(a) Physical and chemical defences

• Epithelial cells form a physical barrier.
• Closely-packed epithelial cells are found in the skin and inner linings of the digestive and respiratory systems.
• Chemical secretions are produced against invading pathogens.
• Secretions include tears, saliva, mucus and stomach acid.
• A pathogen is a bacterium, virus or other organism that can cause disease.

(b) Inflammatory response

• Histamine is released by mast cells causing vasodilation and increased capillary permeability.
• The increased blood flow leads to an accumulation of phagocytes and clotting elements at the site of infection.

(c) Phagocytes

• Phagocytes recognise pathogens and destroy them by phagocytosis.
• Phagocytosis involves the engulfing of pathogens and their destruction by digestive enzymes contained in lysosomes.
• Phagocytes release cytokines which attract more phagocytes to the site of infection.
• Cytokines are protein molecules that act as a signal to specific white blood cells causing them to accumulate at the site of infection.