Have you ever heard of anyone w/true allergy to benadryl (diphenhydramine)? Would this allergy be likely to turn serious? How can someone be allergic to antihistamines?

Yes. Good question. It is very rare but does happen. Sometimes they are not allergic to the Diphenhydramine but the dyes and fillers used to make the medicine. But people have also had reactions to dye free benadryl (diphenhydramine). You can develop an allergy to anything. I have seen people with Benadryl (diphenhydramine) allergy who could take other antihistamines such as hydroxyzine.