What Are Allergies? Symptoms, Causes, Diagnosis, Treatment, and Prevention

Allergies occur when your immune system overreacts to substances called allergens. Common allergens that can trigger allergic reactions include pollen, pet dander, and bee venom. People also have allergies to certain foods and medications.

Allergy symptoms vary depending on the type of allergens.

Allergic rhinitis (commonly known as hay fever), for instance, is associated with the following symptoms:

• Sneezing
• Runny nose
• Congestion
• Itchy eyes, nose, and throat
• Tearing eyes

An allergic food reaction may share some of the above symptoms, but it can also cause:

• Diarrhea, nausea, and vomiting
• Hives, eczema, or itchy skin
• Anaphylaxis, in which a narrowing of the airways makes it difficult or even impossible to breathe

A skin allergy or insect bite can cause the following at the site:

• Redness
• Swelling
• Hives
• Pain
• Itching

The symptoms of a drug allergy may include:

• Hives
• Wheezing
• Light-headedness
• Vomiting
• Swelling of the face or throat

  

Learn More About Signs and Symptoms of Allergies

Your risk of developing allergies is higher if you:

• Have asthma
• Have a family history of asthma or allergies
• Are younger than 18

Children sometimes outgrow allergies as they get older. It's also not uncommon for allergies to go away and then return years later.

Allergic Reactions

Allergens are typically harmless substances that trigger an immune response and cause a reaction in people who are allergic. The allergic reaction occurs if the person inhales, touches, swallows, injects, or somehow comes into contact with the allergen. Allergic reactions can be mild, severe, or even life-threatening.

Normally, the immune system protects the body against harmful substances, such as viruses or bacteria. If you have allergies, "your body responds to allergens as if they were invaders," explains Clifford Bassett, MD, a clinical assistant professor at New York University Langone Health in New York City and the author of The New Allergy Solution. "Your body exaggerates the immune response. That's what causes histamine release and other things that cause allergy misery."

When your immune system reacts to an allergen, it produces an antibody called immunoglobulin E (IgE). The production of IgE is part of your body's attempt to destroy the allergen and protect itself. Your blood vessels dilate and become leaky, so white blood cells that fight infection and other protective substances leave the blood vessels to attack the invader.

In the process, the IgE antibodies signal other cells to release certain chemicals, such as histamine in the local tissue and blood stream. Too much histamine or excessive release in the body can cause an unwanted response that leads to skin, nose, throat, and lung irritation or more severe symptoms of anaphylaxis.

There are other types of allergic reactions, such as delayed hypersensitivity reactions, which includes contact dermatitis from poison ivy or a nickel allergy — these are mediated by T cells in the immune system rather than antibodies.

• Pollen
• Dust mites
• Pet dander or fur
• Cockroaches
• Mold spores
• Foods (eggs, fish, milk, nuts, wheat, soy, shellfish, and others)
• Insect stings or bites (from wasps, bees, mosquitoes, fire ants, fleas, horseflies, black flies, among others)
• Medicines (penicillin, aspirin, and others)
• Latex
• Household chemicals
• Metals (especially nickel, cobalt, and chromates)

  

Seasonal Allergies

Certain allergies can strike at any time of year. Seasonal allergies, on the other hand, occur at times of the year when certain types of outdoor allergens are predominant.

"You can have both," says Dr. Bassett.

Seasonal allergies are most often triggered by mold and pollen from trees, grasses, and weeds such as ragweed. The allergic reaction occurs during the weeks or months when the plant pollinates.

Allergy triggers may vary depending on geographic location and climate, but relocating to avoid seasonal allergies generally doesn't help. Pollen and mold spores travel great distances; and people with allergies often develop sensitivity to other allergens in a different location.

Environmental factors such as pollution and climate change associated with rising temperatures may be contributing to a rise in allergies. Changes in the duration and intensity of pollen and mold seasons mean more people are exposed to allergens for longer amounts of time. "That's a longer period of time for your eyes and nose and throat to become symptomatic as a result," Bassett says.

The first step in diagnosing allergies is an evaluation by your physician to review your symptoms and medical history and to rule out other potential medical problems. You can help your doctor make an accurate diagnosis by keeping a record of your symptoms, including when they started and what triggers them.

If allergies are suspected, your doctor may order tests. These include skin and blood tests. Your doctor may also perform a lung function test or an X-ray of your lungs.

Is It an Allergy or Something Else?

Like allergies, a cold and the flu affect the respiratory system; and they share some symptoms, such as a runny nose and coughing.

One difference between them is the itchiness associated with many allergic reactions.

"Typically, an allergy will have itchiness of the eyes, nose, and throat," says Bassett. "With a cold, you're more likely to have a sore throat, decrease in appetite, and you just don't feel well."

"If you have a cold, there's no real good treatment other than waiting five to seven days," Bassett says. But for a nonsevere allergy, "if you use an antihistamine or nasal steroid spray, they're usually very effective."

The coughing and other respiratory symptoms of COVID-19, the infection caused by the SARS-CoV-2 coronavirus, may also be confused with allergies. One notable difference is that fever, which often accompanies COVID-19 and the flu, isn't usually a sign of allergies. Another difference: Allergic coughing is typically the result of postnasal drip, unlike the dry COVID-19 cough.

Other conditions that produce allergy-like symptoms include:

• Non-allergic rhinitis, which mimics allergic rhinitis but does not involve the immune system (at least one-third of people with rhinitis symptoms don't have allergies)

  

• Sinus infections
• Nasal polyps or a deviated septum

Prognosis of Allergies

There are different prognoses for different types of allergies.

Some allergies last for a number of years and then go into remission, and others can last a lifetime, says John Bosso, MD, the director of the otorhinolaryngology allergy clinic at Penn Medicine in Philadelphia.

"For example, a certain percentage of people with asthma and allergic rhinitis can get better with time — often it improves in puberty, especially in boys," says Dr. Bosso.

There are cases where allergies go away, even if someone has had them a good part of their lives. "A person in their forties or fifties may find that they no longer react to things in the environment," he says.

"Many food allergies are transient; they can last for a few years and then go away," says Bosso. "Milk and egg allergies are frequently outgrown, but not always. Tree nut allergies are permanent in the majority of people — about 80 percent have it long-term," he says.

There are no cures for allergies, but effective treatment can reduce symptoms and improve your quality of life.

Allergy treatments vary, depending on the severity of your condition and the type of allergy you have.

Medication Options

If your allergies are severe enough to significantly interfere with your quality of life, it's a good idea to see a doctor in order to identify what you're allergic to and to gain access to the full range of prescription options.

If your allergies are less severe or merely annoying, you may be able to find an effective over-the-counter (OTC) treatment. A pharmacist may be able to help you choose the best option based on your symptoms.

OTC medicines include antihistamines and decongestants and nasal steroid sprays (which relieve congestion and previously were prescription). Cromolyn is a nasal spray used to help prevent or treat allergy symptoms, and there is also montelukast (Singulair) that is used for asthma and allergic rhinitis.

You may also need medications for asthma. If you have severe allergic reactions and are at risk for anaphylaxis, then your doctor may recommend that you carry a dose of epinephrine, a chemical that narrows blood vessels and opens airways in the lungs.

Alternative and Complementary Therapies

Complementary remedies, such as acupuncture, and nasal irrigation techniques may also help relieve allergy symptoms.

Bosso cautions people about using alternative therapies to treat allergies. "Many things that purport to treat allergies are available because in the supplement/non-pharmaceutical market you don't need to prove efficacy or that it's better than placebo, it just needs to be proven to be safe," he says.

It's often best to discuss with your primary care provider or another healthcare practitioner who is knowledgeable about integrative and complementary medicine and can help you determine if some of these therapies may support your treatment goals safely.

Learn More About Treatment for Allergies: Medication, Alternative and Complementary Therapies, and More

Prevention of Allergies

There are some measures you can take to prevent or limit allergic reactions at home. The following have varying degrees of supporting research but are generally safe and low cost:

• If you're allergic to pollen and you know pollen counts are going to be high, try to remain inside with doors and windows closed. The National Allergy Bureau provides pollen and mold updates by region.
• Use a high-efficiency particulate air filter (HEPA). HEPA filters trap airborne allergens.
• If you already have a cat or dog that you're allergic to, don't sleep near the pet, and wash your hands promptly after contact. Don't touch your eyes, nose, or mouth after touching an animal.
• If you're trying to figure out which allergens might cause or worsen your symptoms, keep a log. Write down what you eat and all your activities to help pinpoint triggers.
• Wash bedding frequently and use hot water to lessen your exposure to dust mites, pet dander, pollen, and other airborne irritants.
• If you have severe allergies, a medical alert bracelet or necklace can aid in getting you medical assistance in an emergency.

People with allergies are at risk of developing complications that range from mild to potentially life-threatening.

Anaphylaxis

One of the most serious allergic complications is anaphylaxis, which is commonly associated with allergies involving food, drugs like penicillin, and insect venom.

Symptoms of anaphylaxis may include:

• A drop in blood pressure
• Loss of consciousness
• Severe shortness of breath
• Skin rash
• Rapid or weak pulse
• Nausea or vomiting
• Light-headedness
• Severe wheezing

Asthma

Asthma is a chronic lung disease characterized by inflamed airways and trouble breathing. An asthma attack causes chest tightness, coughing, wheezing, and episodes of severe shortness of breath.

Allergic asthma causes inflamed airways to become irritated and over-respond when an irritant is inhaled, triggering an asthma attack. The muscles around the airways tighten, making the airways swell and overproduce mucus. The risk of developing asthma if you have allergic rhinitis is significant, Bassett says.

Other Allergic Complications

• Eczema (a skin condition characterized by inflammation)
• Ear or lung infections
• Sinusitis or sinus infection
• Nasal polyps (growths on the lining of the nose or sinuses)

  

• Migraine headaches

  

Black Americans and Hispanics are disproportionately impacted by allergies and asthma and tend to have more serious cases, says Bosso.  

Black and Hispanic Americans and Allergies

Eczema

Obstructive Sleep Apnea (OSA)

RELATED: What Is Eosinophilic Esophagitis (EoE)? Symptoms, Causes, Diagnosis, and Treatment

American Academy of Allergy Asthma & Immunology (AAAAI)

The AAAAI is a professional association of immunologists, asthma specialists, and allergists. The organization offers educational resources, a symptom test to determine if you suffer from allergies or asthma, and a tool to help you locate a specialist in your area.

Asthma and Allergy Foundation of America (AAFA)

This not-for-profit organization is dedicated to saving lives and reducing the burden of asthma and allergies through support, advocacy, education, and research. It also has local chapters and support groups throughout the country.

Food Allergy Research & Education (FARE)

FARE’s mission is to improve quality of life and health for Americans with food allergies. The group advocates for research and provides resources and support for people in the food allergy community.

Additional reporting by Becky Upham.