Glossary of Terms

A glossary of useful terms in alphabetical order

ACE inhibitors

Angiotensin-converting-enzyme inhibitors (ACE inhibitors for short) are medicines used to treat high blood pressure. For people with allergies there is the potential for ACE inhibitors to increase the severity of an acute allergic reaction. This is because these medicines block some of the compensatory mechanisms which provide some protection during severe allergic reactions. 

Adrenaline (also known as epinephrine)

Adrenaline is a naturally occurring hormone released in the human body by the adrenal gland, found at the top of the kidneys. It prepares the body to react to danger, increasing the speed and force of heartbeats and thereby the work that can be done by the heart to maintain blood pressure. This, most importantly, sustains blood flow to the brain. Adrenaline also opens the airways to improve breathing and narrows blood vessels in the skin and intestine so that an increased flow of blood reaches the muscles and allows them to cope with the demands of exercise. Adrenaline has been produced synthetically as a drug since 1900. It remains the drug of choice administered by intra-muscular injection for the treatment of anaphylaxis. 

Adrenaline auto-injector (AAI) 

People with potentially serious allergies are prescribed adrenaline auto-injectors to carry at all times. These can help stop an allergic reaction from becoming life-threatening. They should be used as soon as a serious reaction is suspected, either by the person experiencing the reaction or by someone helping them. Training and support are required to ensure that AAIs are used correctly. 


An allergen is any substance, most often a protein, that causes an allergic reaction. Common allergens include foods, pollen, grasses, dust, insect venoms, and some medications. 


Allergies are inappropriate or exaggerated reactions of the immune system to substances that, in the majority of people, cause no symptoms. The first stage of allergy is sensitisation – where a person is exposed to a food or substance, and their immune system registers it as a potential threat. In some of those people, but not all, a future exposure to that food or substance will cause an allergic reaction. 


Anaphylaxis is a severe, life-threatening allergic reaction. Causes include certain foods, insect stings, prescribed drugs and latex. Occasionally anaphylaxis can occur due to non-allergic mechanisms, for example, in people with mastocytosis (see Mastocytosis, below). 


Angioedema is a swelling underneath the skin, which is usually a reaction to a trigger, such as something you are allergic to. Rarely it can occur as a result of a medical condition involving the immune system. The swelling often affects the lips and tongue, and the area around the eyes. 

Anisakis Simplex 

Anisakis simplex is a parasitic worm which infects marine fish or shellfish. The parasite can also infect humans (known as anisakiasis), as well as trigger allergic reactions in a very small minority of people. Allergic reactions to Anisakis simplex can be mistaken for allergies to fish or shellfish. Anyone experiencing allergy-like symptoms to a particular fish or shellfish that they have previously eaten with no problem should consider the possibility that Anisakis simplex is responsible. 


Antibodies, also known as immunoglobulins, are Y-shaped proteins that are produced by the immune system to help stop invaders from harming the body. When an invader enters the body, the immune system springs into action. These invaders, which are called antigens, can be viruses, bacteria, or chemicals. There are several classes of antibodies including immunoglobulin M (known as IgM) and immunoglobulin G (IgG), which are most important to protect against infections. Immunoglobulin A (IgA) provides a first line of defence on the lining of the bowel and respiratory tract. Immunoglobulin E (IgE) is the allergy antibody. IgE’s main function is to fight parasites. But in people prone to allergy, it reacts against foods or substances that, for most people, are not a threat. 


An antigen is a toxin or other foreign substance that can trigger an immune response, resulting in production of an antibody as part of the body’s defence against infection and disease. Many antigens are foreign proteins (those not found naturally in the body). An allergen is a special type of antigen which causes an allergic response.


Antihistamines are a group of drugs that block the effects of histamine, a chemical released in body tissues and fluids during an allergic reaction. In rhinitis (inflammation of the mucous membranes of the nose), antihistamines reduce itching, sneezing, and runny nose. 


Asthma is a chronic, inflammatory lung disease characterized by recurrent breathing problems. People with asthma have acute episodes when the air passages in their lungs get narrower and breathing becomes more difficult. Sometimes episodes of asthma are triggered by allergens, although infection, exercise, cold air and other factors are also important triggers. Evidence suggests that poorly-controlled asthma will raise the chances of any allergic reaction being severe. If your asthma is troublesome and not well-controlled, and you have an allergic reaction to a food or substance, the reaction could be much more serious. Everyone with asthma should be seen at least once each year by their GP or asthma nurse to ensure their symptoms are well-controlled. 

Beta blockers 

Beta-blockers (beta-adrenoceptor blocking agents) are prescribed medicines that work by decreasing the activity of the heart by blocking the action of hormones like adrenaline. They are used to treat various medical conditions including angina, heart failure, atrial fibrillation (irregular heartbeat), heart attack and high blood pressure. They should not be used in people with asthma as they can worsen symptoms. People who carry adrenaline to treat severe allergic reactions (anaphylaxis) should talk to their doctor if they are also prescribed beta blockers. In such people, the adrenaline is likely to be less effective. 

Biphasic anaphylaxis

The first symptoms of anaphylaxis usually occur very rapidly and most cases are relieved by prompt treatment but occasionally the symptoms return. This usually happens within hours of the initial reaction but on rare occasions they can be more delayed. This is known as biphasic anaphylaxis. It is difficult to predict whether biphasic anaphylaxis will occur after the initial reaction, therefore anyone who suffers anaphylaxis needs to be observed in hospital after they recover.

Bronchodilator drugs

Short-acting bronchodilators (reliever inhalers) are the first line treatment for an acute flare-up of asthma. However, if high doses are not effective, immediate attention is required with the administration of steroids. Some people with asthma are prescribed a long-acting bronchodilator as an “add on” treatment, in addition to a preventer inhaler and reliever inhaler. Bronchodilators are a group of drugs that widen the airways in the lungs, relieving attacks of asthma. They have no preventive properties and in general should not be used continuously.

Coeliac disease

Coeliac disease (spelled celiac in other countries) is a serious illness where the body’s immune system is triggered to attack its own tissues (auto-immune) when you eat foods containing gluten. This causes damage to the lining of the gut and means the body cannot properly absorb nutrients from food. There may be a failure to gain weight in infants and a range of bowel problems and poor nutrition in older sufferers. Cereals containing gluten include wheat, rye and barley. Coeliac disease is not an allergy or food intolerance. However, the same cereals can cause food intolerance and sometimes allergy. Coeliac disease is now diagnosed by detecting an auto-antibody. 

Contact dermatitis 

Contact dermatitis is an inflammation of the skin or a rash caused by contact with various substances of a chemical, animal or vegetable nature. The reaction may be a response of the body’s immune system or it may be a direct toxic effect of a substance. Among the more common causes of a contact dermatitis reaction are detergents left on washed clothes, nickel (in watch straps, bracelets and necklaces, and the fastenings on underclothes), chemicals in rubber gloves and condoms, certain cosmetics, plants such as poison ivy, and topical medications such as those applied on the skin. 

Corticosteroid drugs 

Corticosteroids are a group of anti-inflammatory drugs similar to the natural corticosteroid hormones produced by the cortex of the adrenal glands. Among the disorders that often improve with corticosteroid treatment are asthma, allergic rhinitis, eczema and rheumatoid arthritis. 


Cross-reactivity occurs where the proteins in one food or substance share characteristics with those in another food or substance. A person who is allergic to one may therefore have positive allergy tests to other foods or substances with similar related proteins. Often the cross-reacting proteins come from related species. For example, people with birch-tree pollen allergy (hay fever) will sometimes have a reaction when they eat fresh tree fruits or tree nuts. This is known as pollen food syndrome. In some cases, but not all, they will have an allergic reaction to both foods and substances. 

Desensitisation (also known as allergen immunotherapy)

Desensitisation in the context of allergy is where a person who is allergic to something (such as pollen or insect stings) is exposed to gradually increasing doses of the substances to which they are allergic in order to modify or stop the allergic response. This must be done under medical supervision. Desensitisation is currently being trialled with people with food allergies. While desensitisation was the term used in the past it is now known as allergen immunotherapy.


Eczema is an inflammation of the skin with a distinctive appearance, usually causing itching and sometimes accompanied by crusting, scaling or blisters. A type of eczema often made worse by exposure to an allergen is termed “atopic dermatitis”. 

Extrinsic asthma 

Extrinsic asthma is asthma that is triggered by an allergic reaction, usually something that is inhaled. 

Food Additives 

Allergic reactions to additives may occur if the additive is of natural origin. For example, there have been a few reports of allergic reactions to annatto, a natural colour extracted from a seed and to cochineal, a colouring derived from the dried bodies of cochineal insects.  All additives must be clearly labelled with an “E” number which must be declared in ingredient lists as the functionality followed by either the “E” number or the chemical name (e.g. “Preservative: E210” or “Preservative: sodium benzoate). 

Food Intolerance 

Unlike allergies, food intolerances do not involve the immune system. The symptoms usually occur much more slowly, after eating significant amounts of a particular food, and often because the body is unable to process the food effectively. Many food intolerances involve the digestive system. For example, lactose intolerance is a common condition where a sugar called lactose is poorly digested and thus fermented by gut bacteria. The symptoms caused by food intolerance are varied. Digestive symptoms may include bloating, diarrhoea, nausea or vomiting. Other signs may include fatigue, sluggishness, or joint pains. 

Hay fever 

See Rhinitis. 


Histamine is a chemical present in specific immune cells (mast cells and basophils) throughout the body that is released during an allergic reaction. Histamine is one of the substances responsible for the symptoms of inflammation and is the major cause of the runny nose, sneezing, and itching in allergic rhinitis. It also stimulates the production of acid in the stomach and narrows the airways in the lungs. 


See Urticaria. 


Hypersensitivity is extreme physical sensitivity to particular substances or conditions. 


Hypotension is abnormally low blood pressure.

Idiopathic anaphylaxis 

Anaphylaxis is a severe and potentially life-threatening allergic reaction. If after medical testing and investigation the cause of the reaction is not found, the reaction is then labelled as idiopathic anaphylaxis (which means “cause unknown”). 

Immune system 

The immune system is a collection of cells and proteins that works to protect the body from potentially harmful, infectious micro-organisms (microscopic life-forms), such as bacteria, viruses, parasites and fungi. 

Immunoglobulin E 

Immunoglobulin E (IgE) are antibodies produced by the immune system. If you have an allergy, your immune system overreacts to an allergen by producing antibodies called Immunoglobulin E. These antibodies travel to cells that release chemicals, causing an allergic reaction. These antibodies are important in fighting against intestinal parasites. 


Intrinsic asthma 

Intrinsic asthma is asthma that has no apparent external cause. 

Lactose Intolerance 

Lactose intolerance is not an allergy, but an inability to digest lactose. This can lead to symptoms such as tummy pain, bloating, wind, and loose stools. 

Mast cells 

Mast cells play an important role in the body’s allergic response. Mast cells are present in most body tissues, but are particularly numerous in connective tissue, such as the innermost layer of the skin (dermis). In an allergic response, an allergen stimulates the release of antibodies, which attach themselves to mast cells. The next time the person comes in contact with the allergen, the mast cells release substances such as histamine and a number of other mediators (chemicals responsible for allergic symptoms) into the tissue, causing an allergic reaction. 


In most cases of anaphylaxis there is a trigger such as a food, drug, insect sting or some other agent, but anaphylaxis can also occur in people who have a very rare condition called Mastocytosis, which is caused by too many mast cells gathering in the tissues of the body. These are the main cells that release histamine and other chemicals involved in allergic reactions, causing symptoms such as a skin rash, itchy skin and anaphylaxis. If you have this very rare condition, it’s important that your doctor identifies Mastocytosis as the cause of your symptoms. 


NSAIDs is an abbreviation for Non-Steroidal Anti-Inflammatory Drugs. Examples of NSAIDs include ibuprofen and aspirin. People can have an allergy to NSAIDs, although most symptoms triggered by NSAIDs are not allergic. Read about drug allergies.

Oral Mite Anaphylaxis

Oral Mite Anaphylaxis occurs shortly after the intake of foods made with mite-contaminated wheat flour. The condition is seen most frequently in tropical or subtropical environments, but cases are occasionally seen in the UK. Reactions have been reported when cereals have been stored for extended periods at home and the sufferer may mistakenly believe the cause was wheat allergy. Also known as ‘Pancake Syndrome’

Pancake Syndrome


RAST is an abbreviation for RadioAllergoSorbent Test, a trademark of Pharmacia Diagnostics, from where the test originated. RAST is a blood test used to detect IgE antibodies to specific allergens. As it requires the use of radioactivity this test has been superseded by those using alternative markers.


Rhinitis is an inflammation of the mucous membrane that lines the nose, often due to an allergy to pollen, dust or other airborne substances. Seasonal allergic rhinitis is known as “hay fever,” a disorder which causes sneezing, itching, a runny nose and nasal congestion. Rhinitis is also the main feature of the common cold due to rhinovirus and can also be triggered by irritants such as cigarette smoke. If an allergen is the cause it is known as allergic rhinitis.


Sensitisation is a process by which a person’s immune system responds to its first exposure to an antigen/allergen. This leads to both the generation of sensitised lymphocytes and production of a defensive protein known as an antibody in response to any substance it considers a threat. These substances can include certain foods, pollen, or medications. In some, but not all cases of sensitisation, the production of the antibody leads to symptoms of allergy when that substance or food is encountered. In some people who have been sensitised, there are no symptoms.


The sinuses (paranasal sinuses) are air cavities within the facial bones. They are lined by mucous membranes similar to those in other parts of the airways.


Sinusitis is inflammation of the membrane lining the facial sinuses, often caused by bacterial or viral infection.


Stridor is a harsh vibrating noise that happens when breathing in, caused by obstruction of the windpipe or larynx. It must be distinguished from wheeze, which is usually a higher-pitched sound coming from deep down in the chest when breathing out. Stridor is due to the narrowing of airways in the lung and is the characteristic symptom of asthma.

Threshold dose

The threshold dose for a food allergen is an amount below which the risk of an allergic reaction is negligible.


Urticaria, also known as hives or nettle rash, is a rash of round, red weals on the skin which itch intensely, sometimes with swelling. Urticaria can be a symptom of an allergic reaction but there are many other causes.


This is a high-pitched whistling sound coming from deep down in the chest predominantly heard during breathing out (expiration). It can be associated with bouts of coughing and a feeling of tightness in the chest. In infants, this may only occur during cold virus infections. But if it occurs without infection it is likely to be asthma, particularly if the person has allergies to inhalant allergens such as house mites, cats, dogs and/or pollens.