Inflammation

Inflammation is a local reaction of living tissue to infection, injury or irritation. It is part of innate immunity, and its purpose is to bring immune cells and proteins to the damaged area, destroy any invaders, remove dead cells and begin repair. Its classic signs are redness, swelling, heat, pain and loss of normal function.

Damaged cells and immune cells such as mast cells release chemical messengers, including histamine, bradykinin, prostaglandins and cytokines. These widen nearby blood vessels, raising blood flow, which produces heat and redness, and make the vessel walls leakier, so fluid seeps into the tissue as edema. White blood cells, first neutrophils and later macrophages, squeeze out of the vessels to engulf microbes and debris. Pus is the mix of dead white cells, microbes and tissue fluid that can build up. When signals from the site spread through the body, they can cause fever.

Acute inflammation normally fades once healing is under way. Chronic inflammation lasts for months or years, either because a pathogen persists or because the response fails to switch off. It slowly damages healthy tissue and contributes to conditions such as heart disease, rheumatoid arthritis and some cancers.