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.

Content warning

This article contains sexually explicit language and themes.

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........

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