When mammalian tissue is damaged because of infection or injury, the body responds with inflammation. This response can happen almost anywhere in the body, internally as well as on the surface. Inflammation is an important phenomenon: it isolates the damaged area to stop the spread of the damage; it recruits cells and molecules to the damaged location to kill the invader; and it promotes healing. The first responders to tissue damage are mast cells, which adhere to the skin and the linings of organs and release numerous chemical signals, including:
Tumor necrosis factor, a cytokine protein that kills target cells and activates immune cells.
Prostaglandins, fatty acid derivatives involved in various responses, including the widening of blood vessels. Prostaglandins interact with nerve endings and are partly responsible for the pain caused by inflammation.
Histamine, an amino acid derivative that leads to the important process of dilation of blood vessels, as well as itchy, watery eyes and rashes seen with some types of allergic reactions.
The redness and heat of inflammation result from the dilation and leakiness of blood vessels in the infected or injured area (Figure 41.5). Phagocytes enter the inflamed area, where they engulf the invaders and dead tissue cells. Phagocytes are responsible for most of the healing associated with inflammation. They produce several cytokines, which (among other functions) can signal the brain to produce a fever. This rise in body temperature accelerates lymphocyte production and phagocytosis, thereby speeding the immune response. In some cases, pathogens are temperature-
Following inflammation, pus may accumulate. Pus is a mixture of leaked fluid and dead cells: bacteria, neutrophils (the most abundant white blood cells—