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Rabu, 15 Juni 2011

FOOD ALLERGIES/alergi makanan






Background

Adverse food reactions can be broadly classified into 2 categories.[1] The first category consists of immunologically-mediated adverse reactions to foods that are termed food allergies. Food allergies can result in disorders with an acute onset of symptoms following ingestion of the triggering food allergen (eg, anaphylaxis) and in chronic disorders (eg, atopic dermatitis).

The second category is composed of adverse reactions that are not immune-mediated. An example is lactose intolerance caused by a deficiency of lactase. Adverse reactions to foods can also occur from toxic (eg, bacterial food poisoning) or pharmacologic (eg, caffeine) effects.

Patofisiology

Food allergies are primarily the result of immune responses to food proteins.[2] Normally, noninflammatory immune responses develop to ingested foods in a process called oral tolerance.[3, 4] For reasons that remain unclear, but likely include environmental and genetic factors, tolerance may be abrogated, leading to adverse immune responses. While sensitization (eg, development of an immunoglobulin E [IgE] immune response) to an allergen has been primarily assumed to occur from ingestion, this may not always be the case. For example, oral allergy syndrome (pollen-food related syndrome) describes an allergic response to specific raw fruits or vegetables that share homologous proteins with pollens; the initial route of sensitization is respiratory exposure to pollen proteins rather than oral exposure to food proteins. The skin may be another potential route of sensitization.[5]

IgE antibody – mediated responses are the most widely recognized form of food allergy and account for acute reactions. Patients with atopy produce IgE antibodies to specific epitopes (areas of the protein) of one or more food allergens. These antibodies bind to high-affinity IgE receptors on circulating basophils and tissue mast cells present throughout the body, including the skin, gastrointestinal tract, and respiratory tract.

Subsequent allergen exposure binds and cross links IgE antibodies on the cell surface, resulting in receptor activation and intracellular signaling that initiates the release of inflammatory mediators (eg, histamine) and synthesis of additional factors (eg, chemotactic factors, cytokines) that promote allergic inflammation. The effects of these mediators on surrounding tissues result in vasodilatation, smooth muscle contraction, and mucus secretion, which, in turn, are responsible for the spectrum of clinical symptoms observed during acute allergic reactions to food.

Cell-mediated responses to food allergens may also mediate allergic responses, particularly in disorders with delayed or chronic symptoms. For example, food protein – induced enterocolitis syndrome (FPIES), a gastrointestinal food allergy, appears to be mediated by T-cell elaboration of the cytokine tumor necrosis factor (TNF)-alpha.[6] Persons with atopic dermatitis that flares with ingestion of milk have been noted to have T cells that, in vitro, express the homing receptor cutaneous lymphocyte antigen, which is thought to home the cell to the skin and mediate the response.[7] Celiac disease is the result of an immune response to gluten proteins in grains; this disorder is reviewed in the eMedicine Pediatrics article Celiac Disease.

Food allergens are typically water-soluble glycoproteins resistant to heating and proteolysis with molecular weights of 10-70 kd. These characteristics facilitate the absorption of these allergens across mucosal surfaces. Numerous food allergens are purified and well-characterized, such as peanut Ara h1, Ara h2, and Ara h3; chicken egg white Gal d1, Gal d2, and Gal d3; soybean-Gly m1; fish-Gad c1; and shrimp-Pen a1. Closely related foods frequently contain allergens that crossreact immunologically (ie, lead to the generation of specific IgE antibodies detectable by skin prick or in vitro testing) but less frequently crossreact clinically.[8] Recently, delayed allergic reactions to meat proteins have been attributed to reactions to carbohydrate moieties.[9]

Medication Summary


Despite following stringent avoidance measures for clinically relevant food allergens, accidental or inadvertent ingestions may occur. Therefore, patients must be instructed on actions to take in the event of a reaction. For persons with a potentially severe food allergy, prescription of self-injectable epinephrine is advised.[29]

A concise written plan for the treatment of allergic reactions resulting from accidental exposure to the food should be developed. Examples of such a plan can be downloaded from www.foodallergy.org. For patients with a history of a mild reaction, such as urticaria and pruritus following the ingestion of a food allergen, treatment may be limited to an oral antihistamine. However, the potential for a more severe reaction on subsequent exposures must be taken into consideration because of the possibility of the ingestion of a larger dose than previously ingested or an unexpected or unrecognized increase in the patient’s degree of sensitivity.

If the patient has significant systemic symptoms, the treatment of choice is epinephrine administered by intramuscular injection in the lateral thigh.[63] Examples of systemic manifestations of food allergy include generalized urticaria, laryngeal edema, lower respiratory symptoms (eg, chest tightness, dyspnea, wheezing), and hypotension. Epinephrine should likely be administered to any patient with history of a severe allergic reaction as soon as ingestion of the food allergen is discovered and the first symptoms appear, possibly even before symptoms appear.

Patient must be educated regarding when to use their self-injector and the proper technique. They should be instructed to obtain immediate medical assistance (eg, call 911) in the event of anaphylaxis.

Epinephrine is the primary medication indicated to treat anaphylaxis. Patients should not depend upon bronchodilators or antihistamines to treat anaphylaxis, though these can be used as additional therapies.[64] For more information on the treatment of anaphylaxis, please see eMedicine article Anaphylaxis.

Caregivers of children should be instructed on identification and treatment of allergic and anaphylactic reactions.

Additional therapy during an allergic reaction includes antihistamines. A bronchodilator may be used as an adjunctive therapy for asthma. Although corticosteroids are often given for anaphylaxis, they are not believed to alter the early symptoms; theoretically, they may reduce late symptoms.

Advanced medical therapy of food allergen–induced anaphylaxis may include antihistamines; bronchodilators; histamine 2 (h2) blockers; corticosteroids; administration of intravenous fluids, glucagon, and oxygen; as well as ventilatory and circulatory support in severe anaphylaxis.

source Medscape
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