An allergy is an exaggerated reaction by the immune system in response to exposure to certain foreign substances. The response is exaggerated because these foreign substances are normally seen as harmless by the immune system in nonallergic individuals and do not cause a response in them. In allergic individuals, the body recognizes the substance as foreign, and the allergic part of the immune system generates a response. Allergy-producing substances are called “allergens.” Examples of allergens include pollens, dust mites, molds, animal proteins, foods, and medications. When an allergic individual comes in contact with an allergen. the immune system mounts a response through the IgE antibody. People who are prone to allergies are said to be allergic or “atopic.”
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. These diseases include allergic asthma, anaphylaxis atopic dermatitis, hay fever. Symptoms may include sneezing, runny nose, nasal congestion, wheezing, shortness of breath, swelling, itchy rash and red eyes. Common allergens include pollen and certain foods. Food, insect bites, and medications are common causes of severe allergic reactions. Seasonal allergies result from grass, weed, tree pollen, or molds. Cat and dog dander allergies are common. Food allergies include peanut or milk.
Allergies develop due to both genetic and environmental factors. The underlying mechanism involves immunoglobulin E antibodies (IgE), part of the body’s immune system, binding to an allergen and then to a receptor on mast cells or basophils where it triggers the release of inflammatory chemicals such as histamine.
Signs and Symptoms of Allergy
Allergy symptoms, which depend on the substance involved, can affect the airways, sinuses and nasal passages, skin, and digestive system. Allergic reactions can range from mild to severe. In some severe cases, allergies can trigger a life-threatening reaction known as anaphylaxis.
Hay fever, also called allergic rhinitis, can cause the following symptoms:
- Sneezing
- Itching of the nose, eyes or roof of the mouth
- Runny, stuffy nose
- Watery, red or swollen eyes (conjunctivitis)
A food allergy can cause the following symptoms:
- Tingling in the mouth
- Swelling of the lips, tongue, face or throat
- Hives
- Anaphylaxis
An insect sting allergy can cause the following symptoms:
- A large area of swelling (edema) at the sting site
- Itching or hives all over the body
- Cough, chest tightness, wheezing or shortness of breath
- Anaphylaxis
A drug allergy can cause the following symptoms:
- Hives
- Itchy skin
- Rash
- Facial swelling
- Wheezing
- Anaphylaxis
Atopic dermatitis, an allergic skin condition also called eczema, can cause skin to:
- Itch
- Redden
- Flake or peel
Anaphylaxis
Some types of allergies, including allergies to foods and insect stings, can trigger a severe reaction known as anaphylaxis. A life-threatening medical emergency, anaphylaxis can cause you to go into shock. Following are the signs and symptoms of anaphylaxis:
- Loss of consciousness
- A drop in blood pressure
- Severe shortness of breath
- Skin rash
- Lightheadedness
- A rapid, weak pulse
- Nausea and vomiting
Causes of Allergy
An allergy starts when the immune system mistakes a normally harmless substance for a dangerous invader. The immune system then produces antibodies that remain on the alert for that particular allergen. When you’re exposed to the allergen again, these antibodies can release a number of immune system chemicals, such as histamine, that cause the symptoms of allergy.
Common allergy triggers include:
- Airborne allergens, such as pollen, animal dander, dust mites and mold
- Certain foods, particularly peanuts, tree nuts, wheat, soy, fish, shellfish, eggs and milk
- Insect stings, such as from a bee or wasp
- Medications, particularly penicillin or penicillin-based antibiotics
- Latex or other substances you touch, which can cause allergic skin reactions
When an allergic reaction occurs, allergens bind to antibodies that the body produces called immunoglobin E (IgE). Antibodies combat foreign and potentially harmful substances in the body. Once the allergen binds to IgE, specific types of cells, including mast cells, will release chemicals that trigger the symptoms of the allergic reaction. Histamine is one of these chemicals. It causes the muscles in the airways and walls of the blood vessels to tighten. It also instructs the lining of the nose to produce more mucus.
Risk Factors of Allergy
People may have a higher risk of allergies if they are under 18 years old or have a personal or family history of asthma or allergies. Some researchers have suggested that those born by cesarean delivery may also have a higher risk of allergies, as they do not have exposure to the mother’s microbiome during childbirth.
Diagnosis of Allergy
Following are the allergy tests mostly used for diagnosis:
- Skin tests
- Challenge (elimination-type) tests
- Blood tests
A skin test involves applying a small amount of a suspected allergen to the skin and watching for a reaction. The substance may be taped to the skin (patch test), applied via a small prick to the skin (skin prick test), or injected just under the skin (intradermal test).
A skin test is most valuable for diagnosing:
- Food allergies such as peanuts or shellfish
- Pollen or mold allergies, and animal dander allergy
- Penicillin allergy
- Venom allergy (such as mosquito bites or bee or bug stings)
- Allergic contact dermatitis (a rash you get from touching a substance)
Challenge testing is useful in diagnosing food allergies. It involves removing a food from your diet for several weeks and watching for symptoms when you eat the food again.
A blood test for an allergy checks your blood for antibodies against a possible allergen. An antibody is a protein your body produces to fight harmful substances. Blood tests are an option when skin testing isn’t helpful or possible.
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