Porth's Essentials of Pathophysiology, 4e

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Disorders of the Immune Response

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but frequently is associated with other chronic airway disorders, such as sinusitis and bronchial asthma. 9,10 Severe attacks may be accompanied by malaise (gen- eral discomfort), fatigue, and muscle soreness from sneezing. Fever is absent. Sinus obstruction may cause headache. Typical allergens include pollens from rag- weed, grasses, trees, and weeds; fungal spores; house dust mites; animal dander; and feathers. Allergic rhi- nitis can be divided into perennial and seasonal aller- gic rhinitis depending on the chronology of symptoms. Persons with the perennial type of allergic rhinitis experience symptoms throughout the year, whereas those with seasonal allergic rhinitis (e.g., hay fever) are plagued with intense symptoms in conjunction with periods of high allergen (e.g., pollens, fungal spores) exposure. Symptoms that become worse at night suggest a household allergen, and symptoms that improve or disappear on weekends suggest occu- pational exposure. Diagnosis depends on a careful history and physi- cal examination, microscopic identification of an increased number of eosinophils on a nasal smear, and skin or serum testing to identify the offending allergens. When possible, avoidance of the offending allergen is recommended. Treatment is symptomatic in most cases and includes the use of oral antihistamines and oral or topical decongestants. Intranasal cortico- steroids often are effective when used appropriately. Intranasal cromolyn, a drug that stabilizes mast cells and prevents their degranulation, may be useful, espe- cially when administered before expected contact with an offending allergen. A program of specific immu- notherapy (“allergy shots”) may be used when symp- toms are particularly bothersome. 9,10 Desensitization involves frequent (often weekly) injections of the offending antigens. The antigens, which are given in increasing doses, stimulate production of high levels of IgG, which acts as a blocking antibody by combining with the antigen before it can combine with the cell- bound IgE antibodies. Food Allergies. Virtually any food can produce atopic or nonatopic allergies. The primary target of food allergy may be the skin, the gastrointestinal tract, the respiratory system, or a combination thereof. 11,12 The foods most commonly causing these reactions are milk, eggs, peanuts, tree nuts, fish, and shellfish (i.e., crusta- ceans and mollusks). The allergenicity of a food may be changed by heating or cooking. A person may be allergic to drinking milk but may not have symptoms when milk is included in cooked foods. Both acute reactions (hives and anaphylaxis) and chronic reac- tions (asthma, atopic dermatitis, and gastrointestinal disorders) can occur. Anaphylaxis occurs as a multior- gan response associated with IgE-mediated hypersensi- tivity. The foods most responsible for anaphylaxis are peanuts, 13 tree nuts (e.g., walnuts, almonds, pecans, cashews, hazelnuts), and shellfish. One form of food- associated anaphylaxis occurs with exercise. It may occur when exercise follows ingestion of a particular food to which IgE sensitivity has been demonstrated,

such as the presence of residual amounts of peanut that remain on equipment used for preparing foods con- taining peanuts, can be sufficient to cause anaphylaxis in an extremely sensitive person. Within minutes after exposure, itching, hives, and skin erythema develop, followed shortly by bronchospasm and respiratory distress. Vomiting, abdominal cramps, diarrhea, and laryngeal edema and obstruction follow, and the per- son may go into shock and die unless effective treat- ment is instituted. The initial management of anaphylaxis focuses on the establishment of a stable airway and intrave- nous access, and the administration of epinephrine. 7,8 Epinephrine produces relaxation of bronchial smooth muscle and inhibits the immediate life-threatening car- diovascular effects of anaphylaxis. Persons with a his- tory of anaphylaxis should be provided with preloaded epinephrine syringes and instructed in their use. They should also be instructed to seek immediate professional help regardless of the initial response to self-treatment. Family members and caregivers of young children should be trained to inject epinephrine. Prevention of exposure to potential triggers that cause anaphylaxis is particularly important. Finally, all persons with poten- tial for anaphylaxis should be advised to wear or carry a medical alert bracelet, necklace, or other identifica- tion to inform emergency personnel of the possibility of anaphylaxis. Local (Atopic) Reactions Local or atopic reactions usually occur when the anti- gen is confined to a particular site by virtue of expo- sure. The term atopic refers to a genetically determined hypersensitivity to common environmental allergens mediated by an IgE–mast cell reaction. Persons with atopic disorders commonly are allergic to more than one (often many) environmental allergens. The most common atopic disorders are urticaria (hives), allergic rhinitis (hay fever), atopic dermatitis, food allergies, and some forms of asthma. The discussion in this sec- tion focuses on allergic rhinitis and food allergy. Allergic asthma is discussed in Chapter 23 and atopic dermatitis in Chapter 46. The susceptibility to immediate hypersensitivity disorders tends to be inherited. 2 The genetic basis of atopy is unclear; however, linkage studies suggest an association with cytokine genes on chromosome 5q that regulate the expression of circulating IgE. 1 Persons with atopic allergic conditions tend to have high serum levels of IgE and increased numbers of basophils and mast cells. Although the IgE-triggered response is likely a key factor in the pathophysiology of atopic allergic disorders, it is not the only factor and may not be solely responsible for conditions such as atopic dermatitis and certain forms of asthma. Allergic Rhinitis. Allergic rhinitis is characterized by symptoms of sneezing, itching, and watery dis- charge from the nose and eyes (rhinoconjunctivitis). Allergic rhinitis not only produces nasal symptoms