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Allergy ABC

Immunologically induced hypersensitivity reaction against an otherwise harmless substance such as pollen or house dust mite.

An immediate type hypersensitvity reaction induced by massive release of vasoactive mediators such as histamine. Anaphylaxis can be IgE mediated or non-IgE-mediated. There are 4 grades of anaphylaxis. Symptoms range from local reaction such as urticaria up to severe, life threatening symptoms such as bronchial asthma, drop in blood pressure up to shock. 

Acute or chronic obstruction of the airways due to bronchospasm induced by allergens, infection, cold air, irritants or exercise.

Antibodies (immunoglobulins) are proteins produced by B-lymphocytes binding to specific structures of the surface (epitopes) of an antigen or an allergen. There are 5 classes of immunoglobulins with different functions (IgA, IgD, IgG, IgM, IgE). IgE antibodies are typical for allergies and parasitic infections.

An atopic is someone prone to develop specific IgE against common allergens. Sometimes atopics show strongly elevated IgE levels without specific sensitization.

Atopic dermatitis is a severely pruritic, chronic-eczematous skin disease frequently (but not always) associated to allergic sensitization. The role of allergy in atopic dermatitis sometimes is unclear. 

Atopic diseases are induced by the presence of specific IgE antibodies as there are allergic rhinoconjunctivitis (hayfever), allergic bronchial asthma, atopic dermatitis and food allergy. 

Histamine degrading enzyme in the gut. Besides n-methyltransferase, it is the second most important histamine degrading enzyme. A lack of the enzyme or an impaired function may induce histamine intolerance. 

Tissue-damaging protein produced by activated eosinophilic granulocytes - e.g. in bronchial asthma.

Elevation of eosinophilic granulozytes, which are white blood cells playing a central role in chronic allergic inflammation (e.g bronchial asthma or atopic dermatitis). In addition, eosinophilia is found in parasitic infections. 

Chronic-inflammatory, severely pruritic skin lesions. Eczema may present in various fashions and are of different origin. By definition, there are allergic and non-allergic (irritant, toxic) eczema. 

In this test, liquid allergens or allerens mixed with white petrolatum are applied for 2 to 3 days on special patches on the skin. In case of contact allergy, a local eczematous reaction is elicited. Procedure of choice for diagnosing contact allergy. 

Simple breating test to evaluate impaired carbohydrate uptake or metabolism, e.g. lactose intolerance. After drinking a solution containing lactose or fructose, the amount of hydrogen (H2) in the exhaled air is assessed. An increase in H2 is found in case of impaired lactose metabolism (= lactose intolerance) or impaired fructose uptake of the mucosa (= fructose malabsorption). 

In order to evaluate sensitization, a small amount of allergen is applied on the skin (patch test) or is applied and instilled in the skin (skin prick test, intradermal testing). 

Histamine is a multipotent substance found in every human body. Histamine, released from mast cells and basophils, is the most important mediator of the early phase allergic reaction inducing constriction of smooth muscle (bronchial asthma, diarrhea), dilatation of  capillaries (flush, drop in blood pressure) and drainage of fluid from vessels (oedema, swelling, urticaria). Histamine is also found in certain food (cheese, red wine, pickeld cabbage) by the fermentation process. Some food such as tomatoes naturally contains histamine. 

Histamine intolerance is a disease characterised by allergy like symptoms after ingestion of food rich in histamine. The pathomechanism underlying is an impaired histamine degradation caused either by a lack of the histamine degrading enzyme diamine oxidase or a functional deficit in the enzyme. 

In patients suffering from fructose malabsorption ("fructose intolerance") the fructose uptake impaired in the gut. Fructose remains in the gut and gets digested by bacteria. Patients may experience bloating, abdominal cramps or diarrhea after eating fruits. To test for fructose malabsorption, a fructose provocation test is performed (H2 breathing test). 

In contrast to healthy people, an elevated IgE level is found in atopic patients or in patients suffering from parasitic infections. IgE antibodies bind to specific receptors found on mast cells or basophils. On contact with allergen, these cells release their mediators - especially histamine. This reaction is used for our skin prick test. In case of sensitization, a local reaction consisting of reddening, swelling (wheal) and itching may occur on the testing site.

Amount of all IgE antibodies found in the serum. An elevated total IgE level is an indicator for any kind of allergy, however it is not an evidence. Parasitic infections too go along with elevated IgE levels. In addition, high IgE levels are not necessarily a proof  of severe allergy. 

Specific immunotherapy is the only causative treatment of type-1 allergy. In the classic, subcutaneous immunotherapy (SCIT) increasing amounts of allergen are injected in the upper arm in a weekly interval in order to "desensitize - make the patient unreactive" to the allergen.  After reaching a platform dose (= maintainence dose), the allergy is given in an interval of 4 to 6 weeks for a period of up to three years. SCIT is the golden standard. There is also a sublingual route of administering specific immunotherapy: either by drops (SLIT - sublingual immunotherapy) or by tablets put under the tongue (AIT - allergen immunotherapy tablet, at the moment only available for grass pollen). Due to therapeutic efficacy, AIT is preferred.

Intolerance reactions are non-immunologically mediated hypersensitivity reactions. 

Highly sensitive skin test. A small amount of allergen is injected intracutaneously. Reading is done after 20 minutes. In case of a wheal and flare reaction, the test is positive.   

Acute or chronic eczematous reaction induced by direct contact of the allergen to the skin. Contact dermatitis is not an IgE mediated but a cell mediated reaction. Allergens responsible for the reactions are small molecules such as metals (nickel), frangrances or preservatives. Contact dermatitis is diagnosed by patch testing. In contrast to contact dermatitis, contact urticaria is an IgE mediated reaction induced by latex or animal fur.

Allergic reaction induced by binding of an antibody to a substance mimicking an allergen the patient is sensitized to. Cross reactivity can occur between unrelated allergen sources such as birch pollen and apple or house dust mite and sea food. 

A lung function test enableds to evaluate flow and volume of the lungs. Bronchial obstruction can be found in bronchial asthma or obstructive bronchitis, a restrictive pattern can be found beside others in pulmonary fibrosis. 

Mast cells play a major role in Type-1 allergy. Substances such as histamine and other inflammatory mediators (leucotrienes, prostaglandins, ..) are released from mast cells after cross linking of IgE antibodies on their cell surface. Mast cells can be found in many tissues, but are especially numerous in the skin as well as mucous membranes (bronchial mucosa, intestinal mucosa). 

Patients suffering from lactose intolerance cannot metabolise lactose. Lactose remains in the gut and gets digested by bacteria. Patients may experience bloating, abdominal cramps or diarrhea after drinking milk. To test for lactose intolerance, a lactose provocation test is performed (H2 breathing test). 

A simple, sensitive and rapidly to perform skin test. A drop of allergen solution is applied to the skin and gets pricked with a tiny lancet in order to facilitate the allergen its way to the epidermis of the skin. In case of sensitization, a wheal and flare reaction occurs within 20 minutes. Skin prick testing allows to test 20 allergens in one sitting. 

This test is used to test almost any kind of fresh food such as fruits or vegetable. After pricking the fresh food, the test is identical to the regular skin prick test. (see above) 

Specific IgE antibodies directed against a certain allergen source or allergenic molecule (testing systems frequently used: Immuno CAP, ISAC (allergen chip), Immuolite 2000). 

A variant of specific immunotherapy. The allergen is not injected but applied by drops (SLIT) or by tablets put under the tongue (AIT, at the moment only available for grass pollen). Due to therapeutic efficacy, AIT is preferred. 

Tryptase is released form mast cells in the event of degranulation. Elevated tryptase levels are found in mastocytosis, a disease characterized by an elevated number of mast cells in the body. In addition, tryptase levels transiently increase during anaphylaxis. 

A type of allergic reaction caused by an IgE mediated mechanism. Within minutes after contact to the allergen mast cells start to degranulate and release histamine as well as other inflammatory mediators causing the allergic symptoms. Therefore, Type-1 allergy is also called allergy of the immediate type. Type-1 allergy is mostly caused by proteins from natural sources such as pollen, house dust mite, animal dander or insects venoms or foods. Chemical such as formaldehyde or drugs rarely induce IgE mediated allergic reactions. 

Allergic reaction caused by sensitized T-cells without the involvement of IgE antibodies. Symptoms, occurring after some hours to days (delayed type reaction) are normally restricted to the skin. E.g. contact dermatiti. 

"hives" Local or generalized wheals surrounded by a reddish margin due to degranulation of mast cells. Chronic recurrent urticaria rarely is caused by allergy.