Glossary of Terms

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. 

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. 

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). 

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. 

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.

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. 

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.

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. 

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. 

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.

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

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. 

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. 

Hypersensitivity is extreme physical sensitivity to particular substances or conditions. 

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”). 

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 is asthma that has no apparent external cause. 

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. 

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.

See: Oral Mite Anaphylaxis

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.

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.

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.

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.