Inflammation, in medicine, reaction of the body to tissue injury or infection. The injured site becomes red and warm because of increased blood flow; swelling and tenderness result from fluids seeping into local tissues, causing increased skin tension. Certain body chemicals involved in inflammation also add to the local pain. Within the inflamed area, special defense cells accumulate, including white blood cells, macrophages, and lymphocytes. The white blood cells break down the damaged tissue and signal macrophages; the latter ingest and digest foreign substances and dead tissue (see Immune System). In some diseases this process may be destructive to the host. Treatment depends on the cause.