It is easy to be fascinated by dendritic cells (DCs), not only because of their pivotal role in the immune response, but also because of the elegance with which they perform their tasks. Although DCs comprise multiple subsets (Liu, 2001 [this issue of Cell]), all are unusually effective at antigen processing and presentation. DCs can take up a diverse array of antigens and present them to T cells as peptides bound to both MHC class I and II products. Relative to other antigen presenting cells, DCs are adept at stimulating naı̈ve T cells. DCs also control the quality of the T cell response, driving naı̈ve lymphocytes into distinct classes of effectors. These antigen-specific, adaptive responses are critical for resistance to infections and tumors. Conversely, DCs can also generate regulatory T cells that suppress activated T cells, a function of likely importance in autoimmunity and transplant rejection.

In addition to their role in adaptive responses, DCs play a critical role in innate immunity. In some respects, DCs are as active in responding to microbial challenge as DCs can produce copious amounts of cytokines involved in host defense, such as IL-12 and both type I and II interferons. DCs also activate NK and NKT cells, innate lymphocytes that rapidly kill select targets and produce important cytokines.