Dendritic cells are bone marrow-derived professional antigen-presenting cells which link innate and adaptive immune responses. Acting as sentinels of the immune system, dendritic cells are loaded with a large array of antigen-capturing and pathogen-recognition receptors which constantly sample the surrounding extracellular environment for 'danger signals'. However, dendritic cells also exhibit a naive T-lymphocyte-activating ability and, depending on their maturation status, promote either immunological tolerance or primary immune responses. This functional ambivalence has brought dendritic cells to the focus of attention for immunotherapy protocols in autoimmune diseases, cancer and transplantation.