• Anaphylaxis is likely when all of the following 3 criteria are met [Working Group of the Resuscitation Council (UK), 2008]:
• Sudden onset and rapid progression of symptoms.
• Life-threatening airway and/or breathing and/or circulation problems.
• Skin and/or mucosal changes (flushing, urticaria, angio-oedema).
• There is often a history of anaphylaxis or severe allergic reaction, but it may be the first exposure to the allergen and the first presentation.
• Classical early features of anaphylaxis include:
• Wheeze and hoarseness due to chest tightness, swelling of the tongue and throat.
• An urticarial rash (in less than 50% of people). Skin or mucosal changes alone are not a sign of an anaphylactic reaction.
• Classical advancing features of anaphylaxis include:
• Stridor (high-pitched inspiratory noise caused by upper airway obstruction).
• Respiratory collapse, with rapid or shallow breathing, cyanosis.
• Confusion (caused by hypoxia), anxiety, agitation, and loss of consciousness.
• Hypotension, cyanosis, and circulatory collapse (pale, clammy, rapid pulse followed by low blood pressure).
• The classical signs of anaphylaxis may not always be present:
• The reaction can sometimes be delayed; five percent of children are thought to have biphasic reactions, with further symptoms developing 4–10 hours after the initial presentation.
• Cutaneous features of angio-oedema (swelling of lips and eyes) are commonly present with classical anaphylaxis, but systemic features of an allergic reaction (abdominal pain, vomiting, rhinitis) may also suggest impending anaphylaxis.

[Working Group of the Resuscitation Council (UK), 2008]