Rheumatoid Arthritis Support

Doctors occasionally 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 Greek plural ''granulomata''; modern anglicized plural ''granulomas'') is an organized collection of macrophages.

Macrophages (also known as histiocytes) are the cells that define a granuloma. They often, but not invariably, fuse to form multinucleated giant cells. The macrophages in granulomas are often referred to 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. They also have larger nuclei than ordinary macrophages and their cytoplasm is typically more pink when stained with eosin. These changes are thought to be a consequence of "activation" of the macrophage by the offending antigen.

The other key term in the above definition is the word "organized" that 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.

All granulomas, regardless of cause, 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 may be a clue to coccidioidomycosis or allergic bronchopulmonary fungal disease, and granulomas with numerous neutrophils suggest blastomycosis, Wegener's granulomatosis, aspiration pneumonia or cat-scratch disease.

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, blastomycosis and cat scratch disease. 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.

An important feature of granulomas is whether or not they contain necrosis. Necrosis refers to dead cells that, under the microscope, appear as a mass of formless debris with no nuclei present. 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 ("caseous"), 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 central necrosis in a lung of a person with tuberculosis. Note the Langhans-type giant cells (with many nuclei arranged in a horseshoe-like pattern at the edge of the cell) around the periphery of the granuloma. Langhans-type giant cells are seen in many types of granulomas, and are not specific for tuberculosis.




Adapted from the Wikipedia article Granuloma, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki