Harris, N. L.Ronchese, F.2010-01-072010-01-072010-01-07199910.1046/j.1440-1711.1999.00835.xhttps://infoscience.epfl.ch/handle/20.500.14299/45117CD4(+) T cells are considered to be the major controlling element of the adaptive immune response. They recognize foreign peptides by interaction of the T cell receptor (TCR) with peptide complexed to major histocompatibility complex (MHC) class II molecules on the surface of antigen presenting cells (APC). Once activated CD4(+) T cells orchestrate the various phases of the immune response. They are responsible for the production of numerous cytokines, which activate specific immune effector cell populations including B cells, eosinophils, mast cells and macrophages. Not surprisingly, the activation of CD4(+) T cells needs to be tightly regulated and is subject to finely tuned control mechanisms. The requirement for a second or 'costimulatory' signal, in addition to the antigenic signal, provides a key element for the exquisite control of T cell activation. One of the major signalling pathways responsible for delivery of this costimulatory signal is induced by interaction of CD28 on T cells with B7 molecules found only on APC. The present review outlines our current understanding of the physiological role of B7 costimulatory signals in regulating CD4(+) T cell responses.b7-1b7-2cd28costimulationeffector cellhumoral immunitymemory cellth1/th2 cellaccessory molecule cd28ctla-4 counter-receptorcd28-deficient micelymphocytes-tmessenger-rnaposttranscriptional regulationantigen presentationcytokine productionactivation pathwaylangerhans cellstext::journal::journal article::review article