Yes it is
I've had experiences with people who were "highly allergic" to some common substance, yet it didn't bother them unless they knew about it. Allergic to eggs, yet the cookie didn't bother them until I told them there were eggs in the batter.
My Mom is supposedly allergic to cats, but my cat slept right next to her for several nights without her knowledge and she never had a problem...until Kitty quit being so skitty and revealed himself. She suddenly broke out in these very real hives and had to go to the hospital.