How do the uv rays cause damage?   what exactly causes the skin to burn;   can you be more specific about this mechanism?   3. how do photosensitive medications increase a person's sensitivity to sunlight and therefore increase their risk of getting burned?     what kind of medications can increase photo-sensitivity, can you think of some examples?

1. How do the UV rays cause damage?

UV rays cause a dimerization of the pyrimidines at the level of the genome (DNA).

The aromatic rings of the bases constituting the DNA molecule absorb the energy of photons associated with a wavelength of between 230 and 290 nm, ie in the UVC domain and, to a lesser extent, that of UVB. The energy absorbed at two adjacent pyrimidines (cytosine (C) or thymine (T)) provides the energy necessary for the formation of a covalent bond between these two bases, to the detriment of the hydrogen bonds established between two complementary bases and ensuring the cohesion of the double helix of DNA.

What exactly causes the skin to burn ?

The burns are due to the too intense absorption of the sun rays.

The skin is of course the first target of UV radiation. The epidermis attenuates the transmission of radiation of wavelength <300 nm, but allows the less energetic radiation to pass. UVB is therefore absorbed mainly in the epidermis and superficial dermis while UVA penetrates much deeper into the skin. They are responsible for sunburn, but also cause a warming of the skin and tanning.

Mechanism of photosensitive medications

- Photosensitization consists of a cutaneous reaction occurring during exposure to light.

- The mechanisms of photosensitization reactions differ in:

-> Phototoxic reaction, non-immunological, occurring in principle in all individuals, under two conditions:

* presence in the skin, in sufficient quantity, of a substance endowed with a phototoxic potential;

* light exposure of the skin by an effective wavelength at sufficient dose.

-> Photoallergic, immunological reaction, which is a cell-mediated delayed Type IV hypersensitivity reaction. It occurs only in predisposed subjects and does not take into account either the concentration of the molecule or the radiation dose.

-> Sometimes the origin of photosensitization is idiopathic; the photosensitizing agent is not known; this is the case of lucites.

Examples of photosensitive medications:

Here are the main families of photosensitizing drugs, sorted by order of frequency of photosensitization reactions.

* Antibiotics of the quinolone class

* Medicines containing amiodarone

* Diuretics containing sulfonamides

* Antidepressants and neuroleptics: ...

* Cholesterol treatments

* Anti-inflammatories containing ketoprofen, especially in the form of cream.

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