The people who are 'allergic' to humans

Some people are allergic to others – but the mystery of how this works is only just starting to be unravelled.

Maura believes condoms saved her life.

Now 43 and living in Ohio, US, Maura says the problem first started in her twenties, when it slowly crept up on her. “I noticed that my genitals burned after [unprotected] sex," she recounts. 

Maura (whose name has been changed to protect her privacy) didn't feel comfortable mentioning it to her partner. So, she'd wait until he left and then wash herself thoroughly. She tried changing the personal care products she was using, from soap to lubricant. But the problem only worsened, extending to swelling and redness. And it only happened after she came into contact with semen.

Eventually she broke up with that man, and started seeing one who was dedicated to using condoms. "It wasn't an issue until one night when we were lying in bed after sex and my tongue suddenly began to swell," Maura recounts. "My partner saw what was happening, screamed, 'You're asphyxiating!' and grabbed my inhaler…he was able to cram my inhaler into the corner of my mouth, and he just started firing it. Luckily, I was still breathing enough to draw the medicine into my lungs."

Maura, who also has asthma and a number of allergies, believes that the condom had leaked. She and her long-term partner are now even more careful about condom usage. Until it happened to her, she didn't know it was possible to be allergic to semen, she says.

Though they're extremely rare, some people suffer severe immune reactions to other people's bodies. These often-misunderstood conditions can affect not just health, but also work, relationships and generally how someone moves through the world. But how exactly these reactions unfold, and what causes them, remains largely mysterious. Are they true allergies, or something else? As scientists begin to glean some hints, these strange responses are uncovering insights into the chemistry of our bodies and the quirks of the human immune system. 

Often, sensitivity to another person's body relates to the external products turning up in that body. For instance, the skin can carry synthetic fragrances, including in deodorants and aftershaves. Over 150 fragrances are linked to contact allergies.

The trigger is not always clear. One American woman with a severe version of mast cell activation syndrome, in which infection-fighting cells start malfunctioning, developed debilitating allergic reactions to her husband's scent. Sabine Altrichter, a doctor at Kepler University Hospital in Austria, says that while the link isn't proven, some patients with mast cell disorders suspect that they're sensitive to natural body odours or chemicals emitted by other people's skin.

The skin emits many compounds that contribute to body odour. These skin gases can include chemicals like toluene, which occurs in crude oil and is used to make products including paints and plastics. People can absorb toluene deliberately, for instance when inhaling glue to get high, or unintentionally, for instance through exposure at work. Toluene is also one of numerous chemicals in tobacco smoke.

One group of people who might be able to shed some light on reactions to other humans are those suffering from the mysterious condition People Allergic To Me (PATM). PATM is an unusual and isolating phenomenon where others frequently develop allergy-type symptoms, such as coughing and choking, in their presence. 

In 2023, Yoshika Sekine, a professor of chemistry at Japan's Tokai University, and his colleagues investigated the skin gases emitted by those reporting symptoms of PATM. Of the 75 skin gases the team studied, toluene was especially likely to be present. People in the PATM group emitted 39 times more of this chemical, on average, than those without the condition.

"Toluene is inhaled through the air during breathing. As a harmful compound, it is typically metabolised by the liver and excreted in the urine," Sekine explains. "However, PATM patients have a diminished ability to break down toluene, leading to its accumulation in the bloodstream and subsequent release through the skin," he says.

Sekine notes that the very concept of PATM is still not widely recognised, and there are no diagnostic criteria for it.

Meanwhile, sweat allergy generally involves sensitivity to one's own perspiration, rather than others'. As for hair, in the rare cases where allergies related to human hair have been recorded, the reaction has been caused not by an allergen in the hair itself, but allergens in external substances: for example, formaldehyde derivatives in keratin hair treatments, or a cat protein that gets into cat owners' hair. 

Allergic reactions can also be triggered by specific allergens carried in bodily fluids. In one UK case, a woman with an allergy to Brazil nuts developed hives and shortness of breath after having sex with a man who had eaten mixed nuts a few hours earlier, even though he had cleaned his teeth, nails and skin in between. Nuts have also led to allergic reactions during kissing in people with severe allergies. 

While nuts are the most commonly reported allergen posing an issue during kissing, saliva has also induced allergic reactions following the consumption of fruits, vegetables, seafood and milk. Women with antibiotic allergies have reacted negatively after vaginal and (possibly) oral sex with people who had taken those medicines.

But beyond these external allergens, proteins within certain bodily fluids themselves can also trigger a reaction. The one that some clinicians are familiar with, though there are still major gaps in awareness, is semen.

Semen allergy, or seminal plasma hypersensitivity, involves developing symptoms ranging from a raised, itchy skin rash (hives) to the potentially life-threatening allergic reaction anaphylaxis, after exposure to this bodily fluid. It has been recorded mainly in people in their 20s and 30s, although there have been fewer than 100 documented cases overall, according to a 2024 paper. The allergen most frequently linked to this sensitivity is a prostate-specific antigen, a substance that can trigger an immune response, in the seminal plasma. This is the fluid, apart from sperm, that makes up the majority of semen, and the allergy is to a protein within it, rather than the sperm itself.

It's not clear exactly what is occurring in the body in people with seminal plasma hypersensitivity, explains Jonathan Bernstein, a professor of clinical medicine, focusing on allergy and immunology, at the University of Cincinnati’s College of Medicine in the US. Bernstein says that there aren't good animal models for seminal plasma hypersensitivity, or enough humans with the condition to enable large-scale research.

Cases of semen allergy can be localised or systemic. When the symptoms are localised and limited to around the area of contact, this is usually reported as occurring with or around the vagina. But in one Spanish case report, a woman who had never experienced an allergic reaction after vaginal sex lost consciousness and developed other symptoms of anaphylaxis following anal sex. She was diagnosed with hypersensitivity to seminal fluid. A woman in the US also experienced swelling and rash in a non-sexual situation, when her skin came into contact with ejaculate.

The local symptoms can include severe pain and burning immediately after intercourse. "It's [reportedly] like acid," Bernstein says. One of his patients described it as "like a thousand needles being stuck into your vagina."

A person might be sensitive to the semen of multiple partners or just one, Bernstein notes. Diagnosis typically involves a skin prick test using a fresh sample of seminal fluid from a sexual partner. In his practice, Bernstein typically sees women in monogamous relationships with men, often when they're trying to get pregnant. Some people travel long distances to consult with him, as there aren't many authorities on semen allergy. Many patients get brushed aside or are started on aggressive steroid treatments because medical professionals don't know what to do with them, Bernstein says.

However, Bernstein explains that his practice is able to help just about everyone with a semen allergy.

There is a notable absence of data about semen allergies among men who have sex with men. Bernstein says he has never seen a case of this, though it’s unclear why. He wondered if the symptoms could be related to the particular conditions within the vagina, though this doesn’t explain the case where it occurred after anal sex between a man and a woman. 

One previous treatment Bernstein trialled involved courses of injections of a partner's semen within or under the skin to desensitise the patient. This is similar to the treatment for post orgasmic illness syndrome, a rare and often-debilitating condition where men are sensitive to their own ejaculate. But this was costly: "patients were having to pay for that because there was a lot of laboratory work in preparing the samples," Bernstein says.

Bernstein and his colleagues found that it was reportedly equally safe and effective to localise the treatment, in a one-time procedure of about two hours. First, they separated the sperm from the seminal fluid. Then they diluted the seminal fluid to one part in a million or one in ten million, depending on how severe the patient's reactions were. Then, at intervals of 15 minutes, they inserted the fluid into the patient's vagina. They gradually used stronger concentrations of the fluid, so that the patient developed a greater tolerance. Throughout, they continued monitoring the patient. As a result, "they don't tend to have many systemic reactions and they are able to tolerate unprotected intercourse thereafter" with at least that partner, Bernstein says.

In general, hypersensitivity to seminal plasma remains often misunderstood and misdiagnosed. There's even less information about certain other fluids transmitted during sex. 

There is almost no published research on possible allergy to cervicovaginal fluid – a liquid secreted by the cells of the cervix and vagina, which helps to lubricate this region and provide some protection from pathogens. However, Marek Jankowski believes that he’s seen at least one person with this condition. Jankowski, an assistant professor of dermatology and venereology at Nicolaus Copernicus University in Poland, says he once treated a patient who came to him after seeing a host of other doctors. The patient reported that about 30 minutes after vaginal intercourse, his genitals would become red and itchy. His face would also itch after cunnilingus. To the patient, this seemed like an allergy, but doctors ridiculed or rejected this idea, says Jankowski. 

However, Jankowski kept an open mind, looking up other cases of potential allergy to cervicovaginal fluid secreted by women during sexual activity. "Ultimately the patient responded well to antihistamines," he reports. The case also prompted Jankowski and his colleagues to carry out a study, published in 2017. The researchers surveyed other dermatologists as well as people who might be experiencing this condition. One-fifth of the dermatologists who responded had witnessed such cases, they reported, though many doctors remained sceptical that the condition existed.

According to this research, sufferers reported redness, itch, burning, swelling and hives following contact. Their reports led Jankowski and his colleagues to estimate that allergy to cervicovaginal fluid was as common as semen allergy, which is thought to affect at least tens of thousands of people in the US alone. However, "the current level of evidence for cervicovaginal fluid allergy phenomenon is only circumstantial and further research in this field would be necessary," Jankowski explains. 

One difference between allergies to semen versus cervicovaginal fluid is that condoms would be unlikely to relieve the symptoms of any allergy to the latter, since condoms would not shield the groin and scrotum. But both antihistamines and repeat exposure appeared to help the survey respondents with their allergy to cervicovaginal fluid, according to the survey results of Jankowski and his coauthors. "As [the] majority of cases identified were young adults early in the sexual relationship, the flame of passion was apparently stronger than the discomfort, and repetitive [exposure] to the allergen led to desensitisation," Jankowski notes. This differs from seminal plasma hypersensitivity, which is not known to resolve itself naturally.

For the people who are allergic to some aspect of their partner, the consequences can be grave. Maura believes that her semen sensitivity played a part in her and her partner's decision to not have children, because she expects that it would have been expensive to find a workaround that enabled her to avoid exposure to semen.

The emotional impacts can be complex, for both sufferers and their partners. While Maura's relationship is safe and her partner is happy to use condoms, "he did tell me that he was offended by the idea that I was allergic to his semen," she says. “He doesn't blame me for that, just the universe." 

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