Allergy & Immunology Glossary
Clear, patient-friendly definitions for common allergy and immunology terms — so you can better understand your diagnosis, treatment plan, and conversations with your care team.
This glossary contains 49 terms commonly used in allergy and immunology care. Definitions are written for patients and caregivers — not as medical advice. Always discuss your specific diagnosis and treatment with your allergist.
Allergen
A substance that triggers an allergic reaction in sensitive individuals. Common allergens include pollen, pet dander, dust mites, mold spores, certain foods (such as peanuts or shellfish), insect venom, and some medications. The immune system of an allergic person mistakenly identifies the allergen as harmful and mounts a defensive response.
Allergic Rhinitis
Inflammation of the nasal passages caused by an allergic reaction to airborne substances such as pollen, dust mites, or pet dander. Symptoms include sneezing, runny or stuffy nose, and itchy eyes. When triggered by outdoor seasonal allergens, it is commonly called 'hay fever.'
Anaphylaxis
A severe, life-threatening allergic reaction that occurs rapidly and affects multiple body systems at once. Symptoms may include difficulty breathing, swelling of the throat, a sudden drop in blood pressure, rapid heartbeat, hives, and loss of consciousness. Anaphylaxis requires immediate treatment with epinephrine (adrenaline) and emergency medical care.
Antigen
A substance, typically a protein, that the immune system recognizes as foreign and capable of triggering an immune response. In allergic conditions, antigens that provoke an IgE-mediated reaction are called allergens. The immune system identifies antigens through specialized receptors on immune cells and, in some cases, produces antibodies against them.
Antihistamine
A medication that blocks the effects of histamine in the body, reducing allergy symptoms such as sneezing, itching, runny nose, and watery eyes. Antihistamines are available in both sedating ('first-generation') and non-sedating ('second-generation') forms, over the counter and by prescription.
Antihistamine Drugs
Medications that block histamine receptors to relieve allergy symptoms such as sneezing, itching, watery eyes, and runny nose. These drugs are available in first-generation forms, which can cause drowsiness, and second-generation forms, which are longer-lasting and less sedating. They are used to manage allergic rhinitis, hives, and other histamine-driven reactions.
Anti-Inflammatory Drugs
Medications that reduce inflammation in the body's tissues, including the airways, skin, and nasal passages. In allergy and asthma care, this category includes corticosteroids and leukotriene modifiers, which help control chronic inflammation and prevent flare-ups when used as directed.
Asthma
A chronic respiratory condition in which the airways become inflamed, narrowed, and produce excess mucus, making breathing difficult. Asthma attacks can be triggered by allergens, exercise, respiratory infections, cold air, or irritants such as smoke. Symptoms include wheezing, shortness of breath, chest tightness, and coughing.
Atopy
A genetic predisposition to develop allergic diseases such as asthma, eczema, and allergic rhinitis. Atopic individuals produce elevated levels of immunoglobulin E (IgE) antibodies in response to environmental allergens. Having one atopic condition increases the likelihood of developing others.
Biologic Therapy
A class of medications derived from living cells that target specific components of the immune system involved in allergic and inflammatory responses. Biologics such as dupilumab and omalizumab are used to treat severe asthma, chronic hives, and atopic dermatitis when standard treatments are insufficient.
Bronchitis
Inflammation of the lining of the bronchial tubes, the airways that carry air to and from the lungs. Bronchitis causes coughing, often with mucus, along with chest discomfort and, in some cases, wheezing or shortness of breath. It may be acute, typically following a respiratory infection, or chronic, as seen in long-term irritant exposure or underlying lung disease.
Bronchodilator
A medication that relaxes and widens the muscles around the airways, making breathing easier. Bronchodilators are used to relieve asthma symptoms and come in short-acting forms (for quick relief during an attack) and long-acting forms (for daily maintenance).
Bronchus
One of the two main branches of the trachea (windpipe) that carry air into each lung, further dividing into smaller airways called bronchioles. The bronchi are lined with mucous membranes and muscle that can become inflamed or constricted in conditions such as asthma and bronchitis.
Celiac Disease
An autoimmune condition in which ingestion of gluten, a protein found in wheat, barley, and rye, triggers an immune response that damages the lining of the small intestine. This damage impairs nutrient absorption and can cause digestive symptoms, fatigue, and other complications. Treatment requires strict, lifelong avoidance of gluten in the diet.
Corticosteroid
An anti-inflammatory medication that mimics the effects of cortisol, a hormone naturally produced by the adrenal glands. Used in allergy and asthma treatment, corticosteroids help reduce airway inflammation and are available as inhaled, nasal spray, topical, oral, or injected forms.
Cross-Reactivity
A phenomenon in which proteins found in one allergen are similar enough in structure to proteins in another substance that the immune system reacts to both. For example, people allergic to birch pollen may also react to raw apples, a condition known as oral allergy syndrome or pollen-food allergy syndrome.
Desensitization
The gradual reduction of the immune system's sensitivity to a specific allergen through repeated, controlled exposure. This is the mechanism behind allergen immunotherapy (allergy shots), which aims to retrain the immune system to tolerate allergens rather than react to them.
Digestive System
The group of organs, including the mouth, esophagus, stomach, and intestines, that work together to break down food, absorb nutrients, and eliminate waste. Allergic and immune conditions such as food allergy, celiac disease, and eosinophilic esophagitis can affect the digestive system and produce symptoms such as pain, bloating, or difficulty swallowing.
Dust Mite
Microscopic arachnids that live in household dust, bedding, carpets, and upholstered furniture. Their fecal particles and body fragments are among the most common indoor allergens, triggering symptoms in people with allergic rhinitis, asthma, and eczema.
Eczema (Atopic Dermatitis)
A chronic inflammatory skin condition characterized by itchy, dry, red, and cracked skin. It is closely linked to other atopic conditions (asthma, allergic rhinitis) and is caused by a combination of genetic factors affecting the skin barrier and immune system dysregulation. Flare-ups can be triggered by allergens, irritants, stress, and temperature changes.
Epinephrine (Adrenaline)
A hormone and medication used as the first-line emergency treatment for anaphylaxis. It rapidly reverses the symptoms of a severe allergic reaction by constricting blood vessels, opening airways, and stimulating the heart. It is typically administered via an auto-injector device (such as an EpiPen®) into the outer thigh.
Extrinsic Asthma
A form of asthma triggered by exposure to external allergens such as pollen, pet dander, dust mites, or mold. Also known as allergic asthma, it typically develops in childhood and is associated with elevated IgE antibodies and other atopic conditions. Symptoms occur when the airways react to identifiable environmental triggers.
Food Intolerance
An adverse reaction to a food that does not involve the immune system, unlike a true food allergy. Food intolerances, such as lactose intolerance, result from difficulty digesting certain substances and typically cause gastrointestinal symptoms like bloating, gas, or diarrhea rather than the immune-mediated reactions seen in food allergy.
Gluten Sensitive Enteropathy
Another name for celiac disease, an autoimmune disorder in which the ingestion of gluten damages the lining of the small intestine, impairing nutrient absorption. Management requires strict, lifelong adherence to a gluten-free diet under medical guidance.
Histamine
A chemical released by mast cells and basophils during an allergic reaction. Histamine binds to receptors throughout the body and is responsible for many classic allergy symptoms, including itching, sneezing, runny nose, swelling, and redness. Antihistamines work by blocking these histamine receptors.
Hives (Urticaria)
Raised, itchy, red welts on the skin that appear as an allergic reaction or in response to other triggers such as infections, stress, or medications. Acute hives last less than six weeks; chronic hives persist longer and may require ongoing management. Individual welts typically fade within 24 hours.
IgE (Immunoglobulin E)
An antibody produced by the immune system that plays a central role in allergic reactions. In atopic individuals, the body produces IgE antibodies specific to certain allergens. When the allergen is encountered again, it binds to IgE on mast cells and triggers the release of histamine and other inflammatory chemicals.
Immunoglobulins
Proteins produced by the immune system's plasma cells, also known as antibodies, that identify and neutralize foreign substances such as bacteria, viruses, and allergens. There are five main classes, including IgE, which drives allergic reactions, and IgG, IgA, IgM, and IgD, which play various roles in immune defense.
Immunotherapy (Allergy Shots)
A long-term treatment that gradually desensitizes the immune system to specific allergens by administering progressively increasing doses of the allergen over time. Allergy immunotherapy can reduce symptom severity, decrease medication needs, and in some cases provide long-lasting relief even after treatment ends.
Inhaler
A hand-held device that delivers medication directly to the airways in a fine mist or powder. There are two main types used in asthma management: rescue inhalers (for quick relief of acute symptoms) and controller/maintenance inhalers (used daily to reduce inflammation and prevent attacks).
Intrinsic Asthma
A form of asthma not triggered by identifiable allergens, sometimes called non-allergic asthma. It is often associated with factors such as respiratory infections, exercise, cold air, stress, or irritant exposure, and typically develops in adulthood without the elevated IgE levels seen in extrinsic (allergic) asthma.
Lactose Intolerance
The inability to fully digest lactose, the sugar found in milk and dairy products, due to a deficiency of the enzyme lactase in the small intestine. Symptoms include bloating, gas, abdominal pain, and diarrhea after consuming dairy. Unlike a milk allergy, lactose intolerance does not involve the immune system.
Lymphocyte
A type of white blood cell that is a key component of the immune system. Lymphocytes include B cells, which produce antibodies such as IgE, and T cells, which help regulate immune responses and coordinate defense against foreign substances. Lymphocytes play a central role in both allergic reactions and the body's defense against infection.
Mast Cell
A type of immune cell found in tissues throughout the body, particularly in the skin, lungs, and digestive tract. Mast cells are coated with IgE antibodies and, when activated by an allergen, release histamine and other inflammatory mediators that cause allergic symptoms.
Nasal Polyps
Noncancerous, teardrop-shaped growths that develop in the lining of the nasal passages or sinuses, often as a result of chronic inflammation. They are associated with severe allergic rhinitis, asthma, and aspirin sensitivity. Large polyps can block airflow and reduce the sense of smell.
Patch Testing
A diagnostic procedure used to identify contact allergens that cause delayed allergic skin reactions (contact dermatitis). Small amounts of common allergens are applied to patches placed on the back, left in place for 48 hours, and then evaluated by the allergist for reactions.
Peak Flow Meter
A portable device that measures how fast air can be exhaled from the lungs, reflecting the degree of airway narrowing. Patients with asthma use peak flow meters at home to monitor lung function, detect worsening symptoms early, and guide medication adjustments according to an asthma action plan.
Pollen
Fine powder released by trees, grasses, and weeds as part of their reproductive cycle. Pollen is one of the most prevalent outdoor allergens and is a primary trigger of seasonal allergic rhinitis (hay fever). Pollen counts vary by season: tree pollen peaks in spring, grass pollen in early summer, and weed pollen (including ragweed) in late summer and fall.
Prick Skin Test
A diagnostic test in which a small amount of allergen extract is placed on the skin, typically the forearm or back, and a lancet is used to lightly prick the surface through the drop. A raised, itchy wheal appearing at the test site within 15–20 minutes indicates a sensitivity to that allergen. It is one of the most common and rapid methods used to identify specific allergies.
RAST
Short for radioallergosorbent test, a blood test that measures the level of allergen-specific IgE antibodies in the bloodstream. RAST is used to help diagnose allergies, particularly when skin testing is not possible or practical, such as in patients with extensive skin conditions or those taking medications that interfere with skin test results.
Respiratory System
The organs and structures, including the nose, throat, trachea, bronchi, and lungs, that work together to bring oxygen into the body and remove carbon dioxide. Allergic and inflammatory conditions such as asthma, allergic rhinitis, and bronchitis affect different parts of the respiratory system and can impair normal breathing.
Rhinitis
Inflammation of the mucous membranes lining the nasal passages, characterized by a runny nose, sneezing, and nasal congestion. Rhinitis can be allergic (triggered by allergens) or non-allergic (caused by irritants, infections, hormones, or other factors).
Seasonal Allergies
Allergic reactions that occur during specific times of year when certain plants release pollen into the air. Symptoms typically include sneezing, nasal congestion, runny nose, and itchy eyes. The specific season depends on which plants trigger a person's allergies — spring for tree pollen, summer for grass pollen, and fall for weed pollen.
Sensitization
The first stage of an allergic reaction, in which the immune system is first exposed to an allergen and produces IgE antibodies against it. During sensitization, no symptoms occur. However, on subsequent exposures to the same allergen, the immune system recognizes it and triggers an allergic response.
Sinusitis
Inflammation or infection of the sinuses (air-filled cavities around the nasal passages). Allergic rhinitis can cause chronic sinusitis by blocking sinus drainage. Symptoms include facial pressure or pain, nasal congestion, thick nasal discharge, and reduced sense of smell.
Skin Prick Test
The most common diagnostic test for allergies. A small amount of allergen extract is placed on the forearm or back, and a lancet is used to lightly prick the skin through the drop. A raised, itchy wheal (similar to a mosquito bite) at the test site within 15–20 minutes indicates an allergic sensitivity to that substance.
Triggers
Substances or conditions that provoke allergy or asthma symptoms in sensitized individuals. Common triggers include allergens (pollen, pet dander, dust mites), irritants (cigarette smoke, pollution, strong odors), physical factors (exercise, cold air), and respiratory infections. Identifying and avoiding triggers is a key part of allergy management.
Urticaria
The medical term for hives — a skin reaction producing pale red, raised, itchy bumps or welts. Urticaria can be triggered by allergens, medications, infections, or other stimuli. Chronic spontaneous urticaria (lasting more than six weeks) may not have an identifiable trigger and can require ongoing specialist management.
Wheal-and-Flare Reaction
The characteristic skin response observed during a positive allergy skin prick test. A wheal is a small, raised bump at the test site, surrounded by a flare (redness). The size of the wheal helps the allergist gauge the degree of sensitivity to a specific allergen, though it does not always predict the severity of real-life reactions.
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