Histopathologic examination of the nodule showed a diffuse infiltration of monomorphous mononuclear cells within the upper dermis. The cells contained centrally located oval nuclei. A Giemsa stain demonstrated metachromatic purple granules in the cytoplasm (Figure 4). Immunohistochemical studies showed that the mononuclear cells were positive for common leukocyte antigen and negative for S100 protein, HMB-45, and cytokeratins. The cells were therefore identified as mast cells.
A Red-Brown Plaque on the Nape—Diagnosis. Arch Dermatol. 2004;140(10):1275-1280. doi:10.1001/archderm.140.10.1275-f