question: why scratching sunburn makes the skin turn white, when scratching unburnt soon turns red.
comment: Pressing down on sunburn pushes the blood away. When you stop pressing it turns back to red (rather slowly sometimes).
passages: The condition manifests as an allergic-like reaction, causing a warm red wheal to appear on the skin. As it is often the result of scratches, involving contact with other materials, it can be confused with an allergic reaction, when in fact it is the act of being scratched that causes a wheal to appear. These wheals are a subset of urticaria (hives), and appear within minutes, in some cases accompanied by itching. The first outbreak of urticaria can lead to other reactions on body parts not directly stimulated, scraped, or scratched. In a normal case, the swelling will decrease without treatment within 15–30 minutes, but, in extreme cases, itchy red welts may last anywhere from a few hours to days.
   Direct sunlight makes the lesions resolve more quickly. According to this principle, medical treatment with ultraviolet light has been used to hasten resolution, though studies disagree whether it decreases itching or not. UV therapy is most beneficial in the first week of the eruption.
  
  A 2007 meta-analysis concluded that there is insufficient evidence for the effectiveness of most treatments. Oral erythromycin was found to be effective for treating the rash and relieving the itch based on one early trial; however, a later study could not confirm these results.
  
  Section::::Prognosis.
   PLE appears on areas of the skin newly exposed to sunlight such as the visible part of the neckline, backs of hands, arms and legs, and feet, but less commonly the face. At these areas, there may be feelings of burning and severe itching. Smooth red-topped small papules which merge into plaques, small fluid-filled blisters (papulovesicles) and less commonly target-shaped lesions which look like erythema multiforme may be visible. In addition, it may occur in other parts of the body in some people treated for inflammatory skin diseases with phototherapy. 
   After the exposure, skin may turn red in as little as 30 minutes but most often takes 2 to 6 hours. Pain is usually strongest 6 to 48 hours after exposure. The burn continues to develop for 1 to 3 days, occasionally followed by peeling skin in 3 to 8 days. Some peeling and itching may continue for several weeks.
  
  Section::::Signs and symptoms.:Skin cancer.
   Photoprovocation tests are usually not required but may be undertaken by specialised centres in winter. When a decision to undertake this is made, a small area of the frequently affected skin is exposed to varying doses of UVA and minimal erythema dose (MED) (amount of UV radiation that will produce minimal redness of skin within a few hours following exposure) of broadband UVB for three consecutive days. An examination of the skin to detect the rash is made, however, up to 40% have false negative responses.
  
  Section::::Diagnosis.:Biopsy findings.
   Melanin is a dark pigment of the skin and the melanocytes produce it. The melanocytes produce melanin when their receptors detect UV light. The purpose of the melanin is to block UV light so that it will not cause damage to the body cells under the skin.
  
  Section::::Side effects and complications.
  
  Some patients experience nausea and itching after ingesting the psoralen compound. For these patients PUVA bath therapy may be a good option.
  
  Long term use of PUVA therapy has been associated with higher rates of skin cancer.
   Skin and fingernails are made of a similar type of keratinized protein as hair. That means that drips, slips and extra hair tint around the hairline can result in patches of discolored skin. This is more common with darker hair colors and persons with dry absorbent skin. That is why it is recommended that latex or nitrile gloves be worn to protect the hands.
   The rash caused by dermatitis herpetiformis forms and disappears in three stages. In the first stage, the patient may notice a slight discoloration of the skin at the site where the lesions appear. In the next stage, the skin lesions transform into obvious vesicles and papules that are likely to occur in groups. Healing of the lesions is the last stage of the development of the symptoms, usually characterized by a change in the skin color. This may result in areas of the skin turning darker or lighter than the color of the skin on the rest of the body. Because of the intense itching, patients usually scratch, which may lead to the formation of crusts.
   When fleas bite humans, they may develop an itching rash with small bumps that may bleed. This rash is usually located on the armpit or fold of a joint such as the elbow, knee, or ankle. When the area is pressed, it turns white.
   Thomas Bateman had described, based on findings by Robert Willan, what he called "eczema solare", in the early 19th century, a condition caused by the action of sunlight on abnormally reacting skin. Rasch's interest in "the action of sunlight on the skin" led him, in 1900, to describe this same condition as "eczema-like polymorphic light eruption". He has since been credited with coining the term "polymorphic light eruption", synonymous with "polymorphous light eruption".
   BULLET::::- About one in four people with PR have mild to severe symptomatic itching. (Moderate itching due to skin over-dryness is much more common, especially if soap is used to cleanse the affected areas.) The itching is often non-specific, and worsens if scratched. This tends to fade as the rash develops and does not usually last through the entire course of the disease.
  
  BULLET::::- The rash may be accompanied by low-grade fever, headache, nausea and fatigue.
  
  Section::::Causes.
   BULLET::::- Phytophotodermatitis: UV radiation induces inflammation of the skin after contact with certain plants (including limes, celery, and meadow grass). Causes pain, redness, and blistering of the skin in the distribution of plant exposure.
  
  BULLET::::- Polymorphic light eruption: Recurrent abnormal reaction to UVR. It can present in various ways including pink-to-red bumps, blisters, plaques and urticaria.
  
  BULLET::::- Solar urticaria: UVR-induced wheals that occurs within minutes of exposure and fades within hours.
   A medical case reported in 1957 tells of a fishing captain who worked in the Dogger Bank in the North Sea. The sea chervil, abundant in the area, frequently came up with the fishing nets and had to be thrown back into the water. After doing this repeatedly, the captain "developed an itching red eruption on the flexor aspects of the elbows and forearms which became moist, oozed serum, and spread to involve the backs of the hands, fingers and most of the arms within a few days." Although the rash disappeared after leaving the area, it reappeared with greater severity when he returned to the area and performed the same activities; this time the rash spread to his neck and face, and continued to ooze serum for two months.
   BULLET::::- Immediate photosensitivity. This is typical of XLDPP and EPP. Following a variable period of sun exposure—typically about 30 minutes—patients complain of severe pain, burning, and discomfort in exposed areas. Typically, the effects are not visible, though occasionally there may be some redness and swelling of the skin.
   Section::::Diagnosis.
  
  There is no diagnostic test for calciphylaxis. The diagnosis is a clinical one. The characteristic lesions are the ischemic skin lesions (usually with areas of skin necrosis). The necrotic skin lesions (i.e. the dying or already dead skin areas) typically appear as violaceous (dark bluish purple) lesions and/or completely black leathery lesions. They can be extensive. The suspected diagnosis can be supported by a skin biopsy. It shows arterial calcification and occlusion in the absence of vasculitis. Sometimes the bone scintigraphy can show increased tracer accumulation in the soft tissues. In certain patients, anti-nuclear antibody may play a role.
   Other symptoms can include blistering, swelling (edema), pruritus (itching), peeling skin, rash, nausea, fever, chills, and fainting (syncope). Also, a small amount of heat is given off from the burn, caused by the concentration of blood in the healing process, giving a warm feeling to the affected area. Sunburns may be classified as superficial, or partial thickness burns. Blistering is a sign of second degree sunburn.
  
  Section::::Signs and symptoms.:Variations.
   BULLET::::- if a paste stains skin on the torso black in less than ½ hour, it has PPD in it.
  
  BULLET::::- if the paste is mixed with peroxide, or if peroxide is wiped over the design to bring out the color, it has PPD in it.
  
  Anyone who has an itching and blistering reaction to a black body stain should go to a doctor, and report that they have had an application of PPD to their skin.
   Maculopapular rash
  
  A maculopapular rash is a type of rash characterized by a flat, red area on the skin that is covered with small confluent bumps. It may only appear red in lighter-skinned people. The term "maculopapular" is a compound: "macules" are small, flat discolored spots on the surface of the skin; and "papules" are small, raised bumps. It is also described as erythematous, or red.
   Red skin
  
  Red skin may refer to:
  
  BULLET::::- Redskin, a term referring to Native Americans in the United States and Canada's First Nations
  
  BULLET::::- Flushing (physiology), to temporarily become markedly red in the face or other areas of the skin
  
  BULLET::::- Erythroderma, an inflammatory disease of the skin that causes a red appearance along with scaling
  
  BULLET::::- Erythema, a symptom describing redness of skin or mucous membranes caused by increased blood flow in the superficial capillaries
  
  BULLET::::- Topical steroid withdrawal, also known as red burning skin
  
  Section::::See also.
  
  BULLET::::- Redskin (disambiguation)
   BULLET::::- Changes in pigmentation
  
  BULLET::::- Skin atrophy
  
  Section::::Interactions.
  
  No drug interactions are known.
  
  Section::::Pharmacology.
  
  Section::::Pharmacology.:Mechanism of action.
  
  The efficacy of calcipotriol in the treatment of psoriasis was first noticed by the observation of patients receiving various forms of vitamin D in an osteoporosis study. Unexpectedly, some patients who also suffered from psoriasis experienced dramatic reductions in lesion counts.
   Ultraviolet light (UVA) treatments are normally carried out in a hospital clinic. Psoralen and ultraviolet A light (PUVA) treatment involves taking a drug that increases the skin's sensitivity to ultraviolet light, then exposing the skin to high doses of UVA light. Treatment is required twice a week for 6–12 months or longer. Because of the high doses of UVA and psoralen, PUVA may cause side effects such as sunburn-type reactions or skin freckling.
   BULLET::::- Photoallergic reactions: Uncommon immunological response to sunlight interacting with certain drugs and chemicals in the skin. When in excited state by UVR, these drugs and chemicals form free radicals that react to form functional antigens and induce a Type IV hypersensitivity reaction. These drugs include 6-methylcoumarin, aminobenzoic acid and esters, chlorpromazine, promethazine, diclofenac, sulfonamides, and sulfonylureas. Unlike phototoxic reactions which resemble exaggerated sunburns, photoallergic reactions can cause intense itching and can lead to thickening of the skin.
   The cause is not known, although there are a few lines of thought on what causes it. No cure has been found, but good control with near 100% relief can be achieved. The intense itch/scratch cycle can be broken by applying a topical skin coolant gel like Biofreeze (or a substance containing menthol, camphor or other topical coolant) to affected itchy areas, and then consistently applying 100+SPF sunscreen to affected skin of arms, shoulders, neck, etc., whenever they are expected to be exposed to the sun. When combined, these treatments can bring almost full relief.
   Irritant reactions include miliaria (sweat rash), follicular pustules, and burn-like reactions. Uncertain reactions refer to a pink area under the test chamber. Weak positives are slightly elevated pink or red plaques, usually with mild vesiculation. Strong positives are ‘papulovesicles’ and extreme reactions have spreading redness, severe itching, and blisters or ulcers.
   A recent CBS Cares PSA parodied this in talking about sunburn, which showed raw bacon that accompanied a voice saying "This is your skin", and a slice of it was then placed in a frying pan, cooking it, stating "This is your skin in the sun", and then follows it by a shot of a sun in which the voice says "Any questions?", accompanied by the phrase "Save your bacon. Use sunscreen." superimposed over the sun.
  