A blood blister is a type of blister that forms when subdermal tissues and blood vessels are damaged without piercing the skin. It consists of a pool of lymph, blood and other body fluids trapped beneath the skin. If punctured, it suppurates a dark fluid. Sometimes the fluids are cut off from the rest of the body and dry up, leaving behind dead cell material inside the blister with a texture like putty. Some blood blisters can be extremely painful due to bruising where the blister occurred.

There are also blood blister-like aneurysms as these are known to be located in the supraclinoid internal carotid artery and have been recognized as having unique pathological and clinical features.

Blood blisters are commonly caused by accidents in which the skin is pinched by a tool, mechanism, or heavy weight without protective gear. Blood blisters can also arise from forcible human contact, including grappling.

Blood blisters also may occur with friction caused by constant rubbing of skin against a surface. Because of this, baseball pitchers, rowers, and drummers often contract blood blisters on the fingers and palms. They also form as a result of frostbite.

Blood blisters can also occur in the mouth for a variety of reasons including, but not limited to, side effects to certain medications, nutritional deficiencies, and mouth injuries.

Intense rubbing can cause a blister, as can any friction on the skin if continued long enough. This kind of blister is most common after walking long distances or by wearing old or poorly fitting shoes. Blisters are most common on the hands and feet, as these extremities are susceptible while walking, running, or performing repetitive motions, such as joystick manipulation whilst playing certain video games, digging with a shovel, playing guitar, etc. Blisters form more easily on moist skin than on dry or soaked skin, and are more common in warm conditions. Less-aggressive rubbing over long periods of time may cause calluses to form rather than a blister. Both blisters and calluses can lead to more serious complications, such as foot ulceration and infection, particularly when sensation or circulation is impaired, as in the case of diabetes, neuropathy or peripheral artery disease (PAD).

A blister may form when the skin has been damaged by friction or rubbing, heat, cold or chemical exposure. Fluid collects between the epidermisthe upper layer of the skinand the layers below. This fluid cushions the tissue underneath, protecting it from further damage and allowing it to heal.

Erythema disappears on finger pressure (blanching), while purpura or bleeding in the skin and pigmentation do not. There is no temperature elevation, unless it is associated with the dilation of arteries in the deeper layer of the skin.

Erythema (from the Greek "erythros", meaning red) is redness of the skin or mucous membranes, caused by hyperemia (increased blood flow) in superficial capillaries. It occurs with any skin injury, infection, or inflammation. Examples of erythema not associated with pathology include nervous blushes.

The definitions of the four pressure ulcer stages are revised periodically by the National Pressure Ulcer Advisor Panel (NPUAP) in the United States and the European Pressure Ulcer Advisor Panel (EPUAP) in Europe. Briefly, they are as follows:

- Stage 1: Intact skin with non-blanchable redness of a localized area usually over a bony prominence. Darkly pigmented skin may not have visible blanching; its color may differ from the surrounding area. The area differs in characteristics such as thickness and temperature as compared to adjacent tissue. Stage 1 may be difficult to detect in individuals with dark skin tones. May indicate "at risk" persons (a heralding sign of risk).

- Stage 2: Partial thickness loss of dermis presenting as a shallow open ulcer with a red pink wound bed, without slough. May also present as an intact or open/ruptured serum-filled blister. Presents as a shiny or dry shallow ulcer without slough or bruising. This stage should not be used to describe skin tears, tape burns, perineal dermatitis, maceration or excoriation.

- Stage 3: Full thickness tissue loss. Subcutaneous fat may be visible but bone, tendon or muscle are not exposed. Slough may be present but does not obscure the depth of tissue loss. May include undermining and tunneling. The depth of a stage 3 pressure ulcer varies by anatomical location. The bridge of the nose, ear, occiput and malleolus do not have (adipose) subcutaneous tissue and stage 3 ulcers can be shallow. In contrast, areas of significant adiposity can develop extremely deep stage 3 pressure ulcers. Bone/tendon is not visible or directly palpable.

- Stage 4: Full thickness tissue loss with exposed bone, tendon or muscle. Slough or eschar may be present on some parts of the wound bed. Often include undermining and tunneling. The depth of a stage 4 pressure ulcer varies by anatomical location. The bridge of the nose, ear, occiput and malleolus do not have (adipose) subcutaneous tissue and these ulcers can be shallow. Stage 4 ulcers can extend into muscle and/or supporting structures (e.g., fascia, tendon or joint capsule) making osteomyelitis likely to occur. Exposed bone/tendon is visible or directly palpable. In 2012, the NPUAP stated that pressure ulcers with exposed cartilage are also classified as a stage 4.

- Unstageable: Full thickness tissue loss in which actual depth of the ulcer is completely obscured by slough (yellow, tan, gray, green or brown) and/or eschar (tan, brown or black) in the wound bed. Until enough slough and/or eschar is removed to expose the base of the wound, the true depth, and therefore stage, cannot be determined. Stable (dry, adherent, intact without erythema or fluctuance) eschar on the heels is normally protective and should not be removed.

- Suspected Deep Tissue Injury: A purple or maroon localized area of discolored intact skin or blood-filled blister due to damage of underlying soft tissue from pressure and/or shear. The area may be preceded by tissue that is painful, firm, mushy, boggy, warmer or cooler as compared to adjacent tissue. A deep tissue injury may be difficult to detect in individuals with dark skin tones. Evolution may include a thin blister over a dark wound bed. The wound may further evolve and become covered by thin eschar. Evolution may be rapid exposing additional layers of tissue even with optimal treatment.

The most severe form of miliaria, "miliaria profunda", sometimes referred to as "wildfire" due to the rapid spread and severe burning sensations, generally occurs as a complication of repeated episodes of miliaria rubra. The obstruction is located deep in the structure of the sweat gland, causing the gland's secretions to leak between the superficial and deep layers of the skin. The rash, and associated symptoms, tend to appear within hours of an activity provoking sweating but similarly fade within hours when the stimulus for the sweating is removed. Miliaria profunda is characterised by non-pruritic, flesh-colored, deep-seated, whitish papules. The rash tends to be flesh-coloured as opposed to the prominent redness of miliaria rubra, and the risk of heat exhaustion is larger. Miliaria profunda is also less-commonly known as "Mammillaria"

Pressure ulcers can trigger other ailments, cause considerable suffering, and can be expensive to treat. Some complications include autonomic dysreflexia, bladder distension, bone infection, pyarthroses, sepsis, amyloidosis, anemia, urethral fistula, gangrene and very rarely malignant transformation (Marjolin's ulcer - secondary carcinomas in chronic wounds). Sores may recur if those with pressure ulcers do not follow recommended treatment or may instead develop seromas, hematomas, infections, or wound dehiscence. Paralyzed individuals are the most likely to have pressure sores recur. In some cases, complications from pressure sores can be life-threatening. The most common causes of fatality stem from kidney failure and amyloidosis.

Pressure ulcers are also painful, with individuals of all ages and all stages of pressure ulcers reporting pain.

The most commonly encountered form of the illness is "miliaria rubra", in which obstruction causes leakage of sweat into the deeper layers of the epidermis, provoking a local inflammatory reaction and giving rise to the typical appearance of redness (hence rubra) and larger (but still only a few millimetres), blister-like lesions. This form of the illness is often accompanied by the typical symptoms—intense itching or "pins and needles" with a lack of sweating (anhidrosis) to affected areas. There is a small risk of heat exhaustion due to inability to sweat if the rash affects a large proportion of the body's surface area or the sufferer continues to engage in heat-producing activity. Miliaria rubra is also known as "prickly heat" and "heat rash". Differential diagnosis should be used to rule out polycythemia vera, which is a rare hematological disorder and appears more often in males than females, generally not before the age of 40. Both disorders share the common denominator of appearing after taking a hot shower.

Bullous impetigo can appear around the diaper region, axilla, or neck. The bacteria causes a toxin to be produced that reduces cell-to-cell stickiness (adhesion), causing for the top layer of skin (epidermis), and lower layer of skin (dermis) to separate. Vesicles rapidly enlarge and form the bullae which is a blister more than 5mm across. Bullae is also known as Staphylococcal scalded skin syndrome. Other associated symptoms are itching, swelling of nearby glands, fever and diarrhea. It should also be noted that pain is very rare.

Long-term effects: once the scabs on the bullous have fallen off, scarring is minimal. Possible long-term effects are kidney disease.

Angina bullosa haemorrhagica (ABH) is a condition of the mucous membranes characterized by the sudden appearance of one or more blood blisters within the oral cavity.

The lesions, which may be caused by mild trauma to the mouth tissues such as hot foods, typically rupture quickly and heal without scarring or further discomfort. The condition is not serious except in rare cases where a large bulla that does not rupture spontaneously may cause airway obstruction.

The blisters usually affect the palate or oropharynx and are often long lived to the extent that patients burst them for symptomatic relief.

The condition is diagnosed on the basis of exclusion of other conditions and the typical presentation, particularly the constant presence of blood as the blister fluid. Angina bullosa haemorrhagica does not cause desquamative gingivitis.

Molluscum contagiosum lesions are flesh-colored, dome-shaped, and pearly in appearance. They are often 1–5 mm in diameter, with a dimpled center. Molluscum lesions are most commonly found on the face, arms, legs, torso, and armpits in children. Adults typically have molluscum lesions in the genital region and this is considered to be a sexually transmitted infection; because of this, if genital lesions are found on a child, sexual abuse should be suspected. These lesions are generally not painful, but they may itch or become irritated. Picking or scratching the bumps may lead to a spread of the viral infection responsible for molluscum contagiosum, an additional bacterial infection, and scarring. In about 10% of the cases, eczema develops around the lesions.

Individual molluscum lesions may go away on their own within two months and generally clear completely without treatment or scarring in six to twelve months. Mean durations for an outbreak are variously reported from 8 to about 18 months, but durations are reported as widely as 6 months to 5 years, lasting longer in immunosuppressed individuals.

Molluscum contagiosum (MC), sometimes called water warts, is a viral infection of the skin that results in small, raised, pink lesions with a dimple in the center. They may occasionally be itchy or sore. They may occur singly or in groups. Any area of the skin may be affected, with abdomen, legs, arms, neck, genital area, and face being most common. Onset of the lesions is around 7 weeks after infection. It usually goes away within a year without scarring.

MC is caused by a poxvirus called the "molluscum contagiosum virus" (MCV). The virus is spread either by direct contact including sexual activity or via contaminated objects such as towels. The condition can also be spread to other areas of the body by the person themselves. Risk factors include a weak immune system, atopic dermatitis, and crowded living conditions. Following one infection, it is possible to get reinfected. Diagnosis is typically based on the appearance.

Prevention includes hand washing and not sharing personal items. While treatment is not necessary some may wish to have the lesions removed for cosmetic reasons or to prevent spread. Removal may occur with freezing, opening up the lesion and scraping the inside, or laser therapy. Scraping the lesion can however result in scarring. The medication cimetidine by mouth or podophyllotoxin cream applied to the skin may also be used.

Approximately 122 million people globally were affected by molluscum contagiosum as of 2010 (1.8% of the population). It is more common in children between the ages of one and ten years old. The condition has become more common in the United States since 1966. MC is not a reason to keep a child out of school or daycare.

Bullous Impetigo is a skin condition that characteristically occurs in the newborn, and is caused by a bacterial infection, presenting with bullae.

It can be caused by Exfoliative toxin A. The phyogenic superficial infection can be divided into two other subdivisions; Impetigo, and non-bullous impetigo. Bullous impetigo is caused by "Staphylococcus aureus," which produces exfoliative toxins, whereas non-bullous impetigo is caused by either "Staphylococcus aureus", or "Streptococcus pyogenes." Thirty percent of all Impetigo cases are related to Bullous impetigo. Bullous impetigo, in newborns, children, or adults who are immunocompromised and/or are experiencing renal failure, can develop into a more severe and generalized form called Staphylococcal scalded skin syndrome (SSSS). The mortality rate is less than 3% for infected children, but up to 60% in adults.

Pemphigus vulgaris most commonly presents with oral blisters (buccal and palatine mucosa, especially), but also includes cutaneous blisters. Other mucosal surfaces, the conjunctiva, nose, esophagus, penis, vulva, vagina, cervix, and anus, may also be affected. Flaccid blisters over the skin are frequently seen with sparing of the skin covering the palms and soles.

Blisters commonly erode and leave ulcerated lesions and erosions. A positive Nikolsky sign (induction of blistering in normal skin or at the edge of a blister) is indicative of the disease.

Severe pain with chewing can lead to weight loss and malnutrition.

As the common name for this condition implies, it causes itching or a burning sensation in the groin area, thigh skin folds or anus. It may involve the inner thighs and genital areas, as well as extending back to the perineum and perianal areas.

Affected areas may appear red, tan, or brown, with flaking, rippling, peeling or cracking skin.

The acute infection begins with an area in the groin fold about a half-inch across, usually on both sides. The area may enlarge, and other sores may develop. The rash has sharply defined borders that may blister and ooze.

Tinea cruris does have similar symptoms to Inverse psoriasis.

Sufferers experience very fragile skin, with blisters and skin erosion occurring in response to relatively benign trauma. Blisters may form all over the body, including the mucous membranes. Chronic scarring can lead to the formation of granulation tissue, which may bleed easily, predisposing to infection. Hands and fingers may be affected, as well as various joints.

A corn (or "clavus", plural "clavi") is a specially shaped callus of dead skin that usually occurs on thin or glabrous (hairless and smooth) skin surfaces, especially on the dorsal surface of toes or fingers. They can sometimes occur on the thicker palmar or plantar skin surfaces. Corns form when the pressure point against the skin traces an elliptical or semi-elliptical path during the rubbing motion, the center of which is at the point of pressure, gradually widening. If there is constant stimulation of the tissue producing the corns, even after the corn is surgically removed, the skin may continue to grow as a corn.

The hard part at the center of the corn resembles a funnel with a broad raised top and a pointed bottom. Because of their shape, corns intensify the pressure at the tip and can cause deep tissue damage and ulceration. The scientific name for a corn is "heloma" (plural "helomata"). A hard corn is called a "heloma durum", while a soft corn is called a "heloma molle".

The location of the soft corns tends to differ from that of hard corns. Hard corns occur on dry, flat surfaces of skin. Soft corns (frequently found between adjacent toes) stay moist, keeping the surrounding skin soft. The corn's center is not soft however, but indurated.

The specific diagnostic workup and treatments for corns may differ substantially from other forms of calluses.

Symptoms include skin irritations, which may be itchy or painful, and are sometimes confused with hives. These irritations appear upon or shortly after exposure to sunlight, and may last from 1 to 7 days. Lesions have been photographed showing fluid-filled blisters.

Secondary symptoms include flu-like symptoms; body aches, skin sensitivity, muscle aches, fever, dizziness, exhaustion, vision impairment, and disorientation.

Pustulosis is highly inflammatory skin condition resulting in large fluid-filled blister-like areas - pustules. Pustulosis typically occurs on the palms of the hands and/or the soles of the feet. The skin of these areas peels and flakes (exfoliates).

This condition—also referred to as "palmo-plantar pustulosis"—is a feature of pustular psoriasis.

People with diabetes face special skin challenges. Because diabetes affects the capillaries, the small blood vessels which feed the skin, thickening of the skin with callus increases the difficulty of supplying nutrients to the skin. Callus formation is seen in high numbers of patients with diabetes, and together with absent foot pulses and formation of hammer toe, this may be an early sign of individuals at an increased risk for foot ulcers.

The stiffness of a callus or corn, coupled with the shear and pressure that caused it, may tear the capillaries or adjoining tissue, causing bleeding within the callus or corn. Often, bleeding within a callus is an early sign of diabetes, even before elevated blood sugars may be noticed. Although the bleeding can be small, sometimes small pools of blood or hematoma are formed. The blood itself is an irritant, a foreign body within the callus that makes the area burn or itch. If the pool of blood is exposed to the outside, infection may follow. Infection may also lead to ulceration. Fortunately, this process can be prevented at several places. Diabetic foot infections are the leading cause of diabetic limb amputation.

Transient bullous dermolysis of the newborn (TBDN) is a skin condition that presents in newborns. It is characterized by blister formation secondary to even mild trauma.

It is associated with "COL7A1".

Once pederin is on the skin from the initial beetle contact, it may also be spread elsewhere on the skin. "Kissing" or "mirror-image" lesions where two skin areas come in contact (for example, the elbow flexure) are often seen. Washing the hands and skin with soap and water is strongly recommended, if contact with a rove beetle has occurred.

Initial skin contact with pederin shows no immediate result. Within 12–36 hours, however, a reddish rash (erythema) appears, which develops into blisters. Irritation, including crusting and scaling, may last from two to three weeks.

One study reported best results with a treatment regimen that combined topical steroids with oral antihistamines and antibiotics. The authors hypothesized that antibiotics were helpful because of the possible contamination of skin by pederin-producing bacteria.