Allergy

An allergy is a long term condition in which the immune system mounts an abnormal reaction to an ordinarily harmless substance called an allergen. Allergic reactions vary in severity from relatively minor symptoms to life-threatening anaphylactic shock.

Allergic reaction
An allergic reaction occurs when the person's immune system produces Immunoglobulin E (IgE). When the person comes into contact with an allergen, the IgE travels to cells that release histamine and other chemicals; it is the histamine that causes the allergic reaction.

A severe, systemic allergic reaction is called anaphylaxis or anaphylactic shock, and can be life-threatening.

Symptom recognition
The onset of new allergies is a potential symptom of Long COVID in the World Health Organization's definition.

Notable studies

 * 1988, Allergy and the chronic fatigue syndrome
 * 1998, Chronic fatigue syndrome: identification of distinct subgroups on the basis of allergy and psychologic variables (Full text)
 * 2015, Increased Risk of Chronic Fatigue Syndrome Following Atopy: A Population-Based Study

Learn more

 * 1999, Allergies To Foods, Plants May Predict Chronic Fatigue Syndrome - ScienceDaily
 * 2019, Allergy Season Is Getting Longer and Nastier Each Year - Smithsonian Magazine