ABC of allergies

Anaphylaxis

BMJ 1998; 316 doi: http://dx.doi.org/10.1136/bmj.316.7142.1442 (Published 9 May 1998)
Cite this as: BMJ 1998;316:1442

This article has a correction

Please see: ABC of allergies

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  1. Pamela W Ewan

    Anaphylaxis and anaphylactic death are becoming more common and particularly affect children and young adults. Anaphylaxis can be frightening to deal with because of its rapid onset and severity. Doctors in many fields, but particularly those working in general practice and in accident and emergency departments, need to know how to treat it.

    Features of anaphylaxis

    • Erythema

    • Pruritus (generalised)

    • Urticaria

    • Angio-oedema

    • Laryngeal oedema

    • Asthma

    • Rhinitis

    • Conjunctivitis

    • Itching of palate or external auditory meatus

    • Nausea, vomiting, abdominal pain

    • Palpitations

    • Sense of impending doom

    • Fainting, lightheadedness

    • Collapse

    • Loss of consciousness

    Definition

    Anaphylaxis means a severe systemic allergic reaction. No universally accepted definition exists because anaphylaxis comprises a constellation of features, and the argument arises over which features are essential features. A good working definition is that it involves one or both of two severe features: respiratory difficulty (which may be due to laryngeal oedema or asthma) and hypotension (which can present as fainting, collapse, or loss of consciousness). Other features are usually present.

    The confusion arises because systemic allergic reactions can be mild, moderate, or severe. For example, generalised urticaria, angio-oedema, and rhinitis would not be described as anaphylaxis, as neither respiratory difficulty nor hypotension—the potentially life threatening features—is present.

    Activation of mast cells by allergen crosslinking of adjacent IgE on cell surface in a type I allergic reaction

    Mechanism

    An allergic reaction results from the interaction of an allergen with specific IgE antibodies, bound to Fc receptors for IgE on mast cells and basophils. This leads to activation of the mast cell and release of preformed mediators stored in granules (including histamine), as well as of newly formed mediators, which are synthesised rapidly. These mediators are responsible for the clinical features. Rapid systemic release of large quantities of mediators will cause capillary leakage and mucosal oedema, resulting in shock and asphyxia.

    View this table:

    Effects of mast cell mediators …

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