Granuloma
Chapter: 2
A special form of chronic inflammation. It is formed most often when either a foreign body or persistent microorganism, such as the tubercle bacillus, evades destruction by the unmodified chronic inflammatory response. In its most classical form, a granuloma consists of concentric layers of cells that, together, form the distinctive lesion. At the center, there often is a focus of caseous necrosis, although this need not be the case. This central focus is surrounded by a layer of specialized macrophages, called epithelioid cells, and multinucleated giant cells. The latter form as the result of cytoplasmic fusion of macrophages, due to stimulation by a cytokine. The next layer is predominantly lymphocytes, and the outer layer is most often fibroblasts, that are attempting to wall off the inciting stimulus with fibrous connective tissue. Granulocytes, plasma cells, and other cells types may also be present. If the agent is an inert foreign body, the granuloma will be of the nonimmune type. However, if the inciting agent is antigenic, the lesion will be an immune type of granuloma, in which the antigen-stimulated lymphocytes are producing cytokines, such as interferon-gamma, to activate the macrophages and their specialized forms to heightened levels of microbicidal activity.
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