title: Ia determinants on macrophages.
abstract: It is well established that T cells cannot be activated by antigen alone but only if antigen is presented in context with I region associated (Ia) determinants. As a matter of fact, antigen-presenting cells or accessory cells, which are obligatory for the induction of any type of immune response, all share the same major characteristic of Ia expression. Thus, there seems to be a direct correlation between accessory cell function and Ia expression. Originally, Ia determinants were only detected on a few cell types, B cells and macrophages being the first. However, during the course of time, more and more cells were found to be Ia positive (Ia +) and it is possible that most cells can express Ia, if appropriately induced. The regulation of Ia expression has been best studied in macrophages, where it has been found that positive induction elements include phagocytosis and gamma-interferon, while prostaglandin E and alpha-fetoprotein tend to down-regulate the expression of Ia. The regulation of Ia expression on accessory cells is thus an integrated part of immune regulation. It is highly likely, although not yet directly proven, that the Ia molecules are the products of the immune response (Ir) genes located within the major histocompatibility complex. They may even be the mediators of the Ir genes which determine whether an immune response can take place at all and/or the extent of the response. Recently, it has been shown that not all Ia + cells are able to activate every known T cell function.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000818:Animals; D000938:Antigen-Presenting Cells / Q000276:immunology; D005802:Genes, MHC Class II; D006168:Guinea Pigs; D000949:Histocompatibility Antigens Class II / Q000276:immunology*; D008213:Lymphocyte Activation; D008264:Macrophages / Q000276:immunology*; D051379:Mice; D013601:T-Lymphocytes / Q000276:immunology
chemical_list: D000949:Histocompatibility Antigens Class II