Lymphoid, Hematopoietic, and Histiocytic Infiltrates of Skin
Approach
Dense dermal infiltrates require low-power pattern recognition: top-heavy and band-like suggests T-cell, bottom-heavy and nodular with germinal centers suggests B-cell, and interstitial or periadnexal patterns suggest other lineages. Then assess epidermotropism, cell size, and atypia.
Concepts to master
Mycosis fungoides shows epidermotropism with Pautrier microabscesses, cerebriform lymphocytes, CD4 positivity, and often CD7 loss; early lesions mimic spongiotic dermatitis, so clonality studies are supportive, not decisive. Sezary syndrome is the leukemic form. CD30-positive lymphoproliferative disorders range from lymphomatoid papulosis, which is self-healing with a wedge-shaped infiltrate, to anaplastic large cell lymphoma. B-cell lymphomas include marginal zone lymphoma (centrocyte-like and plasmacytoid cells), follicle center lymphoma (germinal centers, BCL2 within follicles), and leg-type diffuse large B-cell lymphoma (large cells, high proliferation). Blastic plasmacytoid dendritic cell neoplasm expresses CD123, TCL1, and CD4 and is aggressive. Mastocytosis shows tryptase and CD117 positivity and KIT D816V; urticaria pigmentosa shows interstitial mast cells. Langerhans cell histiocytosis shows CD1a, langerin, Birbeck granules, and eosinophils.
Traps
Pseudolymphoma versus lymphoma: mixed infiltrate, germinal centers with tingible-body macrophages, and polyclonal T-cell receptor studies favor benign, but a clonal result never overrides benign morphology. Do not miss systemic involvement in mastocytosis. In Langerhans cell histiocytosis, CD68 and S100 are positive but not specific, so require CD1a or langerin.
Rule of thumb
Top-heavy and epidermotropic means T-cell; bottom-heavy and nodular means B-cell. A plaque with a band-like infiltrate, epidermotropism, CD4-positive cells, and CD7 loss is mycosis fungoides; the same biopsy with germinal centers and a mixed infiltrate favors pseudolymphoma.