Mechanical damage or local chemical changes cause specialised leucocytes called mast cells to be activated by complement proteins, which results in the release of histamine, heparin and other chemicals into the tissue fluid.

Histamine increases blood flow through the area via vasodilation, making the walls of the blood capillaries more permeable – more fluid moves through the capillary walls into the tissue, and the increased blood flow causes heat and redness, and the escape of fluid from the blood causes swelling.

Heparin prevents clotting, so the release of heparin from the mast cells prevents clotting in the immediate area of the injury – a clot of the fluid forms around the damaged area, which slows the spread of pathogens into healthy tissues.

Complement system proteins and some chemicals released by the mast cells attract phagocytes, particularly neutrophils, which actively consume micro-organisms and debris by phagocytosis.

The abnormal conditions in the tissue stimulate pain receptors, and so the person feels pain in the inflamed area.

The phagocytes, filled with bacteria, debris and dead cells begin to die, and the dead phagocytes and tissue fluid form a yellow liquid called pus.

New cells are produced by mitosis, and repair of the damaged tissue takes place.