Inside the Blister
Q. What is the fluid inside a blister?
A. Depending on what caused the blister, it can be a clear fluid called serum, which is the liquid part of blood minus blood cells and the clotting factor; red blood, or pus.
A small blister, called a vesicle if it is less than half an inch across, is a gap between layers of the skin, usually filled with fluid that leaks from damaged blood vessels, the Merck Manual of Diagnosis and Therapy says.
The damage is commonly caused by friction or burns or by irritants like insect bites and some kinds of allergies. It may also result from viral infections, like herpes. A larger blister is called a bulla.
If the trauma that causes a blister is severe enough, it may fill with blood, not just serum. And in cases of infection, the blister may be a pustule, filled with pus, a cloudy liquid containing immune cells recruited to fight infective invaders.
Normally, a blister filled with clear liquid should be protected, not drained, except possibly with a sterile instrument. The fluid is usually sterile, according to The American Medical Association Encyclopedia of Medicine, and as long as the blister stays intact, it is not prone to infection.
C. CLAIBORNE RAY
Drawing (Victoria Roberts)