Gastrointestinal allergy can be classified according to the triggering antigen, the mechanism of immune reaction, or the anatomic site of reaction.
Gastrointestinal allergy may be triggered by food components (e.g. food proteins or glycoproteins), and by other antigen antibody reaction to (e.g. bacterial disease, viral, fungal, and worm antigen), drugs and chemicals, inhaled antigens such as pollens or house dust mites, and any other protein entering the gastrointestinal tract.
Whereas gastrointestinal food allergy has been studied by several laboratories in the past, our knowledge is limited concerning gastrointestinal allergy of other agents. For example, only anecdotal information is available on gastrointestinal food allergy induced by allergens derived from infectious agents or inhaled allergens.
The gastrointestinal tract contains immunity bacteria (approximately 100 times more than the skin), pollens, and other inhaled antigens that are usually swallowed in large amounts and persist during their passage through the gastrointestinal tract. Since most studies on gastrointestinal allergy are concerned with severe food allergy, this will focus mainly on this aspect.
Gastrointestinal allergy is defined as an immune-mediated pathologic reaction toward food or other antigens and it must be distinguished from other forms of adverse reaction to food. In general, drug adverse reactions to food are divided into toxic and non-toxic reactions. Toxic reactions (e.g. in response to food contaminants such as bacterial toxins or chemicals) will occur in any exposed individual provided that the dose is high enough. The occurrence of non-toxic food reactions depends on individual susceptibility to a certain food. The non-toxic adverse reactions to food are either immune-mediated food allergy or not (oral intolerance).
Food intolerance is subdivided into enzymatic, pharmacologic, and undefined food intolerance. The most important enzymatic food intolerance, which affects large numbers of the adult world population, is the acquired lactose intolerance baby due to lactase deficiency.
Pharmacologic food intolerance is present in individuals who are abnormally reactive to substances, such as vasoactive amines present in some foods. Undefined food intolerance reactions may include reactions to food that induce endogenous histamine release, or to food additives such as dyes, tartrazine, sulfates, benzoates, parabens, antioxidants, salicylates, and glutamate. Such reactions may be based on immune- mediated and non-immune-mediated responses (unspecific mediator release, enzyme inhibition), the mechanisms of which are poorly defined.
Gastrointestinal allergy may affect different sites of the gastrointestinal tract, leading to a range of clinical symptoms. Some clinical entities, obviously related to food-induced gastrointestinal allergy, have been described, such as oral allergy syndrome (OAS) which affects the lips and the mucosa of the pharynx, gluten enteropathy which leads to villus atrophy in the duodenum, and malassimilation, or eosinophilic esophagitis and eosinophilic proctitis in children, which causes stool incontinency and pain. In principle, all sites of the gastrointestinal tract may be affected by allergy. The factors determining the site of allergy, which is not necessarily the site of antigen uptake, are unknown
