A granuloma is a mass of immune system cells that have gathered around and sealed off a foreign particle that has resisted the immune cells' attempts to destroy and digest it. This foreign particle could be anything from a tuberculosis bacterium to a splinter. A granuloma is composed primarily of immune cells called macrophages and lymphocytes, but often has fibroblasts and collagen as well. The macrophages and T cells are the mediators of the immune system reaction.
When a foreign organism or particle enters the body, it evokes an immune response. This triggers immune system cells to be attracted to the invader. Sometimes, the invader has defense mechanisms that protect it from the immune cells and keep it from being killed and digested. In this case, more immune cells are recruited to the site. They begin to join together by interconnecting their cell membranes through a folding process called interdigitation to form a wall of cells. Since this wall of cells resembles a layer, or skin, they are often called epithelioid cells.
Some of the cells called macrophages actually fuse to form one giant cell called a multinucleate giant cell. Other cells, called T cells, cause damage to surrounding normal tissue because they release toxic substances in an effort to kill the invader. Over time, these cells build into a nodule called a granuloma.
There are a number of types of granulomas, each caused by a different foreign invader. The tuberculosis bacteria causes granulomas in the lungs. The center of these nodules has an area of necrosis (dead cellular matter). This area has a white or yellowish "cheesy" look to it. In Latin, the word for cheese is caseum, so this area within the granuloma is called caseous necrosis. These granulomas, once formed, are usually present for life. If left untreated, the spread of the disease can cause cavities in the lungs, destroy blood vessels, cause the development of fibrosis, and eventually destroy the lung.
Infections by fungi can cause granulomas in the brain, skin, lungs, and almost any other body organ or tissue. Fungal infections can be very resistant to treatment and relapse can occur.
One complication of late stage syphilis is the development of a granuloma called a gumma. Gummas can also be found in almost any site of the body. Thankfully, since antibiotics can successfully treat syphilis, gummas are rarely seen these days.
In some parts of the world, infection by a blood parasite called Schistosoma is common. Schistosomes are worms of the Trematode family. They live in water, and are able to penetrate the skin of humans and infect them. Mature female worms lay their eggs in blood vessels near the intestines or rectum. These eggs can trigger an immune reaction that leads to the formation of granulomas. These granulomas usually occur in the intestinal, bladder, or kidney tissues.