Accessory Cells and the Activation and Expression of Different T Cell Functions

It is generally accepted that T cells are not activated by antigen alone, but only if it is presented in context of class II major histocompatibility products (also called la products) (1–4) Whether the antigen has to be processed or not before presentation is still controversial (5,6). The cells responsible for processing and/or presentation are called antigen presenting cells (APC) or accessory cells (AC). A major characteristic of functional AC is the expression of la on the cell surface. In vitro, numerous Ia+ cells are reported to function as APC or AC, among them are classical macrophages (Mph), skin-Langerhans cells, dendritic reticular cells, endothelial cells, astrocytes, B cells as well as a large number of tumor cells of various origin (1,2 7–20).