<prompt> Characteristic of acute inflammation isVasodilation and increase permeabilityVasoconstrictionPlatelet aggregationInfiltration by neutrophil  <response>: Ref Robbins 8e p 45 ,9e 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. 