Allergic Reactions: Types and Signs to Look For

January 2024


There’s a common misconception that allergies only happen in the springtime.  The reality is that allergic reactions can happen all year round.  Let’s unpack this more. Before we dive into allergic reactions and what they look like, let's review some foundational allergy info.

First, let’s define allergies. An allergy is a hypersensitive immune response to normally harmless substances, such as pollens or foods. For most people, these substances—also called allergens—pose no problem.

In allergic individuals, however, the immune system identifies them as a threat and produces an inappropriate (i.e., abnormal) response.¹  Hundreds (yes hundreds) of ordinary substances may trigger an allergic reaction in someone who is sensitized to that substance.

Symptoms of allergic reactions cover a range of allergy types and symptoms from watery eyes and a runny nose to eczema (i.e., atopic dermatitis) and anaphylaxis. There are many variables involved that can push symptom severity from one end of the spectrum to the other.

To help provide a better understanding of allergic reactions and how to manage them, we’ll explore the types, causes, and treatments.

What is an allergic reaction?

Let’s bring in some science to clarify. When someone is exposed to an allergen (think pollen, pet dander, milk, etc.) and they become sensitized, the body then produces antibodies called Immunoglobulin E (IgE). The next time the person encounters the same allergen, it binds to the IgE antibodies and causes the mast cell to release histamine and other chemicals.¹

This reaction causes inflammation and triggers allergy symptoms that can range from mild to severe, affecting the skin, gastrointestinal tract, cardiovascular system, and/or respiratory tract.¹

What are the symptoms of an allergic reaction?

The symptoms of an allergic reaction vary from mild to severe and depend on various factors, including the type of allergic trigger and an individual’s immune response.

Common allergic reactions include: 2-3

  • Runny nose
  • Stuffy nose
  • Sneezing
  • Wheezing
  • Shortness of breath
  • Dry, persistent cough
  • Rash (i.e., eczema, contact dermatitis, and hives)
  • Fatigue due to untreated allergies
  • Headache
  • Nausea and vomiting (two symptoms of food allergies)
  • Anaphylaxis (a severe and potentially life-threatening allergic reaction)

If you have asthma, you may also experience symptoms such as:4

  • Difficulty breathing
  • Chest tightness or pain
  • Audible wheezing or whistling when exhaling
  • Trouble sleeping caused by shortness of breath, coughing, or wheezing

If you know which allergens you are sensitized to, you can search our Allergen Fact Sheets to find specific reactions and how to help mitigate them.

How long does an allergic reaction last?

Experiencing symptoms like the ones we’ve talked about can range from slightly uncomfortable to scary, especially when you aren’t sure of the cause, how long the reaction will last, or if the reaction will get worse.

An allergic reaction can last for as long as you are exposed to the allergen. Once you are no longer exposed, your symptoms should go away within a few hours. However, if your reaction is caused by an inhalant allergen, like pet dander, dust mites, or pollens, symptoms can last for multiple days or longer.5

Here are a few of the most common allergen types and related symptoms.


Sneezing, watering eyes, and runny nose in the spring? Those reactions—sometimes called hay fever—may be due to pollen floating in the air. And because trees, weeds, and grasses all pollinate during different months, it can feel like one long ongoing battle—hence the name allergy season.


You can also have those symptoms in the fall and winter, too. Mold, dust mites, and even small amounts of dog and cat dander can trigger the same reactions.


Although most pet allergy symptoms occur within minutes of exposure, symptoms in some allergic individuals build over time and become most severe eight to 12 hours later.6

girl sneezing holding cat


Things get even more complicated when we start talking about food allergies. Most food-related reactions occur within two hours of ingestion—often starting within minutes of eating the suspected allergen. In some cases, the reaction may be delayed by four to six hours or even longer.7

The best way to sort out how long allergic reactions will last is to talk to your healthcare provider. While we can provide estimates, allergy sensitizations and the way our bodies react to them just aren’t an exact science that can be applied to everyone.

How to treat allergic reactions

Treating an allergic reaction will depend on the type of reaction. If you are experiencing anaphylaxis, call an emergency hotline. In some cases, you may be prescribed an epinephrine auto-injector (e.g., EpiPen) to use for severe allergic reactions. Follow your healthcare provider’s instructions for use. Your provider may also prescribe periodic allergy shots, which are typically administered at the doctor’s office.

Over-the-counter medications may also be recommended, to manage symptoms, and can include:8

  • Antihistamines (e.g., Benadryl, Claritin, Allegra)
  • Corticosteroids (e.g., Cortizone 10 Cream, Cortaid)
  • Decongestants (e.g., Sudafed, Mucinex)

How do doctors test for allergic reactions?

We’ve covered allergy sensitization, reaction types, and even some things you can do to treat allergic reactions. But how do you know what you are allergic to? Glad you asked! If you suspect you have a sensitization to an allergic trigger, it’s important to make an appointment with your healthcare provider.

There are a few types of allergen sensitization tests, including skin-prick tests and blood tests. The blood test, called a specific IgE blood test, can test for more than 500 whole allergens and mixes—such as pollens, foods, and animal dander—with a single blood sample.

Once you know what allergens your body is sensitized to, you can work with your healthcare provider to create a treatment plan to help reduce exposure and manage allergic reactions when they do happen. Know your triggers, avoid exposure, and treat reactions when they do happen—and you’ll be ready for anything.

Learn how to get an allergy test.

  4. Discover the connection: Reduce Exposure to Your Allergic Triggers. Thermo Fisher Scientific Inc. 2018. 

Tools for Understanding Allergies


Track allergy symptoms and prepare for a visit with a healthcare provider.

Learn about specific allergens, including common symptoms, management, and relief. 

Are you a healthcare provider? Get comprehensive information on hundreds of whole allergens and allergen components.