"It's really a very rare condition, but it does happen," allergist and immunologist David Resnick at New York Presbyterian Hospital told LiveScience.
Semen allergy symptoms can include itching, burning and swelling in the genitals. In the most severe cases, hives or swelling might appear elsewhere on the body and the woman might experience difficulty breathing.
"Typically symptoms occur within 30 minutes of intercourse, but in rare cases it may be hours or even days later," Resnick explained.
One study from the University of Cincinnati of 1,073 women who sought information on semen allergy concerning their symptoms found 130 had the allergy. In some women, the reaction occurs only with one partner while others are allergic to all partners.
Women are not in all cases allergic to semen itself, but to chemicals in the semen from food, beverages or medications the man has had, ranging from penicillin to compounds in walnuts. However, "this is rare, even more rare than semen allergy itself," Resnick said.
Resnick said about half of all women with semen allergy have other allergies as well, such as skin allergy or hay fever. Most women with the allergy are between age 20 and 30, and 41 percent experience symptoms the first time they have sex.
"In most cases, symptoms gradually worsen and occur sooner with subsequent exposures," he said.
To desensitize a woman's immune system against semen, doctors can either apply diluted samples of semen to a woman's vagina every 20 minutes, gradually increasing the concentration over the course of several hours, or the women can receive allergy shots containing small amounts of semen over the course of several weeks. Both techniques require sex two or three times a week to train their immune system.
There are also cases where women have "outgrown" the allergy without treatment.
The compounds in the semen that appear to trigger this allergy seem to be proteins from the prostate gland. "People are now testing them for use in desensitization therapies," Resnick said.
Resnick discussed the condition today at the annual meeting of the American College of Allergy, Asthma and Immunology in Philadelphia.