Dendritic cells (DC) are widely considered to be the major antigen-presenting cell (APC) type in immune responses. These cells are obtained from adherent cells or are purified CD14⁺ monocytes from peripheral blood mononuclear cells (PBMC) by in vitro stimulation with granulocyte, macrophage-colony-stimulating factor (GM-CSF) plus interleukin (IL)-4. They express high levels of MHC class II and costimulatory molecules, internalize Ag rapidly via Fc receptors and mannose receptors, and, by macropinocytosis, produce large amounts of IL-12 on CD40 ligation, and are potent in presenting soluble Ag and in stimulating allogeneic mixed-leukocyte reactions.