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EOSINOPHILS
A rich component of eosinophils in the epidermis, in areas of spongiosis, or in the cavities of spongiotic vesicles, qualifies as
eosinophilic spongiosis; disorders to be considered include pemphigus, and pemphigoid. Eosinophilic spongiosis may be anticipatory of, or concomitant with, the more characteristic changes of either pemphigus or
pemphigoid. Eosinophils in folliculo-centric, pustular patterns characterize the reaction in erythema toxicum neonatorum. Pustules with neutrophils, and a variable number of eosinophils, are characteristic of
infantile acropustulosis, and eosinophilic folliculitis. Eosinophils are common in inflammatory infiltrates of the skin (S1C30P20-1a, b & c).
In histologic evaluations, the identification of numerous eosinophils tends to prejudice the structuring of a differential diagnosis; eosinophils commonly are equated with an “allergic reaction;” drug eruption and
reaction to insect bite are likely to be considered.
Both eczema and contact dermatitis are manifested in patterns of a cell mediated immune reaction; they share the features of
spongiotic, spongiotic and psoriasiform, and psoriasiform disorders. In the category of spongiotic and psoriasiform dermatitides, the presence of eosinophils in the dermal infiltrates might be taken as evidence
favoring eczema rather than contact dermatitis, but this is not a sure guide. A drug eruption must also be considered in the differential diagnosis.
If, in an inflammatory process, eosinophils are prominent in the interstitium of the reticular dermis, and are associated with a
hyperplasia of fibrocytes, then the infiltrates of eosinophils might be cited as evidence favoring a diagnosis of insect bite. The perivascular infiltrates of a lymphocytic vasculitis, as the most common
manifestation of the common drug eruptions, usually contain a sprinkling of eosinophils. Eosinophils and histiocytes are common in the perivascular infiltrates of angiocentric T cell lymphomas.
MAST CELLS
Mast cells are universally represented in the perivascular spaces of the skin and, in some reactions, are increased in number in
the perivascular spaces, the perifollicular sheaths, and the reticular dermis among collagen bundles. Mast cells have a role in some immune reactions; they may be a feature of the perivascular infiltrates of the
lymphocytic vasculitis of some cell-mediated immune reactions. In making distinctions between mast cell hyperplasia and the mastocytoses, evaluations of the density of the infiltrates of mast cells becomes
important.
Mast cells in perivascular spaces in reactive disorders tend to be oval in outline. Mast cells in the interstitium of the reticular
dermis in association with mucinous changes among the collagen bundles are usually rounded in outline. Mast cells beyond the perivascular spaces may be dendritic in character; they then qualify as "fixed mast
cells." Mast cells in tumoral patterns, as seen in lesions of urticaria pigmentosa, tend to be rounded in outline. Generally, they produce interstitial, or solid infiltrates with little evidence of fibrogenesis
in response to the infiltrate. In adult mastocytosis, mast cells commonly are loosely distributed in the perivascular spaces, and among collagen bundles of the dermis. They tend to be dendritic in outline. In some
examples of mastocytosis, the cells are loosely spaced among delicate fibrous lamellae. In such areas, the mast cells have a spindle-cell configuration.
Mast cells obviously have a role in the maintenance of the mucinous matrix of the reticular dermis. They also have a role as an
inflammatory mediator in some examples of cell-mediated immunity. Degranulation of mast cells may be associated with local infiltrates of eosinophils.
LYMPHOCYTES
Lymphocytes and their progeny are remarkably diverse, functionally and morphologically. The interpretations of cutaneous lymphoid
infiltrates have come a long way from the rather static concepts of the '50's. In the '50's, inflammatory infiltrates were likely to be characterized as simply a reaction to damaged tissue. Their role in
inflammation of the skin was thought to be concerned with little more than a toilet of damaged tissue. The designation, acute dermatitis, had application to disorders in which neutrophils were the preponderant
reacting cell. The designation, chronic dermatitis, had application to disorders in which lymphocytes were the preponderant reacting cell. Currently, the designations, acute dermatitis and chronic dermatitis, if not
additionally qualified, rarely have application. On occasions in the not too distant past, the uncertainty of the nature of an inflammatory process could be even further masked by avoiding either of the two optional
qualifiers, and making no greater a diagnostic commitment than "dermatitis."
The recognition of T & B cell lineages, and a definition of the roles of T & B lymphocytes has greatly facilitated our
understanding of the role of immunity in determining the nature, and distribution of inflammatory infiltrates. Lymphocytes are the common reacting cells in most inflammatory disorders of the skin. In addition, with
rare exceptions, the chief reacting cells locally in the site of an immune-mediated injury are T lymphocytes with their common cooperatives being eosinophils and the histiocytes.
The presence of B lymphocytes and plasma cells in inflammatory infiltrates can be taken as a marker identifying a role for humoral
factors. Even in infiltrates showing B cell markers, T lymphocytes are also likely to be represented. B and T lymphocytes, in their roles in humoral and cell-mediated immune reactions, influence the nature of the
cellular response and, in turn, determine the character of the tissue response. Cell markers have provided insights into the nature of inflammatory infiltrates; in the process of defining the cellular nature of a
reaction, the distinctions between B- and T- cell reactions (i.e., humoral and cellular reactions), as separate processes, have been obscured.
PLASMA CELLS
Plasma cells, if represented in inflammatory infiltrates, are indicative of a role for humoral mechanisms. Interestingly, plasma
cells are rare in the infiltrates of both systemic, and discoid lupus erythematosus; they are a feature of the deep dermal and subcutaneous infiltrates of lupus profundus, as they also are a feature of lesions of
morphea or scleroderma (they also distinguish another sclerosing disorder, namely necrobiosis lipoidica diabeticorum). Plasma cells are common in all forms of leprosy, but are particularly prominent in lesions on
the lepromatous side of the spectrum. They are common in association with infectious granulomas. They also are common in association with inflammatory diseases of the pilosebaceous units. They are a feature of some
lymphoid neoplasias, and their presence in the infiltrates of lesions of macroglobulinemia may lead to a misdiagnosis of such a lesion as an inflammatory process. The identification of Dutcher bodies in the nuclei
of some of the cells in lymphoplasmacytic infiltrates is an aid in the diagnosis of macroglobulinemia.
TRANSFORMED T LYMPHOCYTES
Activated T lymphocytes can be recognized by nuclear convolutions, even in reactive processes. In some examples of inflammatory
infiltrates, the small activated T cells may be numerous; they might be characterized as Lutzner cells. In the face of an infiltrate rich in transformed T lymphocytes, distinctions between a reactive process and an
early, progressive T cell dysplasia become a problem. In examples of inflammatory processes with a high component of transformed, small lymphocytes, the high component deserves recognition as a mild T cell dysplasia
of indeterminate type; atypical cells are, thereby, given recognition but this characterization does not identify the process as a progressive T cell dysplasia.
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