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Inflammation is the response of the immune system to infection or irritation, characterized by the following quintet: redness (rubor), heat (calor), swelling (tumor), pain (dolor) and dysfunction of the organs involved (functio laesa). The first four items were known since ancient times, whereas functio laesa was added to the definition of inflammation by Rudolf Virchow in 1858.

The redness and heat are caused by the increased blood supply to the affected area. The blood vessels are dilated and engorged, and there is a loss of blood plasma from them into the surrounding tissue spaces. This results in edema or swelling. The swelling distends the tissues, compresses nerve endings, and thus causes pain. The white blood cells or leucocytes take on an important role in inflammation; they escape the capillaries, crowd the tissue spaces, and carry on their work as phagocytes picking up bacteria and cellularular debris. They aid in walling off an infection and preventing its spread.

When inflammation subsides, the damaged tissue is repaired. Depending on the severity of the inflammation and the type of tissue involved repairs may or may not be complete; in minor inflammations of the skin, for example, the tissue is capable of complete regeneration whereas in nervous tissue regeneration may be more limited and the damaged cells may be replaced with scar tissue.