photosensitivity photosensitivity photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light. in medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished, photoallergy and phototoxicity. the photosensitive ganglion cells in the mammalian eye are a separate class of light-detecting cells from the photoreceptor cells that function in vision. for other uses, see photosensitive epilepsy, photophobia, and flicker vertigo. skin reactions human medicine main article: photosensitivity in humans