It's rare for someone to be allergic to the sun but it is possible in some people to have an allergic reaction to the sun.
Some peoples skin is just overly sensitive to the suns rays so that can cause problems with their skin.
When someone has allergic reactions to the sun it usually happens in the form of a rash, tiny blisters or, rarely, some other type of skin eruption.
Sometimes it takes awhile of being in the sun for the allergic reactions to happen but sometimes the allergic reaction from the sun can happen in only a few minutes of sun exposure.