Preliminary data from our research group indicated that mucosal CD1a+ dendritic cells (DCs) possessed immune regulatory potentials, and the initial aim of this project was thus to study the functional properties of CD1a+CD1c+ DCs isolated from small intestinal mucosa as well as CD1a+CD1c+ DCs generated in vitro from precursors isolated from blood, in terms of their cytokine production and effect on T cells. However, when we isolated CD1c+ DCs from blood using the CD1c (BDCA-1)+ Dendritic Cell Isolation Kit we discovered that the kit isolates two different populations instead of a homogenous population of DCs. We therefore aimed to determine the identity of these two populations and investigate the functional difference between them first, before proceeding with the primary project. In this study we showed that the CD1c (BDCA-1)+ Dendritic Cell Isolation Kit isolates two distinct populations: CD1c+CD14– DCs and CD1c+CD14+ monocytes. We established that a subpopulation of blood monocytes, consisting of mostly CD14++CD16– classical and CD14++CD16+ intermediate monocytes, expressed CD1c+, and that also small-intestinal CD14hi monocytes expressed CD1c. Next we showed that the isolated CD1c+ DCs and CD1c+ monocytes had strikingly different properties given their differential expression of surface markers, which makes it evident that these two populations should not be treated as one entity. In the next part of the project we induced CD1a expression in blood CD1c+ DCs and CD1c+ monocytes by TGFβ treatment, and saw that their cytokine production was independent of CD1a, but that the CD1c+ monocytes produced higher amounts of TNFα than the CD1c+ DCs. We also analyzed TNFα production of CD1a+ versus CD1a– cells among small intestinal tissue DCs, and observed no differences between these subsets. In terms of T cell activation, we showed that CD1c+ DCs promoted more T cell activation than CD1c+ monocytes, and that within the DCs, CD1a+ DCs promoted less proliferation of T cells than CD1a– DCs. T cells in coculture with CD1c+ monocytes were more likely tBet+ than when cocultured with DCs, indicating that monocytes skewed the T cells towards a TH1 phenotype. These results again emphasized the risks of treating the two functionally different cell populations isolated by the CD1c (BDCA-1)+ Dendritic Cell Isolation Kit as one entity. Collectively, these data did not support the hypothesis that the CD1a+CD1c+ subsets within DCs and monocytes possess a regulatory phenotype.