Allergies and the Immune System: What Happens Inside Your Body 

Allergies are the worst. A nice walk outside? Not while there’s a high pollen alert. A quick cuddle with a corgi? No thank you, pet dander. A large bite of lobster Rangoon? Hope you brought your EpiPen. 

Count yourself lucky if you don’t suffer from seasonal or perennial allergies because, for the rest of us, they’re like a “Must Be This Tall to Ride” sign barring us from life’s simple pleasures.  

So, what makes the human body go haywire every allergy season? In this article, we’re exploring the physiological reactions following the body’s exposure to an allergen.  

Let’s take an in-depth look at allergies and the immune system. 

What Causes Allergies? 

Allergies happen when your immune system misidentifies a harmless substance (allergen) as a dangerous invader. This mix-up triggers a protective chain reaction designed to defend your body. 

When an allergen is inhaled, eaten or absorbed through the skin, your immune system may respond by producing special, allergen-specific immunoglobulin E (IgE) antibodies. These IgE antibodies latch onto mast cells: immune cells that act like tiny chemical alarm systems. This primes your body to react the next time you encounter that allergen. 

After First Exposure: Why the Second Time Hits Harder 

The first time you’re exposed to an allergen, you might not feel much. That’s because your body is essentially “taking notes” and building its defense system. But the second time? That’s when symptoms hit. The allergen latches onto those primed IgE antibodies, triggering mast cells to release inflammatory mediators—and causing your allergic reaction. 

One of the best known mediators is histamine, and it’s a big reason why allergies cause sneezing, itchy or watery eyes, a runny nose and swelling. 
Common allergy symptoms include: 

• Swelling of lips, face or eyes 
• Sneezing and runny nose 
• Itchy skin, eyes or nose 
• Watery, red or swollen eyes (allergic conjunctivitis) 
• Wheezing, coughing or trouble breathing (especially if asthma is involved) 

Symptoms of allergies range from mild to severe, depending on the allergen, the dose and how reactive your immune system is. 

Anaphylaxis: When It’s an Emergency 

Sometimes the body doesn’t just “react”—it goes full alarm mode, triggering anaphylaxis. This potentially life-threatening allergic reaction can involve airway swelling and a dangerous drop in blood pressure. Epinephrine (adrenaline) is the first-line treatment. That’s why people with a known risk often carry an epinephrine auto-injector like EpiPen. 

Why Do Some People Have Allergies (And What Can You Do)? 

Why is it that one person breathe through spring like nothing’s happening while someone else gets taken out by a light breeze? It’s complicated, but IgE-driven allergy mechanisms are a major part of the story. Once your immune system builds that allergen-specific IgE pathway, your symptoms can repeat every time you run into that trigger. 

For symptom relief, antihistamines work by blocking histamine’s effects at H1 receptors, which can help reduce common allergy inflammation and discomfort. There are a handful of solutions out there, ranging from over-the-counter options like Zyrtec, Claritin and Allegra to prescription antihistamines. (The writer of this post is getting weekly allergy shots and desperately hoping that it cures him of his seasonal allergies.) 

Key Takeaways

 Before you brave the elements this allergy season, keep these facts in mind: 

• Why is the second exposure to an allergen usually worse than the first?  
  The first time you’re exposed, your body “takes notes” and produces IgE antibodies that attach to mast cells. On the second exposure, the allergen latches onto these antibodies, triggering the mast cells to release inflammatory chemicals like histamine. 
  
• What are common allergy symptoms?  
  Symptoms can range from mild to severe and include sneezing, a runny or itchy nose, swelling of the lips or eyes, itchy skin and watery or red eyes (allergic conjunctivitis). 
  
• What is anaphylaxis?  
  Anaphylaxis is a “full alarm” emergency reaction. This life-threatening response can involve airway swelling and a dangerous drop in blood pressure. It requires immediate treatment, usually with an epinephrine auto-injector like an EpiPen. 
  
• How do antihistamines work?  
  Antihistamines provide relief by blocking histamine’s effects at the H1 receptors in your body. This helps reduce the common inflammation, itching and sneezing caused by the allergic response. 

Explore How the Human Body Responds to Triggers 

We’ve given you a glimpse of how the body reacts to allergens. But if you really want to understand the body’s complex processes when it encounters microscopic invaders like allergens, you’ll have to take your studies further.  

The University of Florida offers a suite of online medical sciences graduate programs for aspiring medical professionals. If this look at allergic reactions sparked your curiosity about how the body works, our medical anatomy and physiology program digs deeper. In our medical physiology and pharmacology program, you can explore how drugs like antihistamines interact with the body. 

Check out the rest of our online programs if those two don’t pique your interest. When you’re ready, apply to UF. Allergies may be seasonal, but our programs accept students year-round.  

Sources: 
https://www.hopkinsmedicine.org/health/conditions-and-diseases/allergies-and-the-immune-system 
https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/allergies 
https://www.mayoclinic.org/diseases-conditions/allergies/symptoms-causes/syc-20351497