Contrary to our abundant knowledge about the sensitization phase of human contact hypersensitivity, little is known about the cell types orchestrating the effector phase. In order to address this issue, we phenotypically analyzed biopsies from 72 h epicutaneous patch test reactions (n=10) and normal human skin (n=5) for the presence of various leukocyte differentiation antigens. The inflammatory infiltrate was dominated by CD3+/CD4+ T cells with approximately 30% of the cells coexpressing CD25 and CTLA-4, a phenotype consistent with either activated effector or regulatory T cells. In our search for professional antigen-presenting cells, we were surprised to find not only sizeable numbers of CD1a+ dendritic cells and CD1c+ dendritic cells, but also of CD123+, CD45RA+, BDCA-2+, CLA+, and CD62L+ plasmacytoid dendritic cells. Although virtually absent in normal human skin, these cells were detectable already 6 h after hapten challenge and were often found in close proximity to CD56+ natural killer cells, indicative of a functional interaction between these cell types. The detailed knowledge of the cellular composition of the inflammatory infiltrate in allergic contact dermatitis and its kinetics should form the basis for the investigation of the immunologic and molecular events operative in the perpetuation and resolution of the eczematous response.