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How do allergies work?

Medicine & HealthLesson on requestFree to readयह पेज हिंदी में पढ़ें

Quick answer

Allergies are the immune system's overreaction to a harmless substance. It produces IgE antibodies that sit on mast cells; when the allergen appears, these cells release histamine, causing sneezing, itching, swelling or breathing problems. Severe reactions, called anaphylaxis, need emergency medical care.

An allergy happens when the immune system mistakes a harmless substance, called an allergen, for a dangerous invader and attacks it. This misdirected response releases histamine and other chemicals that cause sneezing, itching, swelling or breathing trouble.

Step by step

  1. The immune system first meets a harmless substance such as pollen, pet dander or peanut protein.
  2. Immune cells mistakenly treat it as a threat and produce IgE antibodies specific to that allergen.
  3. These IgE antibodies attach to mast cells found in the skin, airways and digestive tract.
  4. On later exposure, the allergen binds and links nearby IgE antibodies on the mast cell surface.
  5. The mast cell releases histamine and other chemicals, producing allergy symptoms.
  6. In anaphylaxis, the same chemicals act throughout the body, causing dangerous swelling and low blood pressure.

Key things to understand

  1. The immune system normally defends against real threats like viruses and bacteria, but in an allergy it targets harmless things such as pollen or peanuts.
  2. On first contact with an allergen, immune cells make specific antibodies called IgE that attach to mast cells in the skin, airways and gut.
  3. When the allergen returns, it links neighbouring IgE antibodies on a mast cell, triggering the cell to release histamine and other chemicals.
  4. Histamine widens blood vessels and makes them leaky, causing redness, swelling, itching and a runny nose.
  5. In severe cases the whole body reacts at once, dropping blood pressure and narrowing airways, which is a medical emergency needing immediate help.

Labelled diagram

  1. Allergen enters

    Pollen, food or dust enters the body and is seen as a threat.

  2. IgE made

    Immune cells produce IgE antibodies tailored to that allergen.

  3. IgE binds mast cell

    Antibodies attach to mast cells in skin, airways and gut.

  4. Allergen links IgE

    The allergen connects nearby IgE, signalling the mast cell.

  5. Histamine released

    Chemicals pour out, causing swelling, itching and mucus.

  6. Symptoms appear

    Sneezing, rash or, in severe cases, anaphylaxis follows.

How an allergic reaction unfolds

Real-world examples

  • Hay fever in spring, when tree and grass pollen triggers sneezing and itchy eyes in millions of people.
  • A peanut allergy causing hives and throat swelling within minutes of eating a snack.
  • Bee stings that produce a large local swelling in most people but anaphylaxis in some.
  • Dust mite allergy causing year-round sneezing and wheezing indoors, especially in bedding.

Common mistake

✗ Myth: Allergies are just a weak or sensitive immune system.

✓ Fact: Allergies are an overactive, misdirected immune response, not a weak one. The immune system is working strongly but targeting the wrong substance.

People also ask

What causes allergies?
Allergies are caused by the immune system mistakenly identifying a harmless substance as a threat and making IgE antibodies against it. When that substance is met again, mast cells release histamine and other chemicals. Genetics and environment both influence who develops them.
Why do allergies make you sneeze?
Histamine released in the nose irritates nerve endings and triggers sneezing, which expels allergens. It also makes blood vessels leaky, producing a runny or blocked nose. These are the body's attempts to flush out what it wrongly sees as an invader.
What is histamine?
Histamine is a chemical stored in mast cells and released during allergic reactions. It widens blood vessels, makes them leak fluid and stimulates nerves, causing redness, swelling, itching and mucus. Antihistamine medicines work by blocking histamine's effects.
Why do some people have allergies and others don't?
Allergies run in families, so genes that shape the immune system raise the risk. Environment also matters: growing up with certain microbes, diet and pollution can influence whether the immune system becomes oversensitive. No single cause explains every case, and researchers still debate the exact mix.
Can you develop an allergy as an adult?
Yes. Allergies can appear at any age, even to foods or pollen you have tolerated for years. Hormonal changes, illness or repeated exposure may shift the immune response. If new symptoms appear, see a doctor to confirm the cause rather than guessing.
What is anaphylaxis?
Anaphylaxis is a severe, whole-body allergic reaction that can start within minutes. It causes swelling of the throat, difficulty breathing, a sudden drop in blood pressure and dizziness. It is a medical emergency; call emergency services and use prescribed adrenaline if available.
Do allergies ever go away?
Some childhood allergies, especially to milk and eggs, often fade as the immune system matures. Others, like peanut or pollen allergies, usually persist. Doctors can test and monitor changes, and some people outgrow reactions entirely.

Part of the guide: How Your Body Works →