Primary Non-Specific Animal Defences

The animal immune system operates in two broad modes: non-specific (innate) and specific (adaptive). Non-specific defences provide the first, rapid response to any invader — they do not distinguish between different pathogens, but they act within minutes and do not need to "learn" the threat. OCR specification 4.1.1 (e) requires you to describe the primary non-specific defences of animals. These include the skin and mucous membranes, blood clotting, wound repair, expulsion reflexes, inflammation, lysozymes, and fevers.

Key Definitions:

• Innate immunity — non-specific, always-present defences that act rapidly against any pathogen.
• Inflammation — the redness, swelling, heat and pain that follows tissue damage, caused by local blood vessel changes.
• Histamine — a chemical released by mast cells that triggers vasodilation and increased capillary permeability.
• Lysozyme — an enzyme that hydrolyses β-1,4 glycosidic bonds in bacterial peptidoglycan cell walls.

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Physical Barriers

1. The Skin

The skin is the body's largest organ and its most important physical barrier. The outermost layer — the stratum corneum — is composed of dead, flattened, keratinised cells that:

• Provide a thick, tough, waterproof layer.
• Are continuously shed (desquamation), taking attached microbes with them.
• Are replaced from below by dividing cells in the stratum basale.

The skin also supports a resident population of commensal bacteria (e.g., Staphylococcus epidermidis) that compete with and suppress pathogenic species — a phenomenon called competitive exclusion.

2. Mucous Membranes

Mucous membranes line the body surfaces that open to the outside: the respiratory, digestive, urogenital and conjunctival tracts. Goblet cells secrete mucus, a sticky glycoprotein gel that traps pathogens. In the airways, ciliated epithelial cells sweep mucus (with trapped microbes) upwards to the throat to be swallowed and destroyed by stomach acid — the mucociliary escalator.

3. Tears and Saliva

Tears continually wash the surface of the eye, and saliva bathes the mouth. Both contain lysozyme, an enzyme that digests bacterial peptidoglycan cell walls, causing the bacteria to burst.

4. Stomach Acid

Hydrochloric acid in the stomach lowers the pH to 1.5–2.0, killing most ingested pathogens by denaturing their proteins.

5. Earwax

Cerumen traps and kills microbes in the ear canal.

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Blood Clotting and Wound Repair

When skin is broken, the injury site must be sealed before pathogens can enter. This is achieved by the blood clotting cascade:

flowchart LR
    A[Injury exposes collagen] --> B[Platelets bind and release chemicals]
    B --> C[Thromboplastin released]
    C --> D[Prothrombin --> Thrombin]
    D --> E[Fibrinogen --> Fibrin]
    E --> F[Fibrin mesh traps cells]
    F --> G[Clot forms, dries into scab]

Key steps

1. Vascular spasm: damaged blood vessels constrict, reducing blood flow.
2. Platelet plug formation: platelets stick to exposed collagen, release chemicals (ADP, thromboxane) that recruit more platelets.
3. Clotting cascade: damaged cells and platelets release thromboplastin, which (in the presence of Ca²⁺ and vitamin K) converts prothrombin into thrombin. Thrombin catalyses the conversion of fibrinogen (soluble) into fibrin (insoluble fibres). The fibrin mesh traps erythrocytes and platelets, forming the clot.
4. Scab formation: as the clot dries, it contracts and hardens into a scab that protects the underlying tissue while new skin grows beneath.

Meanwhile, fibroblasts move into the wound and lay down collagen, epithelial cells divide to re-cover the wound, and macrophages clear debris.

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Inflammation

If pathogens manage to enter the tissues, a characteristic inflammatory response begins: