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Allergy
Hives2010.JPG
Hives are a common allergic symptom
Symptoms Red eyes, itchy rash, runny nose, shortness of breath, swelling, sneezing
Types Hay fever, food allergies, atopic dermatitis, allergic asthma, anaphylaxis
Causes Genetic and environmental factors
Diagnostic method Based on symptoms, skin prick test, blood test
Similar conditions Food intolerances, food poisoning
Prevention Early exposure to potential allergens
Treatment Avoiding known allergens, medications, allergen immunotherapy
Medication Steroids, antihistamines, epinephrine, mast cell stabilizers, antileukotrienes
Frequency Common

Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity of the immune system to typically harmless substances in the environment. The word "allergy" was first used by Clemens von Pirquet in 1906.

These diseases include hay fever, food allergies, atopic dermatitis, allergic asthma, and anaphylaxis. Symptoms may include red eyes, an itchy rash, sneezing, a runny nose, shortness of breath, or swelling. Food intolerances and food poisoning are separate conditions.

Common allergens include pollen and certain food. Metals and other substances may also cause problems. Food, insect stings, and medications are common causes of severe reactions. Their development is due to both genetic and environmental factors.

Diagnosis is typically based on a person's medical history. Further testing of the skin or blood may be useful in certain cases. Positive tests, however, may not mean there is a significant allergy to the substance in question.

In severe reactions injection of adrenaline (epinephrine) is recommended. Allergen immunotherapy, which gradually exposes people to larger and larger amounts of allergen, is useful for some types of allergies such as hay fever and reactions to insect bites.

Allergies are common. In the developed world, about 20% of people are affected by allergic rhinitis, about 6% of people have at least one food allergy, and about 20% have dermatitis at some point in time. Depending on the country about 1–18% of people have asthma.

There are differences between countries in the number of individuals within a population having allergies. Allergic diseases are more common in industrialized countries than in countries that are more traditional or agricultural, and there is a higher rate of allergic disease in urban populations versus rural populations. Rates of many allergic diseases appear to be increasing.

Allergic reactions

2707 Edema of Right Hand Due to Allergic Reaction
Edema of right hand due to allergic reaction

When people come in contact with something they are allergic to, there can be many different reactions. Their eyes may become very itchy, and their noses may become drippy or stuffy. They may cough and sneeze. They may develop a rash called hives. Their faces may swell up. If it is a food allergy, the person may get an upset stomach.

Sometimes a person's oesophagus, or throat, may swell up so much that the person can no longer breathe. This is called anaphylaxis. When this happens, a doctor must give the person a medicine called epinephrine to make the swelling go down. Anaphylaxis occurs in between 0.05–2% of people. Some people with allergies to very common things, like bee stings or fish, carry this medicine with them so it can be used quickly in an emergency.

Many allergens such as dust or pollen are airborne particles. In these cases, symptoms arise in areas in contact with air, such as eyes, nose, and lungs. For instance, allergic rhinitis, also known as hay fever, causes irritation of the nose, sneezing, itching, and redness of the eyes. Inhaled allergens can also lead to increased production of mucus in the lungs, shortness of breath, coughing, and wheezing.

Signs and symptoms

Affected organ Common signs and symptoms
Nose Swelling of the nasal mucosa (allergic rhinitis) runny nose, sneezing
Sinuses Allergic sinusitis
Eyes Redness and itching of the conjunctiva (allergic conjunctivitis, watery)
Airways Sneezing, coughing, bronchoconstriction, wheezing and dyspnea, sometimes outright attacks of asthma, in severe cases the airway constricts due to swelling known as laryngeal edema
Ears Feeling of fullness, possibly pain, and impaired hearing due to the lack of eustachian tube drainage.
Skin Rashes, such as eczema and hives (urticaria)
Gastrointestinal tract Abdominal pain, bloating, vomiting, diarrhea

Cause

Risk factors for allergy can be placed in two general categories, namely host and environmental factors. Host factors include heredity, sex, race, and age. However, there have been recent increases in the of allergic disorders that cannot be explained by genetic factors alone. Four major environmental options are alterations in exposure to infectious diseases during early childhood, environmental pollution, allergen levels, and dietary changes.

Common allergies

Allergy testing machine
An allergy testing machine being operated in the diagnostic immunology lab
Puerto Rico — San Juan — Shelves with "Allergy-nasal"
Shelves in a pharmacy with anti-allergy products

Some common food allergies are:

Some common environmental allergies are:

Allergic reactions can result from foods, insect stings, and reactions to medications like aspirin and antibiotics such as penicillin.

Symptoms of food allergy include abdominal pain, bloating, vomiting, diarrhea, itchy skin, and swelling of the skin during hives. Food allergies rarely cause respiratory (asthmatic) reactions, or rhinitis.

Insect stings, food, antibiotics, and certain medicines may produce an allergic response that is also called anaphylaxis; multiple organ systems can be affected, including the digestive system, the respiratory system, and the circulatory system. Depending on the rate of severity, anaphylaxis can include skin reactions, inability to breath, swelling, low blood pressure, coma, and death.

This type of reaction can be triggered suddenly, or the onset can be delayed. The nature of anaphylaxis is that the reaction can seem to be subsiding, but may still be going on throughout a period of time.

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