Evidence and conjecture about mechanisms of cutaneous disease in photodermatology.

[erythropoietic protoporphyria]

Photosensitivity disorders are caused by a variety of mechanisms . Three common themes are as follows : excess chromophore allowing visible light energy to cause photodynamic damage , reduced DNA repair capacity to UV-induced DNA damage , and enhanced sensitivity to light-induced allergens mediated immunologically . Although the cause of each condition may be known , the precise pathogenesis underlying the photosensitivity has taken longer to understand . By focussing on three clinical disorders under each of these themes , we have explored the following : why erythropoietic protoporphyria differs so markedly from the other cutaneous porphyrias ; how a DNA repair defect was eventually revealed to be the underlying cause of the vitamin B 3 deficiency disorder of pellagra ; an immunological explanation for the over reactivity to photoallergens in chronic actinic dermatitis .