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 many uncomfortable symptoms such as red eyes, an itchy rash, sneezing, coughing, a runny nose, trouble breathing, or swelling.

Allergies are quite common. In places with advanced healthcare, about one in five people have allergies that affect their nose and eyes, known as hay fever. Food allergies affect about one in ten adults and eight percent of children. Many people also experience skin irritation called atopic dermatitis, and some have trouble breathing due to asthma. 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 can include skin reactions, trouble breathing, swelling, and very low blood pressure. 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 can be grouped into two main types: factors related to the person and factors related to the environment. Personal factors include family history, gender, ethnicity, and age, with family history being the most important. However, the rise in allergic diseases can't be explained by genetics alone.

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

Dust mites

Dust mite allergy is common and can cause reactions like asthma or itching. The allergens come from the mites' droppings, which contain proteins that trigger reactions. Dust mite-proof covers for mattresses, pillows, and duvets can help reduce exposure.

Foods

Many foods can cause allergic reactions, but most reactions are due to a few common foods like 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 multiple nuts. Some allergens can move between foods through genetic changes.

Latex

Latex can cause allergic reactions, especially in healthcare workers due to frequent exposure. The most common reaction is contact dermatitis, but serious reactions can occur. Latex allergy can also affect sensitivity 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 be a source of allergies through bites, stings, or even ingestion.

Toxins interacting with proteins

Contact with plants like poison ivy can cause allergic skin reactions due to a substance called urushiol, which reacts with skin proteins 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 varies. Age also plays a role, with young children being most at risk.

Hygiene hypothesis

The hygiene hypothesis suggests that living in very clean environments might lead to more allergies 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 in sensitive individuals.

Other environmental factors

Allergic diseases are more common in industrialized countries and urban areas. Changes in exposure to microorganisms might explain the rise in allergies. Exposure to certain parasites might also play a role in balancing the immune system.

Pathophysiology

Acute response

When someone has an allergy, their body acts like it’s protecting them from something harmful, even though the thing isn’t actually dangerous. This happens because the body’s defense system makes a mistake. It thinks a harmless thing, like pollen or certain foods, is a threat. This mistake can cause the body to react in ways that make the person feel uncomfortable.

During this reaction, certain cells in the body release chemicals that cause symptoms like a runny nose, itchy eyes, trouble breathing, or swelling. These symptoms can happen all over the body or just in one area, like the lungs or skin.

Diagnosis

Effective management of allergic diseases depends on making an accurate diagnosis. Allergy testing can help confirm or rule out allergies. Correct diagnosis and advice reduce symptoms and the need for medicines, improving quality of life. Two methods are used to check for allergen-specific IgE antibodies: a skin prick test or an allergy blood test. Both methods are recommended and have similar value.

Skin prick tests and blood tests are equally cost-effective. Early and accurate diagnoses save costs by reducing visits to doctors, referrals, and emergency visits.

Allergy changes over time. Regular testing provides information on how to change patient management to improve health. Annual testing is often done to see 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. Results of follow-up testing help decide when it is safe to try eating an allergenic food again.

Skin prick testing

Skin testing involves tiny pricks on the skin with small amounts of suspected allergens. A small device is used to prick the skin. If the patient is allergic, a reaction may occur within 30 minutes, ranging from slight reddening to a hive, similar to a mosquito bite. Results are interpreted by allergists.

Patch testing

Main article: Patch test

Patch testing checks if a substance causes skin inflammation. Adhesive patches with common allergens are placed on the back. The skin is checked for reactions after 48 hours and again a few days later.

Blood testing

An allergy blood test can be done on anyone, regardless of age or skin condition. A sample of blood is sent to a lab for analysis. The test measures specific IgE antibodies in the blood. The results can help determine what a patient is allergic to and predict the risk of severe reactions.

Other testing

Challenge testing: Tiny amounts of a suspected allergen are given to the body under supervision by an allergist.

Elimination/challenge tests: A patient avoids a suspected allergen and then may be challenged by re-introducing it to see if symptoms return.

Some allergy tests, like applied kinesiology and cytotoxicity testing, are unreliable.

Prevention

Giving foods like peanuts early in childhood can help lower the chance of developing allergies. Breastfeeding for the first few months may also lower the risk of skin rashes linked to allergies. While a mother’s diet during pregnancy or breastfeeding doesn’t seem to change allergy risks much, some studies show that irregular exposure to cow’s milk 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, preventing thousands of new cases. 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 linked to allergies.

Management

Managing allergies usually 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

While genes can affect a person's chance of having allergies, the rise in these diseases happened too quickly to be because of genetic changes. Instead, 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 noted a long time ago. For example, three members of the ancient Roman royal family may have had a history of such conditions.

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

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

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

Medical specialty

Allergology is the area of medicine that focuses on studying, diagnosing, and treating allergies. Doctors who specialize in this field are called allergists. They are trained to help manage allergies, asthma, and other related conditions.

In the United States, becoming an allergist takes many years of training. After finishing medical school, a doctor spends three years learning either internal medicine or pediatrics. Then, they complete two more years in a special program focused on allergies and the immune system.

In the United Kingdom, allergy is a part of general medicine or pediatrics, and doctors need to complete additional training and exams to become specialists in this area.

Research

Scientists are working on creating foods that cause fewer allergic reactions. They are also trying to make 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 certain diseases related to the immune system.