Body Surfaces Prevent Pathogen Entry

1. Intact skin forms a tough physical barrier that prevents pathogens reaching living tissues.
2. Blood clotting seals cuts, and the clot dries to form a scab that restores the barrier.
3. Keratinised cells at the skin surface are tightly packed and continually shed, removing microorganisms attached to them.
4. Platelets trigger clotting at a cut, fibrin forms a mesh and trapped blood cells form a clot that dries into a scab.

Mucus and Cilia Clear the Airways

1. Goblet cells produce sticky mucus that traps pathogens and particles in the nose, trachea and bronchi.
2. Cilia beat towards the throat, moving mucus away from the lungs.
3. The mucus is swallowed, allowing stomach acid to kill many trapped pathogens.
4. Mucus contains water and glycoproteins that make it sticky enough to trap inhaled pathogens before they reach alveoli.
5. Coordinated ciliary beating moves mucus to the throat, where swallowing transfers trapped pathogens to the acidic stomach.

Chemical Defences Kill Pathogens

1. Lysozyme is an enzyme in tears, saliva and mucus that breaks down bacterial cell walls.
2. Hydrochloric acid in the stomach creates a very low pH that kills many swallowed pathogens and denatures their enzymes.
3. Lysozyme hydrolyses components of bacterial cell walls, while stomach acid denatures proteins and creates conditions in which many pathogens cannot survive.

Link Each Defence to Its Action

1. Skin blocks entry, mucus traps, cilia move, lysozyme breaks bacterial walls and stomach acid kills.
2. A complete explanation names the defence and gives its biological action.

Defences Work in Sequence

1. Pathogens that pass one barrier may meet another defence deeper in the body.
2. If pathogens enter tissues, white blood cells provide the next line of defence.