HLA-DR-positive leucocyte subpopulations in human skin include dendritic cells, macrophages, and CD7-negative T cells.

The immunophenotypes of the HLA-DR-positive leucocyte populations in normal human skin were studied using an extensive panel of monoclonal antibodies, which included antibodies from the Third International Leucocyte Differentiation Antigen Workshop (3rd LDAW). Langerhans' cells (LC) in the epidermis stained with antibodies from CD15c, Groups 10, 12a, 12b and 15, of the myeloid panel and from CD39 of the B-cell panel. However, LC did not react with CD14 antibodies or 63D3, which are frequently used to stain tissue macrophages. In addition to epidermal LC (26 cells/linear mm) a significant population of CD1a-positive cells was identified in the papillary dermis (7 cells/linear mm of overlying epidermis). The dermal HLA-DR-positive leucocytes consisted of three cell populations. The most numerous cell type stained with antibodies to monocytes/macrophages. There were fewer, though substantial, numbers of T lymphocytes (mainly CD7-negative) and the least numerous was the population of CD1a-positive cells. The CD1a-positive cells and the population of dermal cells that stain with monocyte/macrophage markers are both potential antigen-presenting cells for the skin-associated immune system.