Ref Robbins 8/e p 45 ,9/e p74 The acute inflammatory response rapidly delivers leuko- cytes and plasma proteins to sites of injury. Once there, leukocytes clear the invaders and begin the process of digesting and getting rid of necrotic tissues. Acute inflammation has two major components (Fig. 2-2): * Vascular changes: alterations in vessel caliber resulting in increased blood flow (vasodilation) and changes in the vessel wall that permit plasma proteins to leave the cir- culation (increased vascular permeability). In addition, endothelial cells are activated, resulting in increased adhesion of leukocytes and migration of the leukocytes through the vessel wall. * Cellular events: emigration of the leukocytes from the circulation and accumulation in the focus of injury (cel- lular recruitment), followed by activation of the leuko- cytes, enabling them to eliminate the offending agent. The principal leukocytes in acute inflammation are neu- trophils (polymorphonuclear leukocytes). Stimuli for Acute Inflammation Acute inflammatory reactions may be triggered by a variety of stimuli: * Infections (bacterial, viral, fungal, parasitic) are among the most common and medically impoant causes of inflammation. * Trauma (blunt and penetrating) and various physical and chemical agents (e.g., thermal injury, such as burns or frostbite; irradiation; toxicity from ceain environ- mental chemicals) injure host cells and elicit inflamma- tory reactions. * Tissue necrosis (from any cause), including ischemia (as in a myocardial infarct) and physical and chemical injury * Foreign bodies (splinters, di, sutures, crystal deposits * Immune reactions (also called hypersensitivity reactions) against environmental substances or against "self" tissues. Because the stimuli for these inflammatory responses often cannot be eliminated or avoided, such reactions tend to persist, with features of chronic inflam- mation. The term "immune-mediated inflammatory disease" is sometimes used to refer to this group of disorders. Although each of these stimuli may induce reactions with some distinctive characteristics, in general, all inflamma- tory reactions have the same basic features. In this section, we describe first how inflammatory stimuli are recognized by the host, then the typical reac- tions of acute inflammation and its morphologic features, and finally the chemical mediators responsible for these reactions.