In case anyone landed here after listening to this week's Savage Lovecast, I wanted to point you in the right direction. Please be gentle - that was my first time being interviewed for anything like that, so I was a bit of a nervous wreck. For the uninitiated - a couple of weeks ago on his podcast, Dan Savage fielded a question from a woman that thought a chunk of mushroom managed to make it through her boyfriend's digestive tract untouched and end up in his ejaculate. Dan rightly dismissed this notion, saying that things don't go "from guts to nuts." This is certainly true for solid pieces of undigested food, but as I wrote about earlier this year, allergens actually can go from guts to nuts.
If one partner ingests something that the other is allergic too, small molecules from the digested food will be absorbed into the bloodstream and make their way into secretions, including sweat and semen. This can then cause an allergic reaction in the other partner.
I also mentioned that women can develop allergies to semen itself, or more specifically, seminal plasma.
In this case, the immune response isn't directed at some foreign molecule that makes its way into semen, it's directed at a protein that's part of the semen itself.
If you're interested in what allergies are (other than a huge pain in the ass), I recently wrote a series of posts explaining what exactly is going on, to the extent that we know. If you have any other questions or you want to leave feedback, feel free to leave a comment or send me an e-mail.