What Causes Allergies?

Allergies come in many forms and can be caused by a variety of factors. Allergies occur when your body mistakes an unharmful substance for a harmful one, triggering an unnecessary immune response known as an allergic reaction.

You can experience an allergic reaction to things in the environment, such as pollen, pet dander, mold, insect venom, and dust mites, as well as things you consume—from certain foods to medication. Each allergy can lead to a range of symptoms, such as sneezing, watery eyes, skin rashes, and itching.

Here we’ll discuss what causes someone to develop allergies and what can trigger common allergic responses.

Immune System Reaction

Allergies are caused by your body's immune system reacting to a harmless substance known as an allergen. Your body’s immune system mistakes these substances for dangerous invaders and sends special immune proteins—called antibodies—to fight them off. This leads to an allergic reaction.

The terms “allergy” and “hypersensitivity” are often used interchangeably. However, hypersensitivity describes the actual immune system reaction, while an allergic reaction usually refers to the signs and symptoms you might experience. 

There are two main types of allergies—immunoglobulin E-mediated (IgE-mediated) hypersensitivities and non-IgE mediated hypersensitivities.

IgE-Mediated Allergies

IgE-mediated allergies are also called type 1 hypersensitivity. The first time you are exposed to an allergen that you are sensitive to, your body makes a large amount of IgE antibodies—a type of immune protein. These antibodies attach to other immune system cells called mast cells. 

The second time you are exposed to the same allergen, the mast cells release immunoglobulin E (IgE) and other chemicals, including histamine. This release leads to classic allergic reaction symptoms, like wheezing, trouble breathing, hives, and runny nose.

Severe, life-threatening reactions are known as anaphylaxis. In particular, anaphylaxis due to type I hypersensitivity is called anaphylactic shock. Anaphylaxis can lead to trouble breathing, a rapid heart rate, decreased blood pressure, and other symptoms. Anaphylaxis is life-threatening and requires immediate medical intervention.

Non-IgE-Mediated Allergies 

A non-IgE response involves your immune system but does not involve IgE antibodies. While IgE-mediated responses are immediate or almost always occur within two hours, non-IgE-mediated responses are delayed. These reactions may occur hours to days after exposure to an antigen (a toxic or foreign substance that triggers an immune response).

While anaphylaxis may also occur with this type, it is much more rare than with IgE-mediated responses. The symptoms are similar to anaphylaxis and requires immediate medical intervention.

Common Allergens

Along with how your immune system reacts, the allergen also plays a part in triggering the allergy. The most common allergen types are food, latex, insect venom or droppings, and environmental allergens such as pollen and mold.

Food

Any food can trigger an allergy. However, almost 9 out of 10 food allergies come from the following food:

• Milk and dairy
• Eggs
• Peanuts
• Tree nuts, such as almonds, cashews, and walnuts
• Soy
• Wheat
• Sesame
• Fish
• Shellfish, such as crab, shrimp, and lobster

Usually, certain proteins in the food are the main allergen that triggers an allergy. Food allergies can be IgE-mediated or non-IgE-mediated. IgE-mediated food allergies are more common and cause symptoms within two hours of eating the food. These symptoms may include nausea, abdominal pain, a rash, itching, and swelling in the lips, tongue, nose, throat, or skin. 

Non-IgE food allergies are uncommon and tend to affect babies or toddlers. This type of food allergy usually causes gastrointestinal symptoms. Examples include:

• Food-protein-induced enterocolitis syndrome (FPIES): This usually occurs in babies less than 9 months old. Symptoms include vomiting, diarrhea, and dehydration and typically occur and resolve within 24 hours.
• Food-protein-induced allergic proctocolitis (FPIAP): This allergy is also known as allergic proctocolitis and leads to inflammation of the lower intestine in the gut. This is usually caused by an allergy to cow milk or soy and appears in babies before they turn one year old.
• Food-protein-induced enteropathies: This allergy affects babies and toddlers aged 3 years or younger. Symptoms include diarrhea, oily stool (steatorrhea), and faltering growth (slower weight gain in babies). Cow milk, soy, and rice are the most common triggers.

Environment

Some environmental allergens that can trigger allergies in some people include:

• Pollen from grass, trees, and weeds
• Some molds that often thrive in damp, indoor places, such as Alternaria, Aspergillus, Cladosporium, and Penicillium 
• Pet dander (skin flakes) in animal fur or feathers
• Cockroaches, particularly a protein from the insect’s saliva, fecal matter, and body parts 
• Dust mites (tiny bugs that live in dust), specifically a protein in their fecal matter

These allergens travel through the air and can lead to sneezing, itchy eyes, runny nose, congestion, and other allergy symptoms.

Latex

Latex, also called natural rubber latex, is a flexible material that’s made from the sap of the Amazonian rubber tree (Hevea brasiliensis). Everyday items, such as rubber gloves, balloons, and toys can be made of latex. When someone with a latex allergy comes into contact with these products, it can lead to an IgE-mediated allergic reaction. Symptoms of a latex allergy can include skin rashes, swelling, wheezing, and sneezing.

Insects

Whether a sting or a bite, the main allergen of insect-related allergies is insect venom.

Stings from insects like bees, wasps, and hornets can trigger allergic reactions in some people. Symptoms of an insect sting allergy include swelling around the eyes and lips, hives, and difficulty breathing or wheezing. These allergies can range from mild to life-threatening anaphylaxis. About 85% of insect sting allergies are IgE-mediated reactions.

Insect bites from mosquitoes, fleas, bedbugs, and other insects typically cause mild symptoms such as itching, redness, and swelling on the affected skin. Anaphylaxis is less common. Insect bite allergies may be IgE-mediated or non-IgE-mediated, depending on the insect and person.

Medication

People with a drug allergy have an overactive immune reaction to a medication. 

Drug allergies can be IgE-mediated or non-IgE-mediated. Symptoms can vary by the type of allergy:  

• IgE-mediated: Often, the first exposure to a drug may cause no symptoms and prompts the immune system to become sensitive to it. On the second exposure to the drug, you can experience immediate symptoms, usually within a few minutes to an hour. Common symptoms may include an itchy rash, or in severe cases, signs of anaphylaxis, like wheezing, trouble breathing, and swelling in the mouth and throat.
• Non-IgE-mediated: This is less common compared to IgE-mediated drug allergies. There are several types of non-IgE-mediated allergies, and they can involve other types of immune cells and antibodies that are not IgE. Some allergies may arise from high doses or long-term and frequent use of the drug, such as with nonsteroidal anti-inflammatory drugs (NSAIDs). Symptoms can be varied, including low blood cell counts, inflammation of the blood vessels, fever, or more.

Some drug allergies can be a combination of different immune responses. For example, consider allergies to penicillin—a type of antibiotic (or drug that treats bacterial and fungal infections). Penicillin allergies can be IgE-mediated, non-IgE-mediated, or both.

Not all adverse drug reactions are necessarily allergies—some may be due to side effects of the drug or health complications from an underlying health condition. For example, while some people taking chemotherapy drugs may experience an allergy, including anaphylactic shock, other people may experience non-allergic symptoms and side effects such as hair loss and nausea.

Non-allergic reactions to drugs are much more common than allergic reactions. For example, antibiotics are a commonly reported drug allergen, yet only about 2% of reported allergies in hospital admissions turn out to be actual allergies upon further testing.

At regular visits or before any treatment, your healthcare provider will ask about any medication allergies, followed by the signs and symptoms of a reaction. This information can help your provider determine whether you have an allergic or non-allergic reaction. 

Hygiene Hypothesis

While health experts and researchers understand that allergies are caused by a combination of an overactive immune response and exposure to an allergen, the hygiene theory explores why some people are more prone to develop allergies than others.

The hygiene hypothesis, sometimes called the hygiene theory, suggests that some children might be too protected from germs. Particularly if their living conditions are too clean, some children might not be exposed to the same kinds of microbes that help teach their immune systems to know the difference between harmful and harmless things. 

This theory suggests that many parts of the world have living conditions that are too clean. Those who live on farms or are exposed to farm animals and the environment may have stronger immune systems because they've had that extra exposure.  

However, more research is needed to study this hypothesis.

Are Allergies Hereditary?

Allergies often have a family connection, but that doesn't mean you will have the same ones as your parents or siblings. Oftentimes, allergies can be hereditary, meaning they can be passed down from one generation to the next. Those who have allergies could experience different symptoms than their relatives. Less commonly, you can have an allergy even if no one in your family has a history of allergies.

Who Gets Allergies?

Anyone can have allergies, though certain factors might make you more likely to develop an allergy to common allergens. These include:

• A family history of allergy
• Exposure to household allergens, like dust mites or mold
• Certain jobs that may have latex gloves or other items (e.g., healthcare, lab research) 
• Having a history of another allergy or an allergic condition such as asthma, hay fever (also called allergic rhinitis), or eczema (a skin condition) 

Keep in mind that finding the exact risk factors for an allergy is not easy. This is because your immune system is complex, and multiple factors can contribute to an allergic reaction.

A Quick Review

Allergies are caused by an overactive immune system reacting to certain triggers known as allergens. Some common allergens are pollen, insect venom, dust mites, certain foods, some medications, and more. 

While the exact causes of each allergy are still being studied, allergies are generally due to a combination of an immune response and environmental factors. People who have family members with allergies might also be more likely to develop allergies.

Visit a healthcare provider to find out what's causing an allergy and how to manage it.