Let's say your friend is allergic to peanuts or perhaps she's bothered by mold. These are common allergies, so the news comes as no great shock.
But what if she discovers she's allergic to her husband? Or, more specifically, to his semen?
Yes, an allergic reaction to a man's semen is a real problem, and it's just one of several little-known but serious allergies that afflict people. Some of these can even be deadly.
"It's surprisingly common," said Dr. Larry Borish, professor of medicine at the Asthma and Allergic Disease Center at the University of Virginia Health System in Charlottesville.
"I see a few cases a year," Borish said. "It tends to be particular to one partner. It's the protein in the semen and these are particular enough that they change from person to person."
"These are women who typically present hives and swelling locally, in the vagina," he said.
In severe cases, however, there can be a full-blown anaphylactic reaction, which may cause hives, swelling, difficulty breathing, abdominal cramping and a loss of blood pressure serious enough to be life-threatening.
One way a person might try to solve his or her semen allergy is by using a latex condom during sex. But that could present another set of problems: latex allergies.
Latex allergies were more commonplace in the early days of the AIDS crisis, explained Dr. Harold Nelson, allergist and professor of medicine at the National Jewish Medical and Research Center in Denver.
"There was an epidemic of latex sensitivity among health care workers because of the introduction of latex as a preventive measure against HIV transmission," Nelson said. The main culprit seemed to be the powder used in latex gloves.
Changes in the manufacturing process, however, have since eliminated many of the problems associated with latex allergies.
"This epidemic is definitely over," Nelson said, though there are still people who have an allergic reaction to balloons, condoms and other products made with latex.
For others, just standing near an open window can cause devastating health problems.
Solar urticaria affects a small percentage of the population, but for those who suffer from hives, skin rashes, nausea and vomiting caused by any exposure to the sun, the allergy can wreak havoc on their lives.
For them, any exposure to sunlight must be avoided, forcing them to lead isolated, reclusive lives behind closed doors and windows.
"The assumption is that the sun -- or ultraviolet radiation -- denatures [damages] some protein in human skin, and you develop an allergic reaction to these denatured proteins," explained Borish.
But urticaria can show up from a number of different causes, including cold air. Borish noted that some construction workers can develop an odd form of the disorder: "There are people who work on jackhammers who have vibration urticaria," he said.
Though most people are aware of the allergy problems that dust mites can cause, cockroach allergies affect many city dwellers.
"Cockroach allergy is a huge problem in the inner city," said Borish.
He estimates that up to 50 percent of inner-city dwellers suffer from allergies to cockroaches.
"The main problem has been with inner-city children," added Nelson, who said the problem causes a significant number of missed school days and hospitalizations.
But allergic reactions to insects aren't limited to roaches and other household pests.
"One of the things that we've been seeing here in rural Virginia is ladybug allergies," Borish said.
But researchers may now be on the path to discovering the key to insect allergies, because the problems with roaches and ladybugs seem to be related, even though those insects are not closely related.
Borish explained that the allergic reaction stems from a digestive enzyme in the insects' gut.
"This is the part that's really cool," he said. "The protein that causes the ladybug and cockroach allergy is the same protein."