Late anaphylactic phase

With less severe symptoms in the early anaphylactic phase (EAR), after 6-10 h, the late anaphylactic phase (LAR) occurs, which is triggered by mast cells releasing chemokines, cytokines, and leukotriens—massive chemoattractants for neutrophils and eosinophils. 

Anaphylactic reactions generally begin within 30 minutes but almost always within 2 hours of exposure to the inciting allergen. The risk of fatal anaphylaxis is greatest within the first few hours. After apparent recovery, anaphylaxis may recur 6 to 8 hours after antigen exposure. Because of the possibility of these late phase reactions, patients should be observed for at least 12 hours after an anaphylactic reaction. Fatal anaphylaxis most often results from asphyxia due to airway obstruction either at the larynx or within the lungs. Cardiovascular collapse may occur as a result of asphyxia in some cases, whereas in others cases cardiovascular collapse may be the dominant manifestation from the release of mediators within the heart muscles and coronary blood vessels. 

A number of other agents may be required for the treatment of anaphylactic reactions. Corticosteroids (hydrocortisone sodium succinate intravenously) are recommended to reduce the risk of late-phase reactions. Aminophylline may be used as adjunctive therapy for bronchospasm. Histamine (Hi) receptor blockers (such as diphenhydramine) may be administered to reduce some of the symptoms associated with anaphylaxis however, these agents are not effective as primary therapy. 

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