Photosensitivity is the amount to which an object reacts upon receiving
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
s, especially
visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
. 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 cell
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiat ...
s that function in vision.
Skin reactions
Human medicine
Sensitivity of the
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different ...
to a light source can take various forms. People with particular skin types are more sensitive to
sunburn
Sunburn is a form of radiation burn that affects living tissue, such as skin, that results from an overexposure to ultraviolet (UV) radiation, usually from the Sun. Common symptoms in humans and animals include: red or reddish skin that is h ...
. Particular medications make the skin more sensitive to sunlight; these include most of the
tetracycline antibiotics
Tetracyclines are a group of broad-spectrum antibiotic compounds that have a common basic structure and are either isolated directly from several species of ''Streptomyces'' bacteria or produced semi-synthetically from those isolated compounds. T ...
, heart drugs
amiodarone, and
sulfonamides
In organic chemistry, the sulfonamide functional group (also spelled sulphonamide) is an organosulfur group with the structure . It consists of a sulfonyl group () connected to an amine group (). Relatively speaking this group is unreactive. ...
.
Some dietary supplements, such as
St. John's Wort, include photosensitivity as a possible side effect.
Particular conditions lead to increased light sensitivity. Patients with
systemic lupus erythematosus
Lupus, technically known as systemic lupus erythematosus (SLE), is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue in many parts of the body. Symptoms vary among people and may be mild to severe. Commo ...
experience skin symptoms after sunlight exposure; some types of
porphyria
Porphyria is a group of liver disorders in which substances called porphyrins build up in the body, negatively affecting the skin or nervous system. The types that affect the nervous system are also known as acute porphyria, as symptoms are ...
are aggravated by sunlight. A rare hereditary condition
xeroderma pigmentosum
Xeroderma pigmentosum (XP) is a genetic disorder in which there is a decreased ability to repair DNA damage such as that caused by ultraviolet (UV) light. Symptoms may include a severe sunburn after only a few minutes in the sun, freckling in ...
(a defect in DNA repair) is thought to increase the risk of UV-light-exposure-related cancer by increasing photosensitivity.
Veterinary medicine
Photosensitivity occurs in multiple species including
sheep
Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus '' Ovis'', in everyday usage it almost always refers to domesticate ...
,
bovine
Bovines (subfamily Bovinae) comprise a diverse group of 10 genera of medium to large-sized ungulates, including cattle, bison, African buffalo, water buffalos, and the four-horned and spiral-horned antelopes. The evolutionary relationship betwe ...
, and
horse
The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...
s. They are classified as primary if an ingested plant contains a photosensitive substance, like
hypericin in
St John's wort poisoning and ingestion of biserrula (''Biserrula pelecinus'') in sheep, or
buckwheat plants (green or dried) in horses.
In hepatogenous photosensitization, the photosensitzing substance is
phylloerythrin, a normal end-product of
chlorophyll
Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to ...
metabolism.
It accumulates in the body because of liver damage, reacts with UV light on the skin, and leads to free radical formation. These free radicals damage the skin, leading to ulceration, necrosis, and sloughing. Non-pigmented skin is most commonly affected.
See also
*
Digital camera ISO
Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. A closely related ISO system is used to describe the relations ...
*
Bergaptene
Bergapten (5-methoxypsoralen) is a naturally-occurring organic chemical compound produced by numerous plant species, especially from the carrot family Apiaceae and the citrus family Rutaceae. For example, bergapten has been extracted from 24 ...
*
Heliotropism
Heliotropism, a form of tropism, is the diurnal or seasonal motion of plant parts (flowers or leaves) in response to the direction of the Sun.
The habit of some plants to move in the direction of the Sun, a form of tropism, was already known by ...
*
Photophobia
*
Solar urticaria
Solar urticaria (SU) is a rare condition in which exposure to ultraviolet or UV radiation, or sometimes even visible light, induces a case of urticaria or hives that can appear in both covered and uncovered areas of the skin. It is classified as ...
*
Snow blindness
*
Photosensitizer
Photosensitizers produce a physicochemical change in a neighboring molecule by either donating an electron to the substrate or by abstracting a hydrogen atom from the substrate. At the end of this process, the photosensitizer eventually returns t ...
Notes
External links
Sensor sensitivity (ISO) in digital cameras
{{Authority control
Skin physiology
Clinical pharmacology