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General Features of Allergic Diseases

Allergic diseases are immunoglobulin E (IgE)-mediated immunologic illnesses that can affect any of the body's major organ systems either individually or collectively. Typically, children are not born having allergies because maternal IgE normally does not cross the placenta. In rare instances, neonates and young infants who apparently were sensitized in utero have been born with specific IgE to foods and had allergic reactions to those foods. Development of an allergy requires a familial predisposition and repeat exposure to an allergen (antigen) that provokes specific IgE antibody. Epidemiologic surveys indicate that the familial trait for allergy is inherited as autosomal recessive. Whereas the frequency of positive allergy skin tests is similar in boys and girls, asthma is twice as common in males as in females prior to adolescence, but it appears equal in prevalence thereafter.

A specific immune response gene has been identified for IgE antibody synthesis in rodents, but it has not yet been demonstrated conclusively in humans. With the recent advances in molecular genetics, it is anticipated that the genetic basis of allergy will be understood better in the near future. Allergens sensitize by several potential routes and are categorized as inhalants, ingestants, injectants, and contactants  It is important for the physician to define the route by which any specific allergen provokes clinical allergy in individual patients. Of the inhalant IgE-mediated allergies, allergic rhinitis is by far the most common, affecting approximately 15% of all children. Asthma, of which 80% has an allergic inhalant basis, occurs in more than 5% of children. Gastrointestinal (ingestant) allergy typically is associated with food allergy; however, ingestants also may provoke urticaria and atopic dermatitis and less often may trigger respiratory symptoms. Anaphylaxis is a systemic generalized allergic response consisting of hypotension, urticaria, and angioedema as well as upper and lower airway obstruction; it can be caused by severe allergic reactions to foods (ingestant), insect venom stings (injectant), or medications (ingestant, injectant, or contactant). Although incriminated anecdotally as the cause for hyperactivity, poor school performance, learning disabilities, or abnormal child behavior, there are no definitive, appropriately controlled studies that document an IgE allergic etiology for these predominantly psychosocial or educational problems. The possibility that overgrowth of a yeast such as Candida is important in the pathogenesis of allergy never has been substantiated. The concept of such a candidal syndrome in the context of abnormal child behavior has no validity, and antifungal therapy in the absence of overt clinical candidal infection should be discouraged...