Why Allergies Happen

Tap a term to see what it means.

Allergen

A normally harmless substance — such as pollen, certain foods, or pet dander — that the immune system mistakenly identifies as a threat, triggering an allergic reaction in sensitive individuals.

Why the Immune System Overreacts

In an allergic reaction, the immune system produces a specific antibody (IgE) against a substance that poses no real danger, then reacts aggressively to it on subsequent exposure — essentially a case of mistaken identity.

Histamine

A chemical released by immune cells during an allergic reaction that causes the classic symptoms of allergies — itching, swelling, sneezing, and a runny nose — which is why antihistamine medications are commonly used for relief.

Sensitization

The first exposure to an allergen usually does not cause symptoms but "primes" the immune system, so a later exposure triggers a full allergic reaction — this is why allergies can develop even to substances a person tolerated for years.

Anaphylaxis

A severe, potentially life-threatening allergic reaction affecting the whole body, which can cause difficulty breathing and a sudden drop in blood pressure and requires immediate emergency treatment.

Common Allergens

Frequently reported allergens include pollen, dust mites, pet dander, certain foods (such as peanuts, shellfish, and eggs), and insect stings, though sensitivity varies widely between individuals.

An overprotective immune response

This page is for general information only and is not a substitute for professional medical advice. At its core, an allergy is the immune system misidentifying a harmless substance as dangerous and mounting an unnecessary defense — the exact reasons why some people develop allergies and others do not are still an active area of research, involving both genetics and environmental exposure.

Frequently Asked Questions

Can allergies develop later in life even without a childhood history?

Yes — allergies can develop at any age, and adult-onset allergies to substances a person previously tolerated without issue are not uncommon.

What should someone do if they suspect a severe allergic reaction?

Anaphylaxis is a medical emergency — if someone experiences difficulty breathing, swelling of the face or throat, or a rapid drop in blood pressure after exposure to a suspected allergen, emergency medical help should be sought immediately.