The patient: A 29-year-old woman in Lithuania
The symptoms: The woman had been trying to conceive with her male partner but had not become pregnant. Two rounds of in vitro fertilization (IVF) also failed to produce a pregnancy, but gynecological exams did not find any underlying causes for why she was unable to become pregnant. (When the patient’s doctors described the case in a report, they did not mention if any fertility tests were also conducted on her partner.)
However, the woman had a history of asthma and sensitivity to inhaled allergens, such as mold, cat fur and dust. So she visited a medical facility to see if her allergies might be affecting her fertility.
What happened next: At the facility, blood tests revealed that the woman had unusually high amounts of eosinophils, a type of white blood cell that defends the body against allergens. Skin tests showed that, in addition to the woman’s known allergy triggers, she was sensitive to mites, pollen from weeds and grasses, and allergens from insects and dogs.
The patient was especially sensitive to a protein called Canis familiaris allergen 5 (Can f 5), which is found in dog dander and urine. Sensitivity to Can f 5 can also indicate sensitivity to similar types of proteins found in human semen, the doctors wrote in the report.
In an interview with an allergist at the facility, the patient confirmed that she experienced nasal congestion and sneezing after unprotected intercourse with her male partner. These symptoms were previously overlooked by other specialists during consultations about her inability to conceive, she reported.
The diagnosis: Doctors then conducted further allergy tests using samples of semen collected from the woman’s partner. The patient’s allergic response confirmed the doctors’ suspicions that she had a human seminal plasma allergy. (Seminal plasma is the fluid component of semen that carries the sperm cells.)
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Sensitivity to semen “is a potential cause of female infertility,” because such allergies can trigger inflammation in reproductive organs, according to the report. It’s not immediately clear if the allergy somehow complicated the woman’s IVF treatments, as well, given semen would not have been present in the implanted embryos.
The treatment: Barrier contraception — namely, condom use — is the most common intervention for a semen allergy. However, the patient was still eager to conceive with her partner, so she rejected that strategy.
The only known treatment for reducing sensitivity to semen involves introducing the fluid into the patient’s body in gradually increasing concentrations, to build up their tolerance to the allergens. But this course of treatment was unavailable in Lithuania, the doctors wrote. Instead, they recommended that the woman take antihistamine medication before intercourse to reduce the severity of her allergic reactions.
She followed their instructions but found this approach “ineffective,” the doctors wrote. During a follow-up visit three years later, the woman said that she had still been unable to conceive. What’s more, new allergic symptoms now appeared after contact with her partner’s semen, including a burning sensation in her vulva, puffy eyelids and watery eyes. No further treatments were recommended, according to the report.
What makes the case unique: Worldwide, medical experts have documented about 80 cases of human seminal plasma allergy, and there is still much to be learned about its impact on pregnancy, especially in the presence of other health-related issues that could hinder conception.
The causes of infertility are often difficult to pinpoint. Therefore, “this case serves as a reminder that seemingly unrelated allergic conditions, when combined, can contribute to reproductive health challenges, warranting comprehensive evaluations,” the woman’s doctors wrote.
This article was first published July 23, 2025.
This article is for informational purposes only and is not meant to offer medical advice.
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