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What Is an Allergy? Definition, Symptoms, and Diagnosis

Medically Approved by Dr. Edward Salko

Table of Contents

An allergy is the body’s hypersensitive immune response to a foreign substance. The most common substances that trigger allergic responses (a.k.a. allergens) include pet dander, pollen, molds, or insect bites. Certain foods, drinks, or chemical substances can also cause allergic reactions.

With millions of Americans suffering from allergies, knowing how an allergic reaction occurs, its diagnosis and treatment, as well as management practices, are imperative to prevent its life-threatening complications. In this guide, we spell out the basics of allergy formation and other crucial information you should know to help you better work with your allergist. 

How Does an Allergy Occur?

When a person comes in contact with an allergen, the reaction to the substance is typically not immediate. 

Over time, the immune system builds up sensitivity to the allergen, which it has identified as a foreign body. This process is known as sensitization(1), lasting anywhere from a few days to several years. During this time, the immune system prepares to fight against these foreign bodies by producing antibodies.

Against common belief, allergens are not directly responsible for the symptoms associated with an allergic reaction. The immune system produces histamines that actually cause the inflammation and irritation typically associated with an allergic reaction(2)

Histamines cause the tightening of muscles, including those in the airways and the walls of blood vessels, and cause the nose to produce more mucus.

Spring allergy to pollen.

The production of histamines can cause tightening of the airways.

The severity of a reaction depends on the type of allergen and the area where the allergy is targeted. An allergic reaction can occur in the gut (digestive system), skin, sinuses, eyes, airways, or nasal passages.

Risk Factors for Allergies

You can assess the likelihood of developing allergies by scrutinizing your hereditary inclination and environmental exposure. While fairly common in the US, not everyone might experience allergies in their lifetime. The following factors can determine your risk of developing allergies. 

  • Family history (genetics)

If anyone in your family has an allergy, there is a good chance that you can have one yourself. This is because the immune defect(3) causing allergic reactions is genetic. Hence, keep track of items and substances your family members are allergic to. 

  • Prolonged allergen exposure

Your length of exposure to allergens can also influence your susceptibility to developing an allergic reaction. For example, if you live in a place with high pollen production during spring, you 

  • Existing atopic conditions and asthma

In addition to hereditary factors, people with a preexisting atopic disease or asthma are more susceptible to developing additional allergies. This is because the same DNA defect that typically causes these atopic conditions plays a crucial role in allergen sensitization.  

  • Cesarean birth 

Data suggests that children born via the cesarean section(4) have a higher risk of developing food allergies than those who were born through normal delivery. The likelihood also increases if the parents have a history of allergies. 

Ignoring these allergy symptoms can lead to health issues that require immediate attention. The most popular and alarming complication of an allergy is anaphylaxis – this is an acute form of immune hypersensitivity that can be life-threatening. An anaphylactic shock due to allergy can lead to multi-organ failure, which can be rapidly fatal. 

If you tick all the boxes above, it’s better to be mindful and observant of how your body reacts to certain foods, items, and environments. In doing so, make sure to be cognizant of allergy symptoms and how they typically appear after allergen exposure.

Common Allergy Symptoms

The degree to which allergy symptoms manifest depends on the severity of the immune reaction to the specific allergen. Nonetheless, you might experience the following if you have an allergy.

Respiratory allergy symptoms

  • Coughing
  • Wheezing 
  • Sneezing
  • Runny nose
  • Shortness of breath

Digestive allergy symptoms

  • Stomach cramps
  • Nausea
  • Vomiting
  • Diarrhea 

Dermal (skin) allergy symptoms 

  • Rash
  • Hives
  • Swelling 
  • Eczema 

Optical (eye) allergy symptoms

  • Itchiness
  • Watery eyes
  • Redness 

Other common allergy symptoms

  • Headache 
  • Fatigue 

As many of the common allergy symptoms can also indicate other medical conditions, it’s imperative to confirm the condition by keeping track of the potential allergen you have been exposed to. More than that, however, it’s crucial to undergo allergy testing, especially if you have experienced the symptoms on multiple occasions. 

Allergy Diagnosis 

Allergies can be diagnosed through blood, skin prick, or patch tests – the first two being the most common form of diagnostic tests. The procedure will be performed by a doctor or, in the case of a blood test, by a registered phlebotomist. 

Types of allergy testing

Different types of allergy testing methods

An allergy blood test requires a blood sample after exposure to the allergen to determine the level of IgE antibodies, which are produced by the immune system if you have an allergy. 

On the other hand, the skin prick or scratch test is done by introducing an allergen to your skin via a small prick. The allergic reaction will confirm immune hypersensitivity to the allergen. If you want to learn which of these two tests you should consider, check out our guide on blood tests vs skin tests for allergies.

A patch test is similar to a skin test in that the allergen comes in contact with the skin. But instead of a prick, a patch that contains the allergen will be placed on your skin. While this test is pain-free, it might take a few days for the allergy to develop. 

Treatment for Allergies 

Treating an allergy for good may come a long way. In many cases, managing the condition by avoiding the allergen is the best way to protect yourself from flare-ups. However, your doctor will typically prescribe you a medication to mitigate the allergic reaction once you are exposed to a trigger, which could be any of the following.

  • Over-the-counter antihistamines or histamine blockers
  • Decongestants 
  • Eye drops
  • Nasal and topical corticosteroids (with prescription)
What is an allergy shot? Also known as immunotherapy, an allergy shot or injection is the process of gradually introducing the allergen to your bloodstream with the goal of helping your immune system be more tolerant to the foreign substance. It is designed as a long-term allergy treatment, that reduces (if not eliminates) the allergy symptoms. 

How to Manage Allergy Symptoms?

The first key step to managing allergies is identifying which allergens can trigger a reaction. After you’re diagnosed through allergy testing, make sure to minimize your exposure. 

For example, if you are allergic to certain foods, eliminate them from your diet and be mindful not to accidentally come across the food when eating outside. On the other hand, if you are allergic to latex, educate yourself about products that contain such material. If you have a tree pollen allergy, consider staying indoors during spring or pollen season.

What Are the Most Common Allergies in the US?

According to the American Academy of Allergy, Asthma, & Immunology, more than 100 million Americans(5) are diagnosed with with allergies in the US annually. In 2021 alone, the Centers for Disease Control and Prevention (CDC) reported that one in three adults and about one in four children(6) suffers from at least one allergy. 

The following are some of the most common allergies in the US:

  • Milk allergy
  • Peanut allergy
  • Egg allergy
  • Tree nuts allergy
  • Wheat allergy
  • Soy  allergy
  • Fish allergy
  • Sesame allergy
  • Pollen allergy
  • Mold allergy
  • Animal dander allergy
  • Dust mites allergy

Food substances are widely considered the most common triggers for allergic reactions. While milk, peanuts, and wheat are common allergens, there are other rare food allergies that you might find quite unexpected. Check out these allergies and how to detec them. 

Frequently Asked Questions

How do I know if I’m allergic?

Detecting an allergy starts with identifying common allergy symptoms like rash, coughing, sneeing, and itchiness. However, you cna only confirm if you’re allergic to a specific substance if you undergo allergy testing, as facilitated by an allergist or immunologist. 

How long do allergies last?

How long an allergy flare-up lasts depends on the type and severity. For example, acute allergy symptoms may last within minutes or hours, depending on how fast it is treated. On the other hand, chronic allergies symptoms may be ongoing or recur within days or weeks. In some cases, it could extend to months. 

Can allergies be cured? 

There is still no definitive cure for allergies. However, you can manage the condition and avoid allergic reactions through medications, allergy shots, and avoiding the allergen that trigger the symptoms. 

How is asthma different from an allergy? 

Asthma is a respiratory condition characterized by the chronic inflammation in the trachea (airways). On the other hand, an allergy is the overreaction of the immune system, in which symptoms can extend beyond respiratory nature. However, they overlap in the sense that an asthma attack can also be triggered by allergens. But it can also be due to non-allergic elements. 

The Bottom Line

Understanding the nature of allergies, how they are formed, diagnosed, and treated sets the first steps to proper management of symptoms and prevention of sudden flare-ups. While allergy symptoms provide crucial information about the condition, getting tested for allergies is the most surefire way to detect the specific allergen that causes the immune overreaction.


1 van Ree R, Hummelshøj L, Plantinga M, Poulsen LK, Swindle E. Allergic sensitization: host-immune factors. Clin Transl Allergy. 2014 Apr 15;4(1):12. doi: 10.1186/2045-7022-4-12. PMID: 24735802; PMCID: PMC3989850.

2 Church MK. Allergy, Histamine and Antihistamines. Handb Exp Pharmacol. 2017;241:321-331. doi: 10.1007/164_2016_85. PMID: 28101683.

3 Sokol K, Milner JD. The overlap between allergy and immunodeficiency. Curr Opin Pediatr. 2018 Dec;30(6):848-854. doi: 10.1097/MOP.0000000000000697. PMID: 30407976.

4 Yang X, Zhou C, Guo C, Wang J, Chen I, Wen SW, Krewski D, Yue L, Xie RH. The prevalence of food allergy in cesarean-born children aged 0-3 years: A systematic review and meta-analysis of cohort studies. Front Pediatr. 2023 Jan 17;10:1044954. doi: 10.3389/fped.2022.1044954. PMID: 36733768; PMCID: PMC9887154.

5 AAFA Allergy Statistics. Asthma and Allergy Foundation of America. Retrieved February 14, 2024, from
6 Centers for Disease Control and Prevention (2024, January 28). Diagnosed Allergic Conditions in Children Aged 0–17 Years: United States, 2021. National Center for Health Statistics. Retrieved February 14, 2024, from

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