For three years, a 35-year-old woman in Japan kept suffering severe allergic reactions at night. More than 10 times, the episodes struck one to three hours after she went to bed, bringing hives, facial swelling, trouble breathing, and abdominal pain that required emergency epinephrine injections.
Blood tests pointed to an obvious suspect: very high antibodies to Anisakis, a parasitic worm found in raw or undercooked fish. But the real trigger, according to a case report published July 29 in the Journal of Allergy and Clinical Immunology: Global, was proteins in her partner's semen. Her doctors believe it is also the first full case report of a live birth through in vitro fertilization, or IVF, in a patient with the allergy.
A Fish Parasite That Did Not Fit
Physicians at Showa Medical University in Tokyo noted that Anisakis is a common allergen in Japan, accounting for 10 to 20 percent of adult anaphylaxis there. Her Anisakis-specific IgE measured 51.50 kU/L, a class 5 result, so delayed Anisakis anaphylaxis was the first suspicion. But she had not eaten raw seafood in the period before her attacks, and the pattern did not fit. She also had atopic dermatitis, sensitization to house dust mites and rabbit dander, and a long history of keeping pets.
A detailed sexual history and further testing led the team to a diagnosis of human seminal plasma allergy, a rare IgE-mediated reaction to proteins in semen. Skin prick tests using her partner's freshly collected semen produced positive reactions to whole semen, to its fluid portion and to the sperm pellet. The authors wrote that a basophil activation test, a blood test that measures allergic cell activation, can add supporting evidence using simple semen preparations.
Delayed Reactions Can Hide the Cause
About 30 percent of cases present atypically, the authors wrote, with only whole-body or delayed reactions and none of the classic local symptoms such as burning or itching. The most telling clue in a patient's history, they noted, is that symptoms are completely prevented when a condom is used. Patients may hesitate to share sexual details, and clinicians may not ask.
Only about 80 cases have been documented, according to the report, though the true prevalence is likely higher because the allergy so often goes unrecognized. One extrapolation the authors cite suggests up to 40,000 potential cases in the United States. The Tokyo team concluded that hidden allergens, including semen, should be considered when nighttime anaphylaxis resists explanation. The case also shows how a strongly positive test for one allergen can steer attention away from another.
A Route to Pregnancy Without Exposure
Condoms prevent reactions, but they also prevent conception. Insemination with washed sperm is one alternative, though a Paris team reporting in Fertility and Sterility described a woman who had an allergic reaction immediately after the procedure, treated with steroids, and called the method not totally reliable. That single attempt still produced a pregnancy and a healthy girl.
Other published cases show how varied the workarounds can be. Another report described a woman who conceived after six inseminations with carefully washed sperm and no anaphylactic symptoms. In a report in Clinical and Molecular Allergy, doctors in Jeju, South Korea, treated a 32-year-old woman who developed hives, facial swelling, and low blood pressure after exposure to semen. Because she wanted to conceive a second child naturally, they desensitized her with gradually stronger dilutions of her husband's seminal plasma placed in the vagina. She had no symptoms during six months of observation and was advised to have intercourse more than once a week to maintain tolerance.
Given her age, her wish to conceive soon, and the need to minimize allergic risk, the Tokyo couple chose IVF over intrauterine insemination. IVF kept seminal plasma out of her body entirely, and after two IVF cycles, she had an uncomplicated pregnancy and gave birth to a healthy child.
A Separate Case Raised the Fertility Question
The allergy has also surfaced in fertility clinics. In a report in Frontiers in Medicine, Vilnius University clinicians described a 29-year-old woman with unexplained infertility and two unsuccessful IVF attempts. She had asthma and allergic rhinitis and reported burning, vulvar itching and swelling, along with sneezing and nasal congestion, after unprotected sex.
Testing showed pronounced sensitization to dust mites and to Can f 5, a dog allergen the team suspected of cross-reacting with a human semen protein, and a skin prick test with her partner's semen confirmed the allergy. She also had mild endometriosis, which can affect fertility on its own, so the allergy's role remains uncertain. Her doctors presented the case as a reminder of possible links between allergic conditions and reproductive health.
Mayo Clinic says semen allergy is not a direct cause of infertility and lists treatment to reduce sensitivity, washed-sperm insemination, and IVF among the options for couples trying to conceive.
These reports describe single patients and cannot show how often semen allergy causes anaphylaxis or contributes to infertility. Anyone with recurrent reactions after sex, or with unexplained nighttime anaphylaxis, should discuss it with an allergist, who can test for the condition.
Key Questions Answered
What is human seminal plasma allergy?
It is a rare IgE-mediated allergic reaction to proteins in semen. Symptoms range from local burning and itching to hives, swelling, and anaphylaxis.
Why was the diagnosis missed at first?
Her very high antibodies to Anisakis, a common allergen in Japan, suggested a fish parasite. Her delayed nighttime reactions also lacked the classic local symptoms.
How was the allergy confirmed?
Skin prick tests with her partner's semen, its fluid portion, and the sperm pellet all produced positive reactions.
Can women with this allergy become pregnant?
Yes. Options include desensitization, insemination with washed sperm, and IVF. In this case, IVF avoided exposure entirely and led to a healthy birth after two cycles.
Does semen allergy cause infertility?
Mayo Clinic says it is not a direct cause. A 2024 case raised the possibility of a contribution, but that patient also had endometriosis.
Who should be tested?
People with recurrent reactions after sex, or with unexplained anaphylaxis, can ask an allergist about testing.
