(d) the primary non-specific defences against pathogens in animals

The idea

Teach the primary non-specific defences that prevent pathogen entry: skin, mucous membranes, expulsive reflexes, blood clotting, wound repair and inflammation. The skin is a physical barrier; its outer layer of dead keratinised cells and sebum inhibit microbial growth. Mucous membranes line body openings and trap pathogens in mucus, which is then removed by cilia. Expulsive reflexes such as coughing, sneezing, vomiting and diarrhoea eject pathogens. Blood clotting seals wounds to prevent entry. Wound repair involves inflammation, which increases blood flow and brings phagocytes to destroy pathogens. Learners must be able to name each defence and explain how it acts non-specifically against a range of pathogens.

What earns the marks

• Skin acts as a physical barrier; the outer layer of dead keratinised cells is difficult for pathogens to penetrate, and sebum contains antimicrobial substances.
• Mucous membranes secrete sticky mucus that traps pathogens; ciliated cells then move the mucus away from the lungs or other surfaces.
• Expulsive reflexes include coughing, sneezing, vomiting and diarrhoea, which forcibly remove pathogens from the respiratory or digestive tract.
• Blood clotting forms a scab that seals a wound, preventing pathogen entry and providing a temporary barrier while repair occurs.
• Wound repair involves inflammation: damaged cells release histamine, causing vasodilation and increased permeability of capillaries, so more phagocytes reach the site.
• Inflammation also causes redness, heat, swelling and pain, which are signs of the non-specific response to infection or injury.

Common mistakes and how to put them right

• Omitting expulsive reflexes such as coughing and sneezing.

  Put it right: These are primary non-specific defences required by the specification.

• Omitting wound repair and inflammation.

  Put it right: These are part of the primary non-specific defences, not just secondary responses.

• Stating that skin is a chemical barrier only.

  Put it right: It is primarily a physical barrier, with chemical secretions such as sebum as an additional feature.

• Confusing primary non-specific defences with the specific immune response.

  Put it right: Primary defences act against all pathogens, not a particular pathogen.

Exam tips

• List all primary non-specific defences: skin, mucous membranes, expulsive reflexes, blood clotting, wound repair and inflammation.
• For each defence, state how it prevents pathogen entry or removes pathogens, using precise biological terms such as 'keratinised', 'ciliated' and 'phagocytes'.
• When a question asks for a primary defence, do not give secondary responses such as antibody production or phagocytosis alone; phagocytosis is part of the secondary non-specific response.
• Use examples: coughing removes pathogens from the trachea, vomiting removes pathogens from the stomach, and blood clotting seals a cut.