Antigen presentation at the inflammatory site.

The introduction of antigen into tissues can induce an inflammatory response initiated by antigen-specific T lymphocytes. Central to this process is the recognition of antigen by specific T cells that cannot respond to intact antigen directly, but rather recognize antigenic fragments in association with gene products of the major histocompatibility complex (MHC) displayed by an antigen-presenting cell. To present antigen effectively, a cell must internalize antigen, process it to the immunogenic moiety, present the antigen in the context of a MHC molecule, and deliver various antigen-nonspecific signals required for T-cell activation. At the initiation of an immune response, the cells with the capacity to perform all of these functions are limited to a few specialized cell types, including monocytes/macrophages, dendritic cells, B cells, and Langerhans cells. Once the immune response has been initiated, however, cytokines and other factors, released by activated cells at the inflammatory site, stimulate a variety of changes in various cell types that alter their capacity to function as antigen-presenting cells and facilitate antigen-induced T-cell activation. Therefore, as the immunologically mediated inflammatory response evolves, a variety of changes occur within the local environment that enhance and/or modulate the capacity of T cells to recognize and respond to the inciting antigen. The purpose of this review is to catalog the changes in the function of antigen-presenting cells at inflammatory sites that might alter the nature of the immune response.