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Granulomatous Dermatitis
A granuloma is a inflammatory reaction in which histiocytes are a prominent component. In the most
characteristic patterns of granulomatous inflammation, the histiocytes should be closely aggregated to form sheets, columns (probably representing a concentric layering of histiocytes about a vessel), or expansile,
rounded aggregates. Examples, in which histiocytes are both clustered back to back (to the relative exclusion of both other inflammatory cells (particularly lymphocytes) and connective tissue fibers and, in
addition, form expansile, circumscribed aggregates, are characterized as epithelioid
(tuberculoid) granulomas. In this usage, the term, epithelioid, gives recognition to the close clustering of cells in the absence of an interstitial, fibrous component (S12C11P1-1). The term has acquired other meanings; a popular extension of the term gives recognition to the
size and shape of individual cells. In such usuage, the term gives recognition to cells that are rounded in outline (and generally have acidophilic cytoplasm). It so happens that individual, activated histiocytes of
an epithelioid granuloma, when evaluated in regard to the characteristics of individual cells, have “epithelioid” qualities. In granulomas, there is a tendency for some of the histiocytes to coalesce (fuse) and, in
the act, to form multinucleated giant cells.
The close aggregation of histiocytes in granulomatous patterns is a morphologic arrangement that facilitates the
isolation of foreign, or injurious material, including biologic material such as organisms. Not only is the isolation of such material a physical confinement, but the nature of the physical environment, for some of
the relationships, is well-suited to promote the digestion and, for biologic agents, the death of the injurious material. The environment of an epithelioid granuloma is enzymatically rich. Granulomas tend to enlarge
circumferentially and, having reached a certain size, there is a tendency for histiocytes centrally to undergo necrosis as new histiocytes are recruited at the periphery. For some disorders, the benefits attending a
local population of histiocytes are only relative; some injurious biologic agents are capable of surviving, even in the cytoplasms of some of the clustered histiocytes. They may outlive generations of histiocytes
and may be found in stainable condition in zones of necrosis. Some infectious granulomas are associated with necrosis of vessels and some of the effects of granulomatous inflammation are mediated at a vascular
level. Such lesions, if they approach a surface, may cavitate.
Sarcoidosis
The epithelioid tubercle-like granuloma is symmetrical rounded and tends to be relatively restricted in size. In
such a granuloma, the histiocytes are closely spaced and, early on, show relatively little in the way of connective tissue fibers among the histiocytes (S12C11P1-1). Tubercle-like granulomas often are most prominent in perivascular arrays, but may also be
interstitial in distribution. The tubercle-like aggregates may become confluent. The larger confluent aggregates may show central areas of fibrinoid necrosis. Granulomas heal by the process of fibrosis; in some
examples, such as sarcoidosis, the fibrous product is hyalin. Zones of necrosis, particularly granular areas, may persist as such for years. Some (many) of these qualities are manifested in lesions of sarcoidosis.
Sarcoidosis is prototypic of the pure granuloma. Involvement of the skin is relatively common. In the
subcutaneous fat, the granulomas are numerous; they tend to become confluent. Typically, there is relatively little inflammation associated with the granulomas of sarcoidosis; lymphoid infiltrates tend to be mild
and plasma cells usually are not a prominent feature.The granulomas often are characterized as “naked” (they usually are not associated with prominent infiltrates of lymphoid cells). For granulomas that are
associated with prominent infiltrates of lymphoid cells, the diagnosis of sarcoidosis should be made with circumspection. Sarcoidosis often is characterized as a patheric disorder. In the latter approach, a patient
with the clinical presentation of sarcoidosis is assumed to react to antigenic stimulation in an abnormal fashion; the antigen need not be specific; the antigenic response is granulomatous.
English JC, Patel PJ, Greer KE: Sarcoidosis. J Am Acad Dermatol 2001; 44:725-43.
Lupus Vulgaris
In the skin, lesions of lupus vulgaris can be confused with those of sarcoidosis; necrosis may not be a
prominent feature. The granulomas are not as well organized as those of sarcoidosis (S12C11P1-2); lymphoid
infiltrates with plasma cells tend to be a more prominent feature. Fibrosis also may be a prominent feature (S12C11P1-3).
Organisms often are difficult to identify in lesions of lupus vulgaris (S12C11P1-4).
Atypical Acid Fast Infection
The atypical acid fast organisms cannot be distinguished on the basis of histologic patterns, although some
examples tend to be associated with prominent suppurative components. The most common atypical acid fast infection, in immuno-competent hosts, is that caused by M. marinum.
The basic pattern of M. marinum infection includes: pseudoepitheliomatous hyperplasia, suppurative granulomas,
lympho-histiocytic infiltrates, and fibrosis. Some of the dermal granulomas press upon the extremities of the hyperplastic squamous epithelium (S12C12P2-1 & 2). The organisms often are large and some are beaded (S12C12P2-3).
Small, suppurative granulomas may be incorporated in the hyperplastic epithelium and may eventually be eliminated at the surface (transepithelial elimination) (S12C12P2-4-6).
Some atypical acid fast infections tend to involve the subcutaneous fat; in such lesions, the granulomatous
changes may be complicated by a reaction to necrosis of fat; some of the patterns may be lipogranulomatous (S12C14aP4a-1 & 2).
For some examples, the response in tissue is suppurative.
Foreign Body Granuloma
Some foreign body reactions are pure granulomas with sarcoidal qualities (epithelioid granulomas in
tubercle-like patterns). Silica particles (for example, particles of glass in the tissue may provoke a sarcoidal reaction; the response may first appear years after an injury, such as being thrown against a
windshield. In foreign body granulomas containing particulate material, the tubercle-like granulomas may be compactly aggregated in the dermis and subcutis; under polarized light, birefringent particles can be
identified in some examples (S12C13P3-1-3). Silica-like particles may be found in lesions of sarcoidosis. Larger crystalline
structures, conchoid bodies, may also be a feature.
Smith K, et al: Histologic features of foreign body reactions in patients infected with human immunodeficiency
virus type I. J Am Acad Dermatol 1993;28:470-6.
Filariasis
Parasites in soft tissue may evoke a granulomatous and necrotizing response (S12C14P4-1 & 2).
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