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allergy
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What is an allergy?
An allergy is a hypersensitivity reaction where the body's immune system overreacts to typically harmless foreign substances called allergens. Common allergens include pollens, molds, dust, animal dander, and certain foods.
In an allergic reaction, B cells produce antibodies that bind to allergens, prompting mast cells to release histamine and other chemicals. Histamine causes allergy symptoms such as sneezing, itching, and swelling.
Allergies manifest differently based on whether B cells or T cells are stimulated, with reactions ranging from immediate to delayed. Common allergic reactions include hay fever, skin reactions, asthma, and food allergies.
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What are common symptoms of an allergy?
Allergies occur when the immune system overreacts to a typically harmless substance that has been breathed in, touched, or eaten. Common symptoms include skin reactions, such as hives, and allergic conjunctivitis (an eye reaction).
Other allergic reactions can include:
* Sneezing, wheezing, and a runny nose
* Swelling and redness of the linings of the eyelids and nasal passages, secretion of tears and mucus
* Asthma, a lung condition that causes a person to have difficulty breathing
* Anaphylaxis, which includes symptoms such as itching of the scalp and tongue, difficulty in breathing, skin flush, and a fall in blood pressure5 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What substances can cause allergies?
Allergies arise when the immune system overreacts to a typically harmless substance that has been breathed in, touched, or eaten. These substances, known as allergens, can include a variety of materials such as foods, medications, plants or animals, chemicals, dust, or molds.
Common food allergy triggers are proteins in cow’s milk, eggs, peanuts, wheat, soy, fish, shellfish, and tree nuts. Medications including tetracyclines, digitalis, ibuprofen, aspirin, and penicillin can also cause allergic reactions. In addition, common allergic reactions can be caused by dust mites, cat and dog dander, and pollen.
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How does the immune system react during an allergic reaction?
During an allergic reaction, the immune system mistakenly identifies a harmless substance (allergen) as a threat. This triggers B cells to produce immunoglobulin E (IgE) antibodies that bind to mast cells.
Upon re-exposure to the same allergen, the allergen binds to the IgE antibodies on the mast cells, causing them to release chemicals like histamine. Histamine causes blood vessels to dilate and become more permeable, leading to symptoms such as:
* Sneezing
* Runny nose
* Itching
* SwellingIn severe cases, anaphylaxis can occur, causing a sudden drop in blood pressure and breathing difficulties.
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What is an allergen, and how does it trigger allergies?
An allergen is a substance that triggers an allergic reaction in a person, but is typically harmless to most people. Common allergens include pollen, mold spores, dust, animal dander, and certain foods.
Allergies emerge when the immune system mistakenly identifies a harmless substance as a threat. This prompts B cells to produce IgE antibodies that bind to mast cells. Upon subsequent exposure, the allergen cross-links with IgE molecules, causing mast cells to release chemicals like histamine, leading to allergic symptoms such as itching, sneezing, swelling, and inflammation.
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How can allergies be managed or treated?
Allergy management and treatment involve several strategies to alleviate symptoms and reduce the impact of allergic reactions.
Avoidance of allergens is the primary recommendation for preventing food allergies. Antihistamines can relieve symptoms like sneezing and runny nose by blocking histamine effects, but they may cause drowsiness. For severe reactions like anaphylaxis, epinephrine autoinjectors provide a rapid injection of epinephrine to reverse the effects.
Allergy desensitization, also known as immunotherapy, involves gradually increasing exposure to the allergen through injections to build tolerance. This treatment can diminish hypersensitivity over time. Immunotherapy can have side effects, such as allergic reactions or flu-like symptoms. Other forms of immunotherapy include adoptive cell transfer, monoclonal antibodies, immune checkpoint inhibitors, and therapeutic vaccines.
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allergy, hypersensitivity reaction by the body to foreign substances (antigens) that in similar amounts and circumstances are harmless within the bodies of other people.
Antigens that provoke an allergic reaction are called allergens. Typical allergens include pollens, drugs, lints, bacteria, foods, and dyes or chemicals. The immune system contains several mechanisms that normally protect the body against antigens. Prominent among these are the lymphocytes, cells that are specialized to react to specific antigens. There are two kinds of lymphocytes—B cells and T cells. B cells produce antibodies, which are proteins that bind to and destroy or neutralize antigens. T cells do not produce antibodies; instead, they bind directly to an antigen and stimulate an attack on it. Allergic reactions can have immediate or delayed effects, depending on whether the antigen triggers a response by B cells or T cells.
Allergic reactions with immediate effects are the result of antibody-antigen responses (i.e., they are the products of B-cell stimulation). These can be divided into three basic types.
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Type I reactions, which include hay fever, insect venom allergy, and asthma, involve the class of antibodies known as immunoglobulin E (IgE). IgE molecules are bound to mast cells, which are found in loose connective tissue. When enough antigen has bound with the IgE antibodies, the mast cells release granules of histamine and heparin and produce other agents such as leukotrienes. These potent chemicals dilate blood vessels and constrict bronchial air passages. Histamine is responsible for the visible symptoms of an allergic attack, such as running nose, wheezing, and tissue swelling. A severe, often fatal, type I allergic reaction is known as anaphylaxis. The predisposition of a person to type I allergic reactions is genetically determined. The best protection against such allergies is avoidance of the offending substance. Antihistamine drugs are often used to give temporary relief. Another helpful measure is desensitization, in which increasing amounts of the antigen are injected over a period of time until the sufferer no longer experiences an allergic response.
Type II reactions result when antibodies react with antigens that are found on certain “target” cells. The antigens may be natural components of healthy cells, or they may be extrinsic components induced by drugs or infectious microbes. The resultant antigen-antibody complex activates the complement system, a series of potent enzymes that destroy the target cell.
Type III reactions result when a person who has been strongly sensitized to a particular antigen is subsequently exposed to that antigen. In a type III reaction, the antigen-antibody complex becomes deposited on the walls of the small blood vessels. The complex then triggers the complement system, which produces inflammation and vascular damage. Unlike type I reactions, type II and type III reactions are not dependent on a genetic predisposition. Avoidance of known allergens is the best protection against such reactions.
Delayed, or type IV, allergic reactions are caused by the actions of T cells, which take longer to accumulate at the site where the antigen is present than do B-cell antibodies. The allergic responses appear 12 to 24 hours or more after exposure to an appropriate antigen. A common delayed allergic reaction is contact dermatitis, a skin disorder. The rejection of transplanted organs is also mediated by T cells and thus may be considered a delayed allergic response.
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