[A case of "late-onset" anaphylaxis caused by fermented soybeans; Natto].

Natto is a Japanese traditional food made from fermented soybeans. We report a case of anaphylaxis caused by natto and review the literature. The patient was a 22-year-old man who showed systemic eruption with itching and pectoralgia about 10 hours after eating a meal containing natto. Results of skin tests for soybean allergen were negative, and the allergen remained unidentified. We then used a food elimination trial to confirm the allergy. However the patient did not refrain from eating natto, and he had three anaphylactic reactions might have been caused by natto. Each event occurred 10 to 14 hours after he ate a meal containing natto. We performed detailed examinations to determine the allergen. First, the prick-by-prick tests with natto and its characteristic viscous yarn-like surface were done and yielded positive results. Next, a provocation test with commercial natto (50 g) was performed and caused systemic eruption and pectoralgia about 9 hours after ingestion of the natto. The patients'plasma histamine level was elevated during the anaphylactic event. Anaphylaxis caused by natto was diagnosed. Recent studies have shown that the anaphylaxis caused by natto is of late-onset. Late-onset anaphylaxis can be considered one of IgE-mediated allergic reactions. The viscous surface of natto contains poly-gamma-glutamic acid (PGA). The hypothesized mechanism of late-onset anaphylaxis is delayed absorption or release of PGA into the bowel. In our case, the patient ate heated natto, we therefore speculate that suspect allergens were heat resistant. Patients with natto allergy must not eat natto, whether or not it is cooked or heated. Natto may induce allergic reactions up to a half-day after ingestion; thus, the clinical course and patient's diet must be considered during medical examination. Natto has recently gained popularity as a health food in foreign countries. The existence or natto allergy should be more widely recognized.