9.1 First Line of Defence
The first line of defence is the body's initial, non-specific barrier designed to prevent pathogens (microbes) from entering the body. It consists of external physical and chemical barriers.
Non-Specific Defence
This line of defence acts generally against all types of microbes, rather than targeting specific ones.
Primary Barriers
The two main surfaces exposed to the environment that act as barriers are the skin and the mucous membranes of the digestive and respiratory tracts.
Skin: An Impenetrable Barrier
The skin is an effective multi-layered barrier against microbial invasion.
Structure
- Epidermis: The outermost layer.
- Composed of tightly packed cells, forming a physical wall.
- Contains keratin, a protein that strengthens cells and makes the skin resistant to water loss and abrasion.
- The surface consists of dry, dead cells, which are inhospitable to most microbes due to lack of water and nutrients.
- Dermis: The inner, thicker layer.
- Contains glands, hair follicles, nerves, and blood vessels.
Defensive Mechanisms
| Mechanism Type | Feature | Description |
|---|---|---|
| Physical | Desquamation | The continuous shedding of dead outer skin cells, which helps remove any microbes attached to the surface. |
| Physical | Tightly Packed Cells | The structure of the epidermis creates a physical wall that is difficult for microbes to penetrate. |
| Chemical | Acidic pH | The skin surface has a pH of around 5.5, which is acidic and inhibits the growth of many pathogens. |
| Chemical | Sebum | An oily substance from sebaceous glands. It contains fatty acids that lower skin pH and have antimicrobial properties. |
| Chemical | Sweat | Secreted by sweat glands. It contains lysozyme, an enzyme that breaks down bacterial cell walls, and high salt concentration, which can dehydrate bacteria. |
| Chemical | Natural Antibiotics | Secretions like lactic acid inhibit the growth of bacteria and fungi. |
Mucous Membranes: Internal Barriers
The tracts open to the exterior (digestive, respiratory) are lined with mucous membranes that provide protection.
Epithelium of the Digestive Tract
The gastrointestinal tract (GIT) is protected by various secretions.
- Stomach Acid: Parietal cells in the stomach secrete hydrochloric acid (HCl), creating a highly acidic environment that kills most bacteria ingested with food.
- Digestive Enzymes: Enzymes in gastric, intestinal, and pancreatic juices can digest and destroy bacteria.
- Normal Flora: Beneficial bacteria that compete with pathogens for nutrients and attachment sites.
Epithelium of the Respiratory Tract
The respiratory system has mechanisms to trap and remove airborne particles and microbes. Mechanism of Ventilation→
- Nasal Cavity:
- Hairs in the vestibule (anterior part) filter out large dust particles.
- The cavity is lined with ciliated mucous epithelium. Mucus traps fine particles and microbes.
- Trachea and Bronchi:
- Also lined with ciliated mucous epithelium.
- Cilia are tiny hair-like structures that beat in a coordinated rhythm, sweeping mucus and trapped debris upwards towards the pharynx. This mechanism is often called the mucociliary escalator.
- Once in the pharynx, the mucus is swallowed and the trapped microbes are destroyed by the digestive system.
Biological Significance
These barriers are crucial for preventing infections before they can begin, reducing the burden on the more complex parts of the immune system.