INTRODUCTION
Few publications inliterature report allergic reactions to foods belonging to theLiliacee family (garlic, onion, asparagus). Cross-reactivity amongthe members of this family (1) or between them and grass pollens(2) have been reported. Some cases of contact sensitization havebeen also reported (3, 4, 5). IgE antibodies against onion havebeen demonstrated, both by in vivo and in vitrotests, in four patients having concomitant pollen-inducedrhinoconjunctivitis and asthma, who referred clinical symptomsinduced by the handling of onions, e.g. by chopping, with positiveskin prick test reactions to both heated and non-heated onionextract. In present report a case of anaphylaxis to raw onion (notcooked), confirmed by in vitro tests, isdescribed.
CASEREPORT
A 44-year-old womanreported she had never suffered from allergic symptoms until fouryears ago, except for a hypersensitivity reaction to peach skin atage about 20, which had then regressed. During the last four years,however, she had suffered immediate reactions (at least 5-6episodes) within minutes after eating raw or lightly cooked onion.The reaction had become increasingly severe with intense itching inthe mouth followed by a state of confusion, blurred vision,transient loss of consciousness, profuse sweating, tachycardia andsubsequent transient prostration. The last episode, in June 1998,had been the most serious, lasting several hours, and the patienthad also urticaria on the face and the neck. During the last fewmonths she reported intolerance even simply to contact with rawonion and to inhalation of the steam of onion cooking. She hadnever had any trouble with well-cooked onions.
MATERIALS ANDMETHODS
Skin prick tests havebeen made with extracts of common inhalants and some foods (milk,codfish, egg, peanut, corn, soy, walnut, asparagus, garlic) ofcommercial source (Lofarma SpA, Milan Italy). In vivo testswith onion allergen have been avoided for ethical reasons,considering the severity of symptoms in the referredepisodes.
Total andallergen-specific IgE have been determined in serum with REAST,which briefly consists in the following steps: capture of IgE witha specific antibody absorbed on microtiter wells, incubation withbiotinylated liquid allergens, with streptavidin-peroxidase andchromogenic-substrate, reading of optical density and interpolationon a reference curve prepared with WHO 75/502 internationalstandard for IgE (7). The test has been performed employingreagents of commercial source (Realtest®IgE-Density, Lofarma SpA, Milan, Italy).
The comparative invitro test with unheated and heated onion have been made withan extract obtained by mechanical homogenization of fresh onions at5% weight/volume ratio in phosphate buffered saline, overnightextraction under stirring; the suspension was then centrifuged andthe clear surnatant collected (non-heated onion). Part of theextract was heated for 30' in boiling water, centrifuged and theslight precipitate discarded (heated onion). These extracts havebeen bound to plystyrene beads as solid phase and tested withcommercial RAST reagents, consisting of polyclonal anti-IgE (fromgoat) labeled with 125Iodine and a grass-pollen-positivereference curve (Sferikit® specific-IgE, LofarmaSpA, Milan, Italy).
RESULTS
Skin prick test to commoninhalant and food allergens resulted negative. The REAST revealed acontent of onion-specific IgE in serum of 3.7 kU/L, and a contentof total IgE of 12 kU/L. The density of specific-IgE [calculated aspercent ratio to total IgE, according to (7)] resulted 30.8%. TheRAST with non-heated onion resulted positive with 2.5 RAST Unitscorresponding to class 2, with heated onion resultednegative.
DISCUSSION
Onion is commonly acontact sensitizer (3-5). Reports on IgE-mediated type I reactionsare few, and in all the cases the patients were affected frommultiple sensitization, often to grass pollen (2, 6). This is thefirst report on a monosensitization to onion, characterized bysevere systemic reactions. It is interesting to note that thecommonly observed cross-reactivity in the Liliacee family (garlic,asparagus) was in this case absent, being skin prick testsnegative. The total IgE serum level (12 kU/L) is unusually low foran adult, as well as the low level of onion-specific IgE (3.7 kU/L)is apparently not related to the severity of symptoms subsequent toallergen exposure. As a matter of fact, recent studies showed thatthe clinical severity of allergic sensitization is poorly relatedto the absolute level of circulating allergen-specific IgE, whileis strictly related to their density, calculated as percent ratioto total IgE (8). The affinity of IgE for the receptorFc*RI is so high (Kd of10-10 mol/L) that we can consider circulating IgEantibodies to be the result of overproduction with respect to thewhole receptor capacity of mast cells and basophils, whichconcentrate IgE on their surface (9). On the contrary the densityof allergen-specific IgE is strictly related to the releasabilityof these cells, by rendering easier the bridging and subsequenttype I events. In the patient of present report the density ofonion-specific IgE was 30.8%, value that is sufficient to explainthe severity of clinical picture, according to the experimentalstudy of Crimi, et al, in which allergen sensitivity determined bynasal and bronchial challenge has been related to IgE density (8).An original finding of present report is the negativity of RASTwith heated onion, while in the study of Valdivieso, et al, all thepatients showed a positive skin test reaction both to heated and tonon-heated extracts (6). Evidently the IgE antibodies of ourpatient recognize thermolabile antigenic fractions, different fromthose recognized by patients described in the article ofValdivieso, et al. The existence of thermolabile allergens is wellknown, as in the case of allergy to wheat flour in bakers, whousually have symptoms to inhaled raw wheat flour while can eatwithout any problem bread and other wheat-containing cooked foods(10).
