The inflammatory response is a defence mechanism that evolved in higher organisms to protect them from infection and injury. Its purpose is to localize and eliminate the injurious agent and to remove damaged tissue components so that the body can begin to heal. The response consists of changes in blood flow, an increase in permeability of blood vessels, and the migration of fluid, proteins, and white blood cells (leukocytes) from the circulation to the site of tissue damage. An inflammatory response that lasts only a few days is called acute inflammation, while a response of longer duration is referred to as chronic inflammation.Although acute inflammation is usually beneficial, it often causes unpleasant sensations, such as the pain of a sore throat or the itching of an insect bite. Discomfort is usually temporary and disappears when the inflammatory response has done its job. But in some instances, inflammation can cause harm. Tissue destruction can occur when the regulatory mechanisms of the inflammatory response are defective or the ability to clear damaged tissue and foreign substances is impaired. In other cases, an inappropriate immune response may give rise to a prolonged and damaging inflammatory response. Examples include allergic, or hypersensitivity, reactions, in which an environmental agent such as pollen, which normally poses no threat to the individual, stimulates inflammation, and autoimmune reactions, in which chronic inflammation is triggered by the body's immune response against its own tissues.The most important feature of inflammation is the accumulation of white blood cells at the site of injury. Most of these cells are phagocytes, certain "cell-eating" leukocytes that ingest bacteria and other foreign particles and also clean up cellular debris caused by the injury. The main phagocytes involved in acute inflammation are the neutrophils, a type of white blood cell that contains granules of cell-destroying enzymes and proteins. When tissue damage is slight, an adequate supply of these cells can be obtained from those already circulating in the blood. But, when damage is extensive, stores of neutrophils some in the immature form are released from the bone marrow, where they are generated.To perform their tasks, not only must neutrophils exit through the blood vessel wall but they must actively move from the blood vessel toward the area of tissue damage. This movement is made possible by chemical substances that diffuse from the area of tissue damage and create a concentration gradient followed by the neutrophils. The substances that create the gradient are called chemotactic factors, and the one-way migration of cells along the gradient is called chemotaxis.Large numbers of neutrophils reach the site of injury first, sometimes within an hour after injury or infection. After the neutrophils, often 24 to 28 hours after inflammation begins, there comes another group of white blood cells, the monocytes, which eventually mature into cell-eating macrophages. Macrophages usually become more prevalent at the site of injury only after days or weeks and are a cellular hallmark of chronic inflammation.