The inflammatory response is a defense mechanism of the body against infection, injury, or irritation. It can be acute, lasting days or weeks, or chronic, lasting several months to years. Inflammation helps identify and destroy pathogens and removes damaged tissue elements so the body can heal. This response is seen at the vascular and cellular levels. Both are crucial components of the acute inflammatory process. At the vascular stage, small blood vessels near the injury site initially constrict to prevent blood loss. The arterioles and venules supplying blood to that area dilate, increasing blood flow and causing local redness and heat. In addition, the release of cell mediators, such as histamine, bradykinin, and prostaglandins, in the injured area increases vascular permeability to protein- and immune-cell-rich fluids, which causes swelling, pain and loss of function. During the cellular stage, white blood cells, or WBCs, pass through the blood vessels and rush to the site of injury, causing an increase in the overall WBC count. WBCs, like neutrophils and monocytes, engulf pathogens and ingest cell debris and foreign material. As a result of pathogen phagocytosis, pyrogens are released, causing fever.