Dendritic cells are now recognized as the gatekeepers of the immune response, possessing a unique potential for acquisition of antigens at extremely low exposure levels and for efficient presentation of these in an immunogenic form to the naive T-cell system. Dendritic cell populations throughout the body exhibit a wide range of features in common that are associated with their primary functions, and these are considered in the initial section of this review. In addition, it is becoming evident that the properties and functions of these cells are refined by microenvironmental factors unique to their tissues of residence, a prime example being mucosal microenvironments such as those in respiratory tract tissues, and the latter represents the focus of the second section of this review. (J Allergy Clin Immunol 2000;105:421-9.)