A blister is a bubble of fluid under the skin. The clear, watery liquid inside a blister is called serum. It leaks in from neighboring tissues as a reaction to injured skin. If the blister remains unopened, serum can provide natural protection for the skin beneath it. Small blisters are called vesicles. Those larger than half an inch are called bullae. A blood blister is filled with blood, rather than serum.
Causes of blisters
Blisters are most often caused by skin being damaged by friction or heat. Certain medical conditions also cause blisters to appear.
The damaged upper layer of skin (epidermis) tears away from the layers beneath and fluid (serum) collects in the space to create a blister.
Friction blisters are common in people who are very active, such as sports players and those in the military. They’re usually caused by poor-fitting shoes. A blister can develop if the skin is rubbed for a long period or if there’s intense rubbing over shorter periods.
Friction blisters often occur on the feet and hands, which can rub against shoes and handheld equipment, such as tools or sports equipment. Blisters also form more easily on moist skin and are more likely to occur in warm conditions.
Blisters can appear when skin is exposed to excessive heat – for example, when you have sunburn. Blisters can sometimes form when your skin comes into contact with substances such as cosmetics, detergents and solvents.
A number of medical conditions may cause blisters. The most common are:
- Chickenpox:A childhood illness that causes itchy red spots.
- Cold Sores:Small blisters that develop on the lips or around the mouth, caused by a virus.
- Herpes:A sexually transmitted infection (STI)that most commonly affects the groin.
- Impetigo:A contagious bacterial skin infection.
- Pompholyx:A type of eczema.
- Scabies:A skin condition, caused by tiny mites, which may lead to blisters developing on young children’s feet or palms of their hands.
- Hand, Foot and Mouth Disease:A viral infection that usually affects young children.
Several Rarer Conditions Can also Cause Blisters.
Bullous pemphigoid: A skin disease that causes large blisters and usually affects people over 60 years of age.
- Pemphigus Vulgaris: A serious skin condition where blisters develop if pressure is applied to the skin; the blisters burst easily, leaving raw areas that can become infected.
- Dermatitis herpetiformis: A skin condition that causes intensely itchy blisters, usually on the elbows, knees, back and buttocks; blisters usually develop in patches of the same shape and size on both sides of the body.
- EpidermolysisBullosa: A group of rare inherited skin disorders that cause the skin to become very fragile; any trauma or friction to the skin can cause painful blisters.
- Chronic Bullous Dermatosis of Childhood: A condition that causes clusters of blisters to develop on the face, mouth or genitals.
- Bullous IchthyosiformErythroderma: A type of icthyosis someone is born with, which causes inflamed, scaly skin with blisters.
In general, blisters are round or oval bubbles of fluid under the skin that may be painful or itchy, or they may not cause any symptoms. Symptoms vary depending on the cause.
- Irritation, burns and allergies:Blisters caused by friction or burns are usually painful. Blisters resulting from eczema can be accompanied by redness, severe itching and small bumps on the affected skin.
- Infections:When blisters are caused by an infection, the symptoms depend on the type of infection. Examples include:
- Bullous impetigo:The affected skin can redden, and the blisters may burst easily.
- Herpes simplex virus:When herpes simplex type 1 is the cause, the tiny blisters commonly are known as fever blisters or cold sores. They typically appear on the lips. The affected skin may itch, tingle, swell and become red before the blisters appear. When the blisters eventually break, they leak fluid, and then painful sores develop. Herpes simplex type 2 is the most common cause of genital herpes, a sexually transmitted infection (although type 1 also can cause genital herpes). Generally, small red bumps appear before blisters develop in the affected area, typically the vaginal area or penis, the buttocks and thighs, or the anus. Other symptoms can include fever, muscle aches, headache and burning with urination.
- Varicella zoster virus: When this virus causes chickenpox, the infection starts with a diffuse, itchy rash that develops quickly into itchy blisters. Varicella zoster also can cause shingles (herpes zoster). People with shingles may experience small, painful blisters that usually erupt in a linear pattern along the length of an infected nerve.
- Coxsackievirus: Coxsackievirus A16 can cause a condition commonly called hand-foot-and-mouth disease, in which painful blisters often occur on the hands, on the soles of the feet and in the mouth.
- Skin diseases: Erythema multiforme typically causes blisters on the palms of the hands, the forearms, the soles of the feet, and on the mucous membranes in the eyes, nose, mouth and genitals. Other symptoms include fever, sore throat, cough and muscle pain. The autoimmune diseases (also known as bullous diseases because of the large blisters seen) vary in appearance as well. Dermatitis herpetiformis causes itchy, red bumps or blisters. Pemphigoid, an uncommon condition that primarily affects the elderly, results in large, itchy blisters, and pemphigus, an uncommon disease that tends to strike in middle age, causes blisters inside the mouth and on the surface of the skin. The blisters of pemphigus break easily and leave painful areas.