An abscess is an enclosed collection of liquefied tissue, known as pus, somewhere in the body. It is the result of the body's defensive reaction to foreign material.
There are two types of abscesses, septic and sterile. Most abscesses are septic, which means that they are the result of an infection. Septic abscesses can occur anywhere in the body. Only bacteria and the body's immune response are required. In response to the invading bacteria, white blood cells gather at the infected site and begin producing chemicals called enzymes that attack the bacteria by first marking and then digesting it. These enzymes kill the bacteria and break them down into small pieces that can travel in the circulatory system prior to being eliminated from the body. Unfortunately, these chemicals also digest body tissues. In most cases, bacteria produce similar chemicals. The result is a thick, yellow liquid—pus—containing dead bacteria, digested tissue, white blood cells, and enzymes.
An abscess is the last stage of a tissue infection that begins with a process called inflammation. Initially, as invading bacteria activate the body's immune system, several events occur:
Blood flow to the area increases.
The temperature of the area increases due to the increased blood supply.
The area swells due to the accumulation of water, blood, and other liquids.
It turns red.
It hurts, due to irritation from the swelling and the chemical activity.
These four signs—heat, swelling, redness, and pain—characterize inflammation.
As the process progresses, the tissue begins to turn to liquid, and an abscess forms. It is the nature of an abscess to spread as the chemical digestion liquefies more and more tissue. Furthermore, the spreading follows the path of least resistance, commonly, the tissue that is most easily digested. A good example is an abscess just beneath the skin. It most easily continues along immediately beneath the surface rather than traveling up through the outermost layer or down through deeper structures where it could drain its toxic contents. The contents of an abscess can also leak into the general circulation and produce symptoms just like any other infection. These include chills, fever, aching, and general discomfort.
Sterile abscesses are sometimes a milder form of the same process caused not by bacteria but by non-living irritants such as drugs. If an injected drug such as penicillin is not absorbed, it stays where it is injected and may cause enough irritation to generate a sterile abscess. Such an abscess is sterile because there is no infection involved. Sterile abscesses are quite likely to turn into
hard, solid lumps as they scar, rather than remaining pockets of pus.
L. Fleming Fallon Jr., MD, DrPH, The Gale Group Inc., Gale, Detroit,