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Allergies are brought on when the immune system acts as though foreign substances are more harmful than they really are.
According to the Centers for Disease Control and Prevention, eight types of food account for more than 90 percent of allergic reactions in affected individuals. The eight: milk, eggs, peanuts, tree nuts, fish, shellfish, soy and wheat.
However, some people react to some very unusual allergens. Here are nine of the weirdest allergies.
SemenSlide 2 of 19
While it's possible for women to be allergic to their partners' semen, it's also possible, if even less common, for men to be allergic to their own semen.
Dutch researchers reported two cases of postorgasmic illness syndrome in the January issue of the Journal of Sexual Medicine. In both cases, the men experienced allergic symptoms around their eyes and nose, and transient flu-like illness within seconds, minutes or hours after sex, masturbation or spontaneous ejaculation. Skin-prick tests confirmed they were indeed allergic to their own semen.
Their allergies were eventually treated by hyposensitization therapy, in which a person is injected with small amounts of their allergen in the case, their own semen over time, in gradually increasing doses. The researchers reported that the men showed significant improvement in their symptoms after three years of treatment.Slide 3 of 19
BeefSlide 4 of 19
Allergies to beef are unusual, especially in adults. But according to a case study reported in the Journal of Investigational Allergology and Clinical Immunology in 2001, a man with no apparent history of allergies suffered two such reactions to beef. One came immediately after eating sirloin steak, and the other, some days later, occurred after eating a hamburger made with cow's meat. A skin-prick test showed he was allergic to beef, pork, lamb and rabbit meat, but not allergic to cats, horses, mites, soybeans or cow's milk, the study said.
Because the patient could tolerate milk and poultry products, the doctors recommended he eat a meat-free diet, with the exception of poultry. However, he had trouble sticking to his new diet. Four months later he had two new breakouts, after eating lamb and pork sausage, respectively.Slide 5 of 19
Female hormonesSlide 6 of 19
It's relatively common to hear women complain of worsening acne and water retention at certain points of their menstrual cycle. However, a small number of women suffer a condition called autoimmune progesterone dermatitis (APD) a skin disorder that's made worse by progesterone hypersensitivity during the luteal phase of the menstrual cycle, which occurs after ovulation.
APD usually occurs in adult life and rarely during pregnancy or the postmenopausal period.
A typical case was reported in the European Journal of Dermatology in 2002. A 27-year-old woman was admitted to a clinic for recurrent facial hives that always started about three days before her period and faded within seven days. Her skin lesions would disappear spontaneously without any residual markings until her next cycle.
The diagnosis was confirmed when a skin-prick test showed that progesterone caused redness and swelling within 30 minutes. Once the woman was diagnosed, her symptoms were resolved by hormone-based therapy.Slide 7 of 19
WaterSlide 8 of 19