Allergy

An allergy is when the body’s defense system reacts too strongly to something that is usually harmless. This can happen with things like pollen from plants, certain foods, insect stings, or even materials like latex. When this happens, it can cause uncomfortable symptoms such as red eyes, an itchy rash, sneezing, coughing, a runny nose, trouble breathing, or swelling.

Allergies are quite common. Many people have allergies that affect their nose and eyes. Food allergies affect some adults and children. Some people also have skin irritation, and others may have trouble breathing. In severe cases, a reaction can become very dangerous very quickly, a condition called anaphylaxis.

Doctors figure out if someone has an allergy by asking about their health history and sometimes doing tests on the skin or blood. While there is no cure, people can manage allergies by avoiding what triggers them and using medicines like steroids or antihistamines. For very serious reactions, a special medicine called adrenaline can help save a life. In some cases, a treatment called allergen immunotherapy can help by slowly getting the body used to small amounts of the allergen.

Signs and symptoms

Allergies happen when the body thinks something harmless, like dust or pollen, is dangerous. This can cause many different signs and symptoms. For example, when tiny particles get into the air we breathe, they can make our eyes red, our noses itchy and runny, and make us sneeze. These are signs of allergies like hay fever. Sometimes, breathing in allergens can also make it hard to breathe, cause coughing, or create a wheezing sound.

Allergies can also come from things we eat, like certain foods, or from insect stings and some medicines. Food allergies might cause stomach pain, bloating, vomiting, diarrhea, itchy skin, or red, raised spots called hives. In some cases, an allergy can affect many parts of the body all at once. This is called anaphylaxis and it is important to get help right away if this happens.

The skin can also react to allergens that touch it, like latex. This can cause rashes, swelling, or red, itchy spots. When an insect sting breaks the skin, it can cause a large red area that lasts a day or two. Allergies affecting the airways, like hay fever or asthma, can cause sneezing, a stuffy or runny nose, itching around the eyes, and trouble breathing. Common triggers include pollen, dust, animal fur, and mold. Some reactions happen right away, while others take a few hours. Severe reactions that make breathing very hard need quick medical help.

Cause

Risk factors for allergies are split into two groups: personal factors and environmental factors. Personal factors include family history, gender, ethnicity, and age. Family history is the most important. But genetics alone can't explain why allergies are increasing.

Environmental factors include early childhood infections, pollution, allergen levels, and diet.

Dust mites

Dust mite allergy is common. It can cause asthma or itching. The allergens come from mite droppings. Using dust mite-proof covers for beds can help reduce exposure.

Foods

Many foods can cause allergic reactions. The most common are cow's milk, soy, eggs, wheat, peanuts, tree nuts, fish, and shellfish. Children may outgrow some food allergies, like egg or peanut allergies. Milk-protein allergies are common in children but different from lactose intolerance. Tree nut allergies can involve many types of nuts.

Latex

Latex can cause allergic reactions, especially in healthcare workers. The most common reaction is skin irritation, but serious reactions can happen. Latex allergy can also make someone sensitive to foods like banana, avocado, kiwi, and chestnut.

Medications

Some people report being allergic to penicillin, but serious reactions are rare.

Insect stings

Insects can cause allergies through bites, stings, or even when eaten.

Toxins interacting with proteins

Contact with plants like poison ivy can cause skin reactions. This happens because of a substance called urushiol, which reacts with skin and triggers an immune response. This can cause redness, swelling, and blisters.

Genetics

Allergies often run in families. Children of allergic parents are more likely to have allergies. The risk of developing allergies is inherited, but the specific allergen can vary. Young children are most at risk.

Hygiene hypothesis

The hygiene hypothesis suggests that very clean environments might lead to more allergies. This is because the immune system isn't exposed to enough germs. This theory explains why allergic diseases are more common in developed countries. Exposure to certain bacteria and parasites might help balance the immune system.

Stress

Long-term stress can make allergic conditions worse. Managing stress might help improve symptoms.

Other environmental factors

Allergic diseases are more common in industrialized countries and cities. Changes in exposure to microorganisms might explain the rise in allergies. Exposure to certain parasites might also help balance the immune system.

Pathophysiology

Acute response

When someone has an allergy, their body thinks something harmless, like pollen or certain foods, is dangerous. This can make the body react and cause uncomfortable feelings.

During this reaction, the body makes chemicals that can cause a runny nose, itchy eyes, trouble breathing, or swelling. These symptoms can happen anywhere in the body or just in one place, like the lungs or skin.

Diagnosis

Getting the right diagnosis is important for managing allergies. Allergy testing can show if someone has an allergy. This helps reduce symptoms and the need for medicines, making life better. Two ways to test for allergies are a skin prick test or an allergy blood test. Both are good choices.

Skin prick tests and blood tests cost about the same. Getting the right diagnosis early can save money by reducing doctor visits and trips to the emergency room.

Allergies can change over time. Regular testing helps doctors decide how to best care for the person. Testing once a year can show if allergies to milk, egg, soy, and wheat have gone away. Testing every 2–3 years is done for allergies to peanut, tree nuts, fish, and shellfish. Follow-up tests help decide when it might be safe to try eating a food that once caused an allergy.

Skin prick testing

Skin testing uses tiny pricks on the skin with small amounts of possible allergens. If the person is allergic, a reaction may happen within 30 minutes, from slight redness to a bump, like a mosquito bite. A doctor reads the results.

Patch testing

Main article: Patch test

Patch testing checks if a substance causes skin irritation. Patches with common allergens are put on the back. The skin is looked at for reactions after 48 hours and again a few days later.

Blood testing

An allergy blood test can be done on anyone, no matter their age or skin condition. A blood sample is sent to a lab for testing. The test looks for specific IgE antibodies in the blood. The results can help find what a person is allergic to and predict the risk of serious reactions.

Other testing

Challenge testing: Tiny amounts of a suspected allergen are given to the body while being watched by an allergist.

Elimination/challenge tests: A person avoids a suspected allergen and then may be given it again to see if symptoms come back.

Some allergy tests, like applied kinesiology and cytotoxicity testing, are not reliable.

Prevention

Giving foods like peanuts early can help lower the chance of allergies. Breastfeeding for the first few months may also help lower skin rashes from allergies. A mother’s diet during pregnancy or breastfeeding doesn’t change allergy risks much. But some studies show that not eating cow’s milk often might increase the risk of cow’s milk allergy.

Recent studies show that giving peanut products early to babies can greatly reduce the chance of peanut allergies. Because of this, guidelines now suggest introducing peanuts to babies at risk of allergies between 4 to 6 months old. These changes have led to fewer cases of peanut allergies. However, some experts remind parents to be careful when trying these new guidelines. Fish oil during pregnancy may lower the risk of food sensitivities, and probiotic supplements might help prevent skin rashes from allergies.

Management

Managing allergies means staying away from what causes them and using medicines to help with symptoms. For some allergies, a special treatment called allergen immunotherapy might help.

Different medicines can be used to help with allergy symptoms. Common ones include antihistamines, glucocorticoids, and mast cell stabilizers. In serious cases, a medicine called epinephrine might be needed.

Allergen immunotherapy can help with allergies to things in the environment, like pollen, and some types of asthma. It works by slowly giving a person tiny amounts of what they’re allergic to, helping their body change its reaction. This treatment can keep helping even after it’s finished. Another way is called sublingual immunotherapy, where the allergen is placed under the tongue.

Epidemiology

Allergic diseases like hay fever and asthma have become more common in the Western world over the past few decades. These increases started in the 1960s and 1970s and continued through the 1980s and 1990s. In developing countries, the number of new cases each year has stayed much lower.

Changing frequency

Genes can affect a person's chance of having allergies, but the rise in these diseases happened too quickly to be because of genetic changes. Changes in our environment and lifestyle seem to play a big role. For example, living in cleaner conditions and spending more time indoors may be part of the reason. One idea, called the hygiene hypothesis, suggests that living in very clean conditions means children are exposed to fewer infections. This might change how the immune system develops, making it more likely to react to harmless things. Other factors, like changes in what we eat, being less active, and changes in the gut from microbes, may also be important. Exposure to certain infections, like hepatitis A, or living on a farm, has been linked to a lower risk of allergies.

History

Some signs of allergic diseases were noticed a long time ago. For example, three members of an ancient Roman royal family may have had such conditions.

The word "allergy" was first used in 1906 by a doctor named Clemens von Pirquet. He saw that some patients reacted more strongly the second time they received certain injections. He made the term "allergy" from ancient Greek words meaning "other" and "work."

Later, scientists learned that many health problems happen when the immune system reacts the wrong way. In 1963, two researchers created a system to sort these reactions into four types. Today, the word "allergy" refers to one fast type of reaction that involves special antibodies called IgE.

In the late 1960s, researchers discovered IgE antibodies, which are important in allergies. New ways to test for allergy antibodies in blood were developed over time.

Medical specialty

Allergology is a part of medicine that studies and treats allergies. Doctors who do this work are called allergists. They help people with allergies, asthma, and other related health problems.

In the United States, becoming an allergist takes many years. After medical school, a doctor learns either internal medicine or pediatrics for three years. Then, they spend two more years in a special program about allergies and the immune system.

In the United Kingdom, allergy is part of general medicine or pediatrics. Doctors need extra training and exams to become allergy specialists.

Research

Scientists are trying to make foods that cause fewer allergic reactions. They are also working on better tests to predict how the body will react to things like insect stings. New medicine tablets that dissolve quickly are being tested, and researchers are studying treatments for some immune system diseases.