Why are immune responses primarily directed towards infectious agents, and how can the immune system be manipulated to attack for instance malignant cells? The role of the dendritic cells in the immune system may provide the answers. We present a review of a field in which results from basic science are rapidly applied in clinical trials. We searched the Medline database using the terms dendritic cells combined with ontogeny, subpopulations, vaccine or review. Results from our own experimental work are also described. The cited studies show that dendritic cells take up material from their surroundings and migrate to lymphoid tissue where the material is presented to T-cells. Dendritic cells have the ability to selectively direct immune responses towards potentially harmful agents such as bacteria and viruses. Clinical trials show that vaccines based on the use of dendritic cells induce tumor-specific immunity and clinical remission. Experiments conducted by the authors and others indicate the existence of subpopulations of dendritic cells with specialized functions. Dendritic cells play a central role in the initiation of immune responses and may be used to manipulate the immune system. Their use in the treatment of diseases such as cancer is highly promising.