DermNet NZ: Photosensitivity
Some people develop a rash because their skin is sensitive to sunlight.
www.dermnetnz.org/dna.photo/photo.html
InteliHealth: Sun Allergy (Photosensitivity)
Explains various immune system reactions to sunlight, including polymorphous light eruption, actinic prurigo, photoallergic eruption, and solar urticaria.
www.intelihealth.com/IH/ihtIH/WSIHW000/9339/25934.html