Module 4: Non-Specific Defences in Animals
These free OCR A Level Biology Non-Specific Defences in Animals revision notes have been written for specification point 4.1.1(d).
Primary Non-Specific Defences Against Pathogens
Primary nonspecific defences keep microorganisms from entering the body.
Primary defences are non-specific because they will target and respond to a wide range of pathogens.
Examples of primary defences include:
| Type | Examples |
|---|---|
| Physical Barriers | Skin, mucous membranes, ear wax, and mucus plug in the cervix. |
| Chemical Barriers | Sebum, stomach acid, and lysozymes in tears and saliva. |
| Mechanical Reflexes | Coughing, sneezing, vomiting, diarrhoea. |
| Physiological Responses | Blood clotting, inflammation. |
Physical Barriers
The Skin
The outer layer of the skin is the epidermis, a tough, waterproof layer.
The epidermis is mostly made up of keratinocytes, which specialise into dead cells containing keratin, which makes them tough and waterproof; this is known as keratinisation.
Because the keratinocytes are dead (and flake off), they must be continuously replaced.
Skin Damage
When the skin is damaged, it provides an entry point for pathogens. The body prevents infection by sealing the wound temporarily (blood clotting) and permanently (wound repair).
Blood clotting
Blood clotting involves platelets releasing substances that trigger a reaction chain that produces fibrin, an insoluble protein. Fibrin forms a mesh-like network that captures extra platelets and forms a stable clot.
Wound repair
A clot hardens into a scab, creating a temporary seal and drawing the wound edges together. Underneath, collagen is laid down while skin stem cells divide to repair the tissue, and new blood vessels grow. Once healing is complete, the scab is shed.
Inflammation
Inflammation is swelling and redness. It occurs to help contain pathogens and encourage the action of white blood cells.
Mast cells in the skin detect microorganisms if they have entered the area, and release the cell-signalling molecule histamine.
Histamine mainly causes vasodilation, so that the capillary dilates and becomes more permeable to white blood cells and plasma proteins.
More white blood cells and blood plasma can now enter the infected area from the capillaries, forming more tissue fluid and causing oedema (swelling).
Inflammation also occurs during allergic reactions when mast cells mistake substances for pathogens.