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Symptoms of type I allergic reactions

In this article Andreas J. Bircher presents the symptoms associated with immediate allergic reactions.
© University of Basel

Type I (immediate type) allergies present themselves with characteristic symptoms in different organs. The release of histamine, leukotrienes, and other inflammatory mediators from mast cells and basophils act on skin nerves (causing itch), on small vessels (causing vasodilation and swelling), on the lungs, and the gut (causing constriction), and by acting on the heart or large vessels they increase heart rate and lower blood pressure.

Skin and mucosal symptoms

In the skin, sudden itch, flushing, redness, elevated wheals (ie nettle rash or urticaria), and localized swellings (ie angioedema) occur. On mucosal surfaces such as the lips, the oral cavity, and the tongue, angioedema may cause considerable swelling and obstruction, and can interfere with swallowing and breathing. Direct contact of the oral cavity with a large amount of allergen. An apple for example may cause itching and swelling of lips, tongue, and throat. This is called the ‘oral allergy syndrome’. If volatile agents (eg pollen, dust mites, or animal dander) land in the eyes, the conjunctiva turns itchy and red. In the nose, itching, swelling, and hypersecretion cause sneezing and a running or stuffy nose.

Respiratory symptoms

If the lung is involved (eg upon inhalation of cat allergens), the bronchi tighten, the mucous secretions thicken and turn sticky. The patient may turn blue, begin to cough, get short of breath, and start wheezing. These are the typical signs of an asthmatic attack.

Gastrointestinal symptoms

Symptoms in the gastrointestinal tract include nausea, vomiting, swelling, obstruction, cramps, and sudden diarrhoea. Gastrointestinal symptoms are not very typical for an allergy – an acute food poisoning may cause very similar symptoms but is often accompanied by fatigue and fever.

Cardiovascular symptoms

If the cardiovascular system gets involved, the heart rate increases. In more severe cases, the blood pressure starts to drop, resulting in dizziness, vertigo, and faint. In case of a rapid cardiovascular collapse, the patient may suddenly fall and hurt himself.

Further symptoms

In severe cases, further symptoms may occur. These include a metallic taste in the mouth, sudden headaches, anxiety or agitated behaviour, sudden loss of urine, uterine cramps, and bleeding.


The symptoms may remain limited to one organ only (eg generalized urticaria in food allergy). They may concomitantly occur in several organs or become generalized, involving the whole organism. If two or more organs are involved (eg a severe urticaria combined with asthma), it is called anaphylaxis. If additionally, blood circulation collapses, it may result in an anaphylactic and sometimes lethal shock.

Symptom severity

Depending on the susceptibility of the patient, the type, amount, and ‘port of entry’ of an allergen, allergic symptoms may vary. Volatile allergens such as pollen, mites, and animal dander mainly affect the eyes, nose, lungs, and rarely the skin. Food allergens ingested in large amounts, getting in direct contact with the oral cavity and intestines, cause initial local symptoms, however, they may cause general symptoms upon their uptake into the circulation, too. Bee and wasp venoms as well as drug allergens, which are rapidly absorbed into the blood (eg by ingestion or injection), more frequently cause systemic symptoms.

© University of Basel
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