The patient: A 20-year-old woman in the United Kingdom

The symptoms: The patient had unprotected vaginal sex with her habitual male partner, and shortly afterward, her vulva and vagina began itching and swelling. She developed angioedema — swelling of the tissues below the surface of the skin — and hives appeared across her body. She also felt faint and short of breath.

What happened next: The woman visited a hospital and was given 10 milligrams of cetirizine, an oral antihistamine that’s typically used to treat hives and allergies. She was already aware that she was severely allergic to Brazil nuts, a type of tree nut, but it was unclear what triggered her allergic reaction in this instance.

They conducted skin prick tests on the patient using different semen samples from her partner; this involved exposing small regions of skin to each sample. For one test, the doctors used a sample taken when the man had not recently eaten nuts. The other test used a sample collected about 2.5 hours after he ate Brazil nuts. In the latter test, a 0.28-inch-long (7 millimeters) welt appeared on the woman’s skin. That suggested that Brazil nut allergens were behind her severe postcoital allergic reaction.

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What makes the case unique: There are other documented cases of individuals with severe allergies developing local allergic reactions after intimate contact. However, the culprit is usually the direct transfer of allergens through touch or kissing. In other words, the allergen is present on a person’s hands or mouth and then gets passed over.

“To our knowledge this is the first case of a severe food allergic reaction transferred by normal vaginal intercourse,” doctors at the hospital wrote in a report.