What Are Allergies, Exactly?

This informational guide, part of POPSUGAR's Condition Center, lays out the realities of this health concern: what it is, what it can look like, and strategies that medical experts say are proven to help. You should always consult your doctor regarding matters pertaining to your health and before starting any course of medical treatment.

Allergies are incredibly common. They're the sixth leading cause of chronic illness in the US and affect more than 50 million people every year, reports the Asthma and Allergy Foundation of America (AAFA). While some allergies are easy to manage, others cause severe symptoms. Food allergies alone are responsible for roughly 30,000 emergency department visits and around 200 deaths every year, according to the Center for Disease Control and Prevention (CDC).

If you've ever experienced an allergic reaction before, then you know how inconvenient (and scary) the symptoms can be — from unexpected hives and itchiness to severe swelling of the mouth. But what causes allergies in the first place? What treatment options are the best? And what's the best diagnostic method? Ahead, 2 allergists weigh in.

Allergies are the result of the body's immune system reacting to a substance, says Neha Sanan, DO, allergist and immunologist. The body identifies the substance as harmful and releases immune chemicals like histamine, which leads to allergy symptoms, says Shawn Nasseri, MD, Mayo Clinic trained ENT and allergy expert.

Allergies can be experienced by anyone and can develop at any time in life. They might be seasonal (as with pollen allergies) or perennial (occurring year-round). But the symptoms a person experiences depends on the type of allergen they're exposed to, and can range from itchiness, hives, sneezing, or diarrhea to severe swelling of the mouth, nose, and throat.

The most severe type of reaction is known as anaphylaxis, which is potentially fatal. It happens when an allergic reaction causes the body goes into shock, leading to a drop in blood pressure and a narrowing of the airways, according to Mayo Clinic. It can occur within seconds, which is why people with severe allergies often carry epinephrine autoinjectors (commonly referred to as an EpiPens); EpiPens inject the body with epinephrine, which keeps the airways open for long enough for the person to get to the emergency room.

The type of allergy you have is determined by how your body reacts to certain allergens. In other words, allergies are essentially an immune reaction to a foreign substance or allergen; when you're exposed, you have a reaction. Common allergens, according to the Mayo Clinic, include:

Generally, there are factors that can make a person more likely to develop allergies in their lifetime.

If you think you have allergies, visit an allergist to determine what you're allergic to. Unchecked allergies can lead to inflammation and infection, says Dr. Nasseri: "Things like ear infections and sinus infections can start to occur more frequently, leading to asthma or structural sinus issues."

Allergies can be diagnosed in a number of ways. An allergist will typically perform a skin prick or blood test, two of the most common allergy tests out there. A skin prick involves pricking the skin with several different allergens to see how the body reacts, while a blood test involves taking blood samples to examine IgE antibodies, which are produced during an allergic reaction.

When it comes to allergic reactions, preventive measures are key, Dr. Sanan says. Once you know what you're allergic to, you can make a plan for avoiding known triggers and dealing with any future exposures (such as by carrying an EpiPen or antihistamines just in case).

It may also be helpful to keep overall inflammation down, since allergic reactions can trigger inflammation, Dr. Nasseri says. He suggests eating an anti-inflammatory diet, filled with green vegetables, nuts, fish, and olive oil. That said, diet tweaks like this alone likely can't treat many allergies, like pollen allergies.

You can also ask your doctor about immunotherapy. This preventive treatment involves having a doctor give you increasing doses of an allergen (starting with a very small dose) in order to make your immune system less reactive to it, according to ACAAI. It can take time, and it must be done under the supervision of a healthcare professional. But for people with severe allergies, immunotherapy can be life-changing.

Alexis Jones is the senior health editor at POPSUGAR. Her areas of expertise include women's health, mental health, racial and ethnic disparities in healthcare, diversity in wellness, and chronic conditions. Prior to joining POPSUGAR, she was the senior editor at Health magazine. Her other bylines can be found at Women's Health, Prevention, Marie Claire, and more.