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Granuloma
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A granuloma is a medical term for a ball-like collection of immune cells trying to destroy a foreign substance. It represents a special type of inflammatory reaction common to a wide variety of diseases, both infectious and non-infectious. The adjective "granulomatous" refers to diseases or inflammatory reactions that are characterized by granulomas.
ors often use the term "granuloma" loosely to mean "a small nodule".
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A granuloma is a medical term for a ball-like collection of immune cells trying to destroy a foreign substance. It represents a special type of inflammatory reaction common to a wide variety of diseases, both infectious and non-infectious. The adjective "granulomatous" refers to diseases or inflammatory reactions that are characterized by granulomas.
Definition
Doctors often use the term "granuloma" loosely to mean "a small nodule". Since a small nodule can represent anything from a harmless nevus to a malignant tumor, this usage of the term is not very specific. The correct use of the term "granuloma" requires a pathologist to examine surgically removed and specially colored (stained) tissue under a microscope. The following is a more technical definition of a granuloma.
In pathology, a granuloma (classical Latin plural granulomata; modern anglicized plural granulomas) is an organized collection of macrophages .
Macrophages (also known as histiocytes) are the cells that define a granuloma. The other key term in the above definition is the word "organized", which refers to a tight, ball-like formation. The macrophages in these formations are typically so tightly clustered that the borders of individual cells are difficult to appreciate. Loosely dispersed macrophages are not considered to be granulomas.
The macrophages in most granulomas have a special appearance that is described as "epithelioid". This term refers to the vague resemblance of these macrophages to epithelial cells. Epithelioid macrophages differ from ordinary macrophages in that they have elongated nuclei that often resemble the sole of a slipper or shoe. Epithelioid macrophages also have larger nuclei than ordinary macrophages and their cytoplasm is more pink. These changes are thought to be a consequence of "activation" of the macrophage by the offending antigen.
All granulomas, regardless of etiology, may contain additional cells and matrix. These include lymphocytes, neutrophils, eosinophils, multinucleated giant cells, fibroblasts and collagen (fibrosis). The additional cells are sometimes a clue to the cause of the granuloma. For example, granulomas with numerous eosinophils are a clue to coccidioidomycosis or allergic bronchopulmonary fungal disease, and granulomas with numerous neutrophils suggest blastomycosis, Wegener's granulomatosis or aspiration pneumonia.
In terms of the underlying cause, the difference between granulomas and other types of inflammation is that granulomas form in response to antigens that are resistant to "first-responder" inflammatory cells such as neutrophils and eosinophils. The antigen causing the formation of a granuloma is most often an infectious pathogen or a substance foreign to the body, but often the offending antigen is unknown (as in sarcoidosis).
Granulomas are seen in a wide variety of diseases, both infectious and non-infectious . Infections that are characterized by granulomas include tuberculosis, leprosy, histoplasmosis, cryptococcosis, coccidioidomycosis andblastomycosis. Examples of non-infectious granulomatous diseases are sarcoidosis, Crohn's disease, berylliosis, Wegener's granulomatosis, Churg-Strauss syndrome, pulmonary rheumatoid nodules and aspiration of food and other particulate material into the lung. This classification of granulomas (infectious vs. non-infectious) should be used with caution as both infection as well as non-infectious mechanisms may contribute in some diseases.
An important feature of granulomas is whether they contain necrosis or not. Necrosis refers to dead cells, and, under the microscope, appears as a mass of formless debris. A related term, "caseation" (literally: turning to cheese) refers to a form of necrosis that, to the unaided eye (i.e., without a microscope), appears cheese-like, and is typically (but not uniquely) a feature of the granulomas of tuberculosis. The identification of necrosis in granulomas is important because granulomas with necrosis tend to have infectious causes. There are several exceptions to this general rule, but it nevertheless remains useful in day-to-day diagnostic pathology.
Image:Granuloma 20x.jpg|Granuloma without necrosis in a lymph node of a person with sarcoidosis.
Image:Necrogran10x.jpg|Granuloma with necrosis in a lung of a person with tuberculosis.
Diseases Characterized By Granulomas
Tuberculosis
The granulomas of tuberculosis tend to contain necrosis ("caseating tubercule"), but non-necrotizing granulomas may also be present. Multinucleated giant cells with nuclei arranged like a horseshoe (Langhans giant cell) are often present, but are not specific for tuberculosis. The diagnosis of tuberculosis requires identification of the causative organism by special stains or culture.
Leprosy
In leprosy, granulomas are found in the skin and tend to involve nerves. The appearance of the granulomas differs according to the precise type of leprosy.
Histoplasmosis
Granulomas are seen in most forms of histoplasmosis (acute histoplasmosis, histoplasmoma, chronic histoplasmosis). Histoplasma organisms can sometimes be demonstrated within the granulomas by biopsy or microbiological culture.
Cryptococcosis
When Cryptococcus infection occurs in persons whose immune systems are intact, granulomatous inflammation is typically encountered. The granulomas can be necrotizing or non-necrotizing. The organisms can be seen within granulomas even without special stains.
Sarcoidosis
Sarcoidosis is a disease of unknown cause characterized by granulomas in multiple organs and body sites, most commonly the lungs, lymph nodes, liver, spleen, skin and eyes. The granulomas of sarcoidosis are similar to the granulomas of tuberculosis and other infectious granulomatous diseases. However, in most cases of sarcoidosis, the granulomas do not contain necrosis, are distributed along lymphatic pathways in the lung and are surrounded by concentric fibrosis. Sarcoid granulomas often contain star-shaped structures termed asteroid bodies or lamellar structures termed Schaumann bodies. However, these structures are not specific for sarcoidosis. Sarcoid granulomas can resolve without sequelae or heal with residual scarring. In the lungs, this scarring can cause pulmonary fibrosis; in the heart, it can impair function and lead to rhythm disturbances, heart failure and even death.
Aspiration pneumonia
Aspiration pneumonia is typically caused by aspiration of bacteria from the oral cavity into the lungs, and does not result in the formation of granulomas. However, granulomas may form when food particles or other particulate substances like pill fragments are aspirated into the lungs. Patients typically aspirate food because they have esophageal, gastric or neurologic problems. Intake of drugs that depress neurologic function may also be causal. The resultant granulomas are typically found around the airways (bronchioles) and are often accompanied by foreign-body-type multinucleated giant cells, acute inflammation or organizing pneumonia. The finding of food particles in lung biopsies is diagnostic .
See also
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