How Allergies Develop: Why Your Immune System Sometimes Overreacts

Have you ever wondered why one person can cuddle a cat without any problems while another starts sneezing within minutes? Or why some people can eat peanuts safely while others experience a life-threatening reaction?

The answer lies in the fascinating—and sometimes misguided—workings of the immune system.

What Is an Allergy?

An allergy occurs when your immune system mistakes a harmless substance for a dangerous invader.

Normally, your immune system protects you from bacteria, viruses, and parasites. But in people with allergies, everyday substances such as pollen, dust mites, pet dander, certain foods, insect venom, or medications are incorrectly identified as threats. These harmless substances are called allergens.

The Two-Step Process

Step 1: Sensitisation

The first time you're exposed to an allergen, you usually don't develop symptoms.

Instead, your immune system creates a special type of antibody called IgE (immunoglobulin E). These IgE antibodies attach themselves to immune cells known as mast cells and basophils, which are found throughout the skin, airways, nose, and digestive tract.

At this point, you've become sensitised.

Step 2: The Allergic Reaction

The next time you encounter the same allergen, it binds to the IgE antibodies sitting on mast cells.

This triggers the cells to release powerful chemicals, including:

• Histamine

• Leukotrienes

• Prostaglandins

• Cytokines

  
  

These chemicals cause the familiar symptoms of allergies:

• Sneezing

• Itchy eyes

• Runny nose

• Skin rashes

• Hives

• Wheezing

• Swelling

• In severe cases, anaphylaxis

Interestingly, these symptoms are not caused by the allergen itself—but by your own immune system's response.

Why Do Some People Get Allergies?

There's no single cause, but several factors increase the risk.

Genetics

Allergies often run in families. If one parent has allergies, the risk for their children increases. If both parents are allergic, the likelihood is even higher.

Environment

Modern lifestyles may contribute.

Children raised in very clean environments may have less exposure to microbes during early life. According to the hygiene hypothesis, this reduced exposure may make the immune system more likely to react to harmless substances later on.

Other environmental factors include:

• Air pollution

• Cigarette smoke

• Changes in diet

• Reduced exposure to nature

• Climate change, which is lengthening pollen seasons in many regions

Why Can Allergies Develop Later in Life?

Many people assume allergies only appear during childhood, but adults can develop new allergies at any age.

Possible reasons include:

• Repeated exposure over many years

• Hormonal changes

• Moving to a different climate or country

• Viral infections that alter immune responses

• Changes in the gut microbiome

  
  

Food Allergies vs Food Intolerances

People often confuse these two conditions.

A food allergy involves the immune system and can potentially cause life-threatening reactions.

A food intolerance does not involve the immune system. Instead, it usually results from difficulty digesting certain foods—for example, lactose intolerance due to reduced lactase enzyme.

While intolerances can be uncomfortable, they generally do not cause anaphylaxis.

Can Allergies Be Prevented?

There is no guaranteed way to prevent allergies, but research suggests that certain strategies may help reduce the risk.

Current evidence supports:

• Avoiding smoking during pregnancy and around children

• Breastfeeding where possible

• Introducing common allergenic foods (such as peanuts and eggs) during infancy when appropriate and according to current medical guidance

• Maintaining a healthy, diverse diet

• Spending time outdoors and encouraging normal environmental exposure during childhood

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How Are Allergies Treated?

Treatment depends on the type and severity of the allergy.

Options include:

• Avoiding known allergens

• Antihistamines

• Steroid nasal sprays

• Asthma inhalers when needed

• Adrenaline (epinephrine) auto-injectors for people at risk of anaphylaxis

• Allergen immunotherapy ("allergy shots" or tablets), which gradually retrains the immune system to become less reactive over several years

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The Bottom Line

Allergies develop when the immune system mistakenly labels a harmless substance as dangerous. After becoming sensitised, future exposure triggers the release of histamine and other inflammatory chemicals, producing symptoms that range from mild sneezing to severe allergic reactions.

Although allergies can't always be prevented, modern treatments are highly effective, and for some people, immunotherapy can significantly reduce symptoms—or even change the course of the disease.

Understanding how allergies develop is the first step toward managing them safely and effectively.

This article is for educational purposes only and should not replace personalised medical advice. If you experience difficulty breathing, swelling of the face or throat, or signs of anaphylaxis, seek emergency medical care immediately.

For more medical content, visit Askadoc Youtube.

Stay healthy,

Dr. Joanna