Human defence systems: Students should be able to describe the non-specific defence systems of the human body against pathogens, including the:

The idea

Non-specific defence systems protect the body against any pathogen without recognising a particular type. They act as barriers and rapid responses. The skin is a physical barrier: its outer layer of dead cells is tough, and it produces antimicrobial secretions. The nose traps particles in hairs and mucus, and cilia in the airways sweep mucus containing pathogens away from the lungs. The trachea and bronchi are lined with ciliated cells and mucus-secreting cells that trap and remove pathogens. The stomach produces hydrochloric acid, which kills many pathogens swallowed in food or drink. These defences are non-specific because they do not distinguish between different pathogens; they simply prevent entry or destroy invaders quickly.

What earns the marks

• Non-specific defences act against any pathogen and do not target a specific type.
• The skin acts as a physical barrier and produces antimicrobial secretions.
• The nose contains hairs and mucus that trap pathogens and particles.
• The trachea and bronchi are lined with cilia and mucus that trap and move pathogens away from the lungs.
• The stomach produces hydrochloric acid that kills pathogens in swallowed food or drink.

Common mistakes and how to put them right

• Confusing non-specific defences with the immune system's specific antibody response.

  Put it right: Non-specific defences do not recognise particular pathogens.

• Stating that the skin is only a physical barrier and ignoring its antimicrobial secretions.

• Forgetting that cilia in the airways move mucus and trapped pathogens upwards, away from the lungs.

Exam tips

• Name each defence system and state its role in preventing infection.
• Use correct terms such as cilia, mucus, hydrochloric acid and antimicrobial secretions.
• Link each defence to the part of the body where it acts, such as skin, nose, trachea, bronchi or stomach.