When a person experiences a photosensitivity reaction they actually undergo one of two separate reactions, known as either a phototoxic reaction or a photoallergic reaction. In a phototoxic reaction, which is much more common than a photoallergic response is, a change in the skin is usually observed within minutes to hours following exposure to the offending substance. As for photoallergic reactions, however, a response is not observed for 1 - 3 days after the substance has come into contact with the body.
Phototoxic reactions are the result of a release of energy by photosensitizing agents, causing potentially long term damage to the skin surrounding these molecules. This excessive energy is initially absorbed by the molecules from the sun's ultraviolet rays, explaining how sunlight can indirectly result in the death of skin cells. Although the medication or agent that causes the reaction might be stopped quickly, such a reaction can occasionally last 20 years after the substance has been removed. Medications that are taken orally, topically (i.e. a cream applied to the skin) or injected can all cause phototoxic reactions
Photoallergic reactions, as mentioned, are much less common. These responses are specifically caused by topical medicines or photosensitizing agents. As hinted by the name of the reaction, photoallergic responses occur when UV rays cause the shape of a molecule to transform into a new substance, eliciting a response of the immune system to attack these foreign antigens. As antibodies are produced and attack the photosensitizer a rash develops, usually a few days following application of the substance. The outbreak is not limited to sun exposed areas but can spread to all parts of the body. (http://www.webmd.com/a-to-z-guides/sun-sensitizing-drugs)
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