ABSTRACT
I. Introduction The epithelial surfaces of the respiratory tract are continuously exposed to a wide variety of pathogenic and non-pathogenic antigens present in ambient air, and the maintenance of immunological homeostasis within this fragile tissue microenvironment requires finely tuned mechanisms for regulation of T cell-dependent immune responses to the inhaled agents. Recent evidence (reviewed later) suggests that local immunoregulatory functions are partitioned in the steady state between two major bone marrow-derived cell populations, dendritic cells that control the induction of primary immunity and macrophages that control the expression of secondary immunity.
