Why do allergies cause symptoms?
Short answer
An allergy happens when the immune system treats a usually harmless substance (an allergen) as a threat and releases chemicals such as histamine. These chemicals can lead to symptoms such as a runny nose, sneezing, itching, or hives.
How it works
Triggers and symptoms differ between people. Pollen, dust, food, and medicines are common examples. Depending on how the allergen enters the body and how strong the reaction is, symptoms can affect the nose, eyes, skin, airways, or digestive system.
A rapid, severe reaction affecting the whole body is called anaphylaxis. Sudden breathing difficulty, swelling of the lips, tongue, or throat, repeated vomiting, collapse, or a change in consciousness require emergency help. Contact the local emergency number.
Key point
A runny nose, cough, or rash can also have causes other than an allergy, such as an infection. Do not diagnose the cause from symptoms alone; consult a healthcare professional when symptoms keep returning or are severe.