Marshall E. Kadin
Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts, U.S.A.
Inflammatory reactions in the skin usually contain a polymorphous infiltrate of T cells, B cells, plasma cells, tissue macrophages, Langerhans cells, dermal dendritic cells, and variable numbers of myeloid cells. The composition of the inflammatory cell infiltrate depends on the nature of skin injury or infection. In some conditions, such as Borrelia burgdorferi infections, secondary follicles with germinal centers are often seen. Reactive germinal centers in inflammatory conditions appear polarized with dark and light zones, have a starry sky appearance due to macrophages with ingested nuclear material from apoptotic cells, a high mitotic rate, and a distinct mantle zone of small round lymphocytes. In contrast, follicular lymphomas have a loss of polarity, few or absent starry sky macrophages, a low mitotic rate and an indistinct to absent mantle zone. In marginal zone lymphomas, the germinal centers may be partially replaced by centrocyte-like cells which are elongated B lymphocytes with small nuclei. Numerous plasma cells, with light chain restriction by immunohistochemistry, often in the superficial portion of the infiltrate, are characteristic of cutaneous marginal zone lymphomas, in contrast to the polyclonal plasma cells randomly dispersed and concentrated in perivascular sites in inflammatory conditions. Some degree of tissue eosinophilia and or neutrophilia is a common feature of hypersensitivity and arthropod bite reactions.
In nearly all inflammatory conditions, a mixture of cell types is seen. In contrast, lymphomas and leukemias show a clear predominance of one cell type or lineage. Usually the cells comprising the infiltrate represent a particular stage of cellular differentiation. Tumors derived from immature cells or blasts are revealed by a uniform population of medium to large cells with round to oval nuclei, fine evenly dispersed chromatin, and one to several small distinct nucleoli. The mitotic rate is high. In myeloid leukemias, some number of cytoplasmic granules can be detected in cells showing maturation. This is often most apparent in eosinophilic myelocytes. Chloracetate esterase or Leder stains for primary granules are helpful in establishing a diagnosis of myeloid leukemia.
Primary cutaneous T-cell lymphomas usually show some degree of epidermo-tropism in which malignant T cells with highly convoluted or cerebriform nuclei, often surrounded by halos, are found in the basal layer of the epidermis and sometimes concentrated in the upper layers of the epidermis comprising Pautrier microabscesses. The cerebriform cells are often found in juxtaposition with Langerhans cells and one must be careful to distinguish benign collections of Langerhans cells with pale elongated and convoluted nuclei and pale cytoplasm, in inflammatory conditions, from collections of malignant T cells with dark round to oval highly convoluted nuclei, best appreciated by focusing up and down with 60-100 X oil objective lenses or electron microscopy.
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