A bulla, bullae in plural, is the medical term for a large, thin-walled and fluid-filled blister. The walls of this type of blister are translucent, and the fluid inside can consist of lymph, blood, pus or serum, meaning any kind of clear bodily fluid. A bulla most commonly occurs on the skin, but can also form on the corneas and on the membranes lining the inside of the body, such as the mouth, nose, throat and lungs. Common causes include insect bites, infection, burns, the herpes simplex virus and allergic skin reactions. Recommended treatments include protecting the blister with a bandage and the application of medicated creams or lotions.
In Latin, bulla means a bubble, stud or knob, and the term is particularly used for a rounded protrusion that is hollow or filled with fluid. In medical terms, a blister has to have a diameter greater than 0.2 inches (0.5 cm) to be called a bulla. Smaller blisters are called vesicles. A blister formed on the membranes lining the inside of the lungs is called a bleb.
A bulla is often formed as the result of a skin injury such as frostbite or some kind of burn, including sunburns, chemical burns and friction burns. The herpes simplex virus commonly causes this type of blister, and it is then often called a cold sore. These kinds of blisters can also be caused by contact with certain poisonous plants and creatures, such as poison ivy and jellyfish. In some cases, more serious medical conditions such as chicken pox, thyroid disease and lupus can also lead to the formation of bullae.
A blister can be epidermal or subepidermal, depending on whether it is formed between two layers of skin or underneath the skin. An epidermal blister occurs when two layers of the outer-most layer of the skin, called the epidermis, separate and fluid is trapped between them. In the case of a subepidermal blister, there is a separation between the epidermis and the skin's underlying structure, called the subepidermis, and fluid then fills the affected area.
In order to avoid pain and possible infection, the skin covering a bulla should not be removed. Recommended treatments include applying an antibiotic or antiseptic cream and protecting the blister with a bandage or other form of skin dressing. Tincture of benzoin is also a recommended treatment for blisters. Home remedies like tea tree oil, lavender oil, aloe vera gel and petroleum jelly can also be used.