Work Together
The innate barriers function as an integrated defence system.
Examples
- Mucus traps microbes, while cilia move them out of the respiratory tract.
- Tears flush away pathogens, while lysozyme destroys bacteria.
- Normal microbiota inhibit pathogen colonisation and support chemical defences.
When Barriers Are Compromised
Barrier function can be weakened by:
- Skin damage (cuts, abrasions, burns).
- Cilia damage (e.g. smoking).
- Antibiotic use disrupting normal microbiota.
- Reduced stomach acid.
Commensal Organisms
Commensal organisms:
- Cause little to no harm to the host.
- Form part of the microbiome.
- Exist in a symbiotic relationship with the host.
- Are also called commensal microorganisms.
Key Concepts Summary
Innate Immunity
- Rapid response.
- Non-specific.
- No immunological memory.
Chemical Defenses
- Antimicrobial peptides.
- Lysozyme.
- Stomach acid.
- pH barriers.
- Normal microbiota.
Bridge to Adaptive Immunity
- Dendritic cells present antigen to T cells.
- NK cells eliminate infected host cells.