Non specific defence

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.

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.

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.