Quick Answer
An allergy is an immune system hypersensitivity reaction to a normally harmless substance. When the immune system mistakenly treats an allergen as a threat, it triggers the release of histamine and other chemicals — causing symptoms ranging from sneezing and hives to life-threatening anaphylaxis.
Key Takeaways
- Allergies are immune system overreactions to harmless substances called allergens, mediated primarily by IgE antibodies and mast cells
- The most common allergens include pollen, dust mites, pet dander, mold, certain foods, insect stings, and medications
- Genetic predisposition (atopy) increases allergy risk — but environment determines which specific allergy develops
- Adult-onset allergies are common and can develop at any age, even to previously tolerated substances
- Diagnosis requires a board-certified allergist using skin prick testing or specific IgE blood testing
- Allergen immunotherapy is the only treatment that modifies the underlying immune response for long-term relief
How Your Immune System Triggers an Allergic Reaction
The Medical Definition of Allergy
Allergies represent a hypersensitive immune response to substances that are generally tolerated by most people. This misidentification by the immune system triggers a cascade of chemical releases, primarily governed by Immunoglobulin E (IgE) antibodies. Understanding the mechanism is critical — read our complete allergy symptoms guide to understand how these reactions present clinically.
The Sensitization Process
Before a true allergic reaction occurs, a patient must undergo sensitization. The immune system encounters the allergen, falsely flags it, and manufactures specific IgE antibodies. These attach to mast cells and basophils. Upon subsequent exposure, the allergen binds to these antibodies, causing the cells to degranulate and release histamine. This process is explained in detail in our allergy diagnosis timeline.
Genetic and Environmental Factors
The development of allergies is multifactorial. Atopy is the genetic tendency to develop allergic diseases. However, environmental exposure, the timing of allergen introduction in infancy, and viral infections all play a significant role. Learn how pediatric allergies develop and why adult-onset allergies are increasingly common.
Types of Allergic Reactions
| Type | Mechanism | Example |
|---|---|---|
| Type I (IgE-mediated) | IgE + Mast cells | Peanut allergy, hay fever |
| Type IV (Delayed) | T-cell mediated | Contact dermatitis, latex allergy |