Answer: Blue nevus in a lymph node

Histology: Collections of pigmented cells, some of which demonstrate fine dendritic processes, are seen in the capsule and fibrous trabeculae of this axillary lymph node. The cells are not present in the sinus of the node.

Discussion: Nodal blue nevi have been recognized for decades. They are sometimes appreciated grossly as black streaks in the capsule of the lymph node. Microscopically heavily pigmented dendritic and bipolar cells are present in the capsule and intranodal fibrous trabeculae. Macrophages that have ingested pigment are also often present. Electron microscopy, if performed, will reveal mature melanosomes. Nodal blue nevi are entirely benign and should not be mistaken for metastatic malignant melanoma. The location of the pigmented cells in the capsule rather than the sinus of the node helps establish the correct diagnosis.