Allergy is an inappropriate reaction of the immune system to a substance that is ordinarily harmless under normal exposure conditions. Such a substance is called an allergen. Allergic reactions may affect the nose, eyes, skin, airways, or digestive tract, and their severity ranges from mild, localized symptoms to life-threatening systemic reactions. Many familiar allergies involve immunoglobulin E, but others arise through different immune mechanisms. Allergy therefore describes a group of immune-mediated conditions rather than a single disease. (nhs.uk)
Immune mechanisms
In immediate-type allergy, the immune system produces antibodies called immunoglobulin E (IgE) that recognize a particular allergen. IgE associates with immune cells involved in allergic responses, especially mast cells and basophils. Allergen exposure can activate these cells, releasing substances such as histamine and other mediators that contribute to itching, swelling, mucus production, and airway symptoms. This process links allergen recognition to inflammation in affected tissues. (aaaai.org)
The development of allergen-specific immune recognition is called sensitization. Sensitization and clinical allergy are not equivalent: a person may have detectable allergen-specific IgE without developing symptoms on exposure. Consequently, the presence of an antibody response alone does not establish that an allergen causes disease. (education.aaaai.org)
Not all allergies are IgE-mediated. Allergic contact dermatitis is a delayed, cell-mediated reaction involving T cells. Small chemicals, such as some metal compounds or cosmetic ingredients, can interact with skin proteins and initiate an immune response. This differs from irritant contact dermatitis, in which substances directly damage the skin rather than trigger allergen-specific immunity. (education.aaaai.org)
Allergens and exposure
Common airborne allergens include tree and grass pollen, house dust mite material, and substances associated with animals. They commonly produce nasal, eye, or respiratory symptoms. Other important triggers include insect venom, certain medicines, and foods. Exposure may occur through inhalation, ingestion, skin contact, or an insect sting; the route and the immune mechanism help determine the resulting clinical pattern. (nhs.uk)
Food allergy can involve milk, eggs, peanuts, tree nuts, wheat, soy, sesame, fish, or crustacean shellfish. It differs from food intolerance, which does not involve an allergic immune response. Intolerance commonly causes digestive symptoms, whereas food allergy may also cause skin, airway, or circulatory effects. Similar symptoms can therefore arise from biologically different processes. (medlineplus.gov)
Clinical manifestations
Allergic rhinitis, often called hay fever, causes symptoms such as sneezing, nasal itching, and a runny or blocked nose. Eye involvement may produce watering and itching. Allergens can also contribute to asthma, with cough, wheezing, and breathlessness. Skin manifestations include urticaria, or hives, and swelling of tissues. Food-related reactions may include mouth itching, abdominal cramps, vomiting, or diarrhea. These symptoms are not individually specific to allergy and must be interpreted in context. (nhs.uk)
Atopic dermatitis, also called atopic eczema, is associated with allergic conditions but has a complex inflammatory biology; its presence does not by itself identify a particular allergen. Allergic contact dermatitis typically produces an itchy, sometimes blistered or cracked rash following contact with the responsible substance. Delayed skin reactions can emerge hours or days after exposure rather than immediately. (education.aaaai.org)
Anaphylaxis is a serious systemic allergic reaction that can progress rapidly. Its manifestations may include airway narrowing, throat swelling, a marked fall in blood pressure, shock, or loss of consciousness. Foods, medicines, and insect stings are important triggers. Anaphylaxis is clinically distinct from an isolated mild rash because breathing or circulation may be compromised. (medlineplus.gov)
Diagnosis
Diagnosis combines the exposure history, timing and reproducibility of symptoms, physical examination, and appropriately selected tests. Skin-prick testing introduces small quantities of allergens into superficial skin and assesses local reactions. Blood tests measure allergen-specific IgE. Both can help identify sensitization, but results require interpretation alongside the clinical history; a positive result is not sufficient on its own to establish clinical allergy. (aaaai.org)
An oral food challenge assesses whether ingestion produces a clinical reaction under medical supervision. It is the most accurate test for food allergy and differs from simply measuring antibodies. Patch testing is used in the evaluation of allergic contact dermatitis. Testing methods are therefore selected according to the suspected mechanism. Food-specific IgG testing is not an established diagnostic method for food allergy. (medlineplus.gov)
Treatment and clinical course
Clinical management varies with the allergen, affected organs, and reaction severity. Approaches include reducing exposure to a confirmed trigger and using medicines that control symptoms or inflammation. Antihistamines and corticosteroids have roles in selected allergic conditions. In anaphylaxis, intramuscular epinephrine, also called adrenaline, is first-line treatment; other medicines are adjunctive rather than replacements for it. (nhs.uk)
Allergen immunotherapy uses controlled allergen exposure to modify the immune response. Established applications include selected respiratory allergies and insect-venom allergy, with evidence of reduced symptoms or reactions. Outcomes and suitable treatment methods differ among conditions. Food allergies can begin in childhood or adulthood; some childhood food allergies resolve, while others persist. Family history, eczema, and asthma are associated with an increased likelihood of food allergy, but they do not establish whether a specific food causes a reaction. (aaaai.org)