Freezing does not alter antigenic properties of fresh fruits for skin testing in patients with birch tree pollen-induced oral allergy syndrome.

To the Editor: The oral allergy syndrome (OAS) describes an allergic reaction that occurs on ingestion of certain fresh fruits and vegetables in pollen-sensitized subjects. The culprit allergens found in associated fresh foods are rapidly inactivated by digestion and cooking. As a result, symptoms are usually limited to the mouth and throat and preferably observed with raw forms of the food. When evaluating patients with OAS, skin prick tests (SPTs) with commercial extracts will often yield negative responses because the allergens are thought to be altered in the fabrication process of food extracts. Specific serum IgE (ssIgE) assays are also of limited value, and although recombinant allergens appear to be promising, they are not yet widely available. At present, the preferred method for evaluating patients with OAS is the use of SPTs with fresh fruits and vegetables using the prick-prick technique, which consists of pricking the fruit directly with a lancet and then using the same lancet immediately for the skin prick. This method has been shown to be much more sensitive than SPTs with commercial extracts for most foods implicated in the syndrome. A practical limitation to this approach is the access to fresh food at the clinic. One solution is to have the patient return for a second appointment with his own fresh food, but this is time consuming for both the patient and the physician and delays diagnosis. Another potential pitfall is that the culprit fruit might not be available in its fresh form depending on the harvest season. Although the effect of cooking is well established, much less is known about the effect of freezing on the allergenic properties of the proteins responsible for OAS. During freezing, soluble proteins are progressively entrapped in newly formed crystals and might experience shear tension at the ice-liquid interface. Although this phenomenon has been shown to decrease the enzyme activity of some soluble proteins, the effect on antigenicity has not been studied. We hypothesized that frozen fruits would have the same value as fresh fruits for the purpose of skin testing patients with OAS. Twenty-three patients with OAS were recruited from the allergy clinic at our center. Inclusion criteria were as follows: (1) sensitivity to birch pollen on skin testing, (2) oral symptoms to a plant food related to birch, (3) and sensitivity to this fresh plant food on skin testing. Demographic data and medical history were collected for each subject. They were also asked to fill out a questionnaire regarding their upper respiratory tract symptoms, and scores for rhinoconjunctivitis symptoms were calculated. Fruits were bought fresh and frozen in a regular home freezer at 2188C for 48 to 72 hours. Frozen fruits were thawed at room temperature for 15 minutes before evaluation. Skin tests were performed with aeroallergens and commercial fruit extracts (Omega Laboratories, Montreal, Quebec, Canada), as well as with fresh and thawed fruits, by using the prick-prick method. ssIgE levels were measured for each fruit by using UNICAP technology (Phadia, Uppsala, Sweden). Twenty-three control subjects without symptoms of OAS and with negative SPT responses to birch pollen were also recruited from the clinic staff and underwent skin testing. Agreement between allergy tests was analyzed by using the Pearson correlation coefficient (PCC). Sensitivity was calculated as described elsewhere. All statistical analyses were performed with the SPSS software package (SPSS, Inc, Chicago, Ill). The study was reviewed and approved by the local ethics committee. All subjects provided written informed consent before enrollment into the study. Demographic and clinical data are presented in Table I. Table II shows allergy test results and agreement between tests for each fruit. Although correlation between fresh and frozen fruit SPT responses is shown to be systematically high and significant, correlation between fresh fruit and extract SPT responses remains very low and seldom significant. Somewhat lower PCCs were found between fresh and frozen plum, peach, nectarine, and Golden Delicious apples when compared with other fruits. This could be explained in part by the fact that these fruits elicited some of the largest SPT responses because marginally high results are expected to have lower reproducibility (regression toward the mean phenomenon). When considering all Bet v 1–related fruits (ie, excluding cantaloupe and watermelon, which are associated with allergy to the birch allergen Bet v 2), the size of the wheal from fresh fruit tests did not correlate with that obtained with birch tree extracts or with rhinoconjunctivitis scores (PCC, 0.011 and20.021, respectively; not significant). There was, however, a poor but significant correlation between fresh fruit SPT responses and ssIgE values (PCC, 0.227;P5 .003). Similar findings were observed for watermelon and cantaloupe, although correlation with ssIgE levels was nonsignificant, possibly because of the small numbers of patients sensitized to these fruits. Overall sensitivities of SPTs with fresh (81%) and frozen (82%) fruits were similar and in line with those reported in the literature. Overall sensitivity of SPTs with fruit extracts (12%) and ssIgE levels (69%) also fell within the expected range. Interestingly, ssIgE levels and extract SPT responses did not correlate TABLE I. Clinical characteristics of subjects