# 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. - **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.