As with nearly any substance, it is possible for people to be allergic to semen. This condition, referred to as human seminal plasma hypersensitivity, is estimated to affect 5% of women. Since semen naturally occurs in the male body, it is quite rare for men to suffer from this condition, though there are reported cases of patients receiving reverse vasectomy operations who have later developed allergies to their own seminal fluids.
This condition was first reported by gynecologists in 1958, but was not well known until 1996 when a study was conducted by Dr. Jonathan A. Bernstein at the University of Cincinnati College of Medicine. The study connected semen allergies with food allergies because of the similar protein compositions in each.
The average male ejaculate can contain aboutonia, ascorbic acid (vitamin C), blood-group antigens, calcium, chlorine, citric acid, cholesterol, choline, creatine, deoxyribonucleic acid (DNA), fructose, glutathione, hyaluronidase, inositol, lactic acid, magnesium, nitrogen, phosphorus, phosphatase, potassium, purines, pyrimidines, pyruvic acid, sodium, sorbitol, spermidine, spermine, urea, uric acid, vitamin B12, selenium, and zinc - any of which can cause an allergic reaction. Though obviously natural, and required for survival, semen does perform many actions in a woman's body that may be interpreted as "hostile" by the immune system. Clotting agents increase the chance that the sperm will stay in the vagina long enough to make their way to the egg. Prostaglandins stimulate the muscles surrounding the vagina to contract and draw the semen upwards. Either of these, as well any of the contents, could possibly be identified as a foreign substance in the body. Since the quantities of any of these substances vary from man to man, women with this condition may not experience problems with all sexual partners.
The hypersensitivity can cause localized or systemic reactions. The localized reactions can include vaginal burning, pain, swelling, redness, or blisters which usually occur 30-60 minutes after exposure to semen. The systemic reactions can include generalized itching, hives, angioedema, wheezing, and in very rare cases anaphylaxis (shock). Systemic reactions are much more rare than localized, and most patients reported that the symptoms from either reaction resolved themselves (assumably once the semen was expelled from the body or neutralized by the immune system).
There is no correlation between human seminal plasma hypersensitivity and fertility problems in the male or female. However a woman with this condition can experience numerous problems and frustrations in trying to conceive because of the discomfort.
It should be noted that other aspects of your life can cause similar symptoms, and one should not immediately assume it is semen allergies. Many women are sensitive or allergic to the contents of birth control pills, latex condoms, lubricating gels, spermicides, or even their own or their partner's hygiene products. In addition, yeast infections can cause similar effects, and can be inflamed by sexual activity.
The simplest (though inconvenient) method of treatment is the use of condoms. There are other options which are similar to allergy shots. The woman is systematically exposed to larger and larger samples of the proteins found in semen*. For many women, these treatments either cure them of their hypersensitivity, or make it tolerable.
*Get your mind out of the gutter.