How does drug-induced photosensitivity work?

Question

Some drugs (tetracyclines, for instance) can cause photosensitivity reactions—that is, some patients become extremely sensitive to the sun, developing rashes or inflammation after spending time in bright sunlight. Sources I've read describe this as an immune reaction similar to an allergy, but I've also heard that antihistamines are generally ineffective at treating such reactions.So my question is, what are the biological processes behind this reaction, and how does it differ from a normal skin allergy that could be treated by antihistamines?

Answer 1

This is an interesting question because it involves a process most people don't think about much, When you are exposed to sunlight, some of the ultraviolet radiation penetrates your skin. Radiation can knock the delicate little machinery in cells apart. Thus you get sunburn,

Drugs that cause photo-sensitivity reactions are drugs that are chemicals that can be knocked apart into bits by the ultraviolet radiation of the sunlight. What results is not an immune reaction, as you have been led to believe. It is a reaction to the now toxic components of the chemicals.

Normal allergic reaction are when your body over-reacts to a stimulus, thinking it a threat all out of proportion to it's real danger. Histamine is one of the major mediator chemicals that signal this type of response.

Some fraction of a photosensitivity reaction may be immunological in nature, but most of it is not. This means histamine release is not involved in the mechanism producing the symptoms, even though the end-result is similar looking to when it is.