One characteristic common to all organisms is the dynamic ability to coordinate constantly one's activities with environmental changes. The function of communicating with the environment is achieved through a number of pathways that receive and process signals, not only from the external environment but also from different regions within the cell. Individual pathways transmit signals along linear tracts resulting in regulation of discrete cell functions. This type of information transfer is an important part of the cellular repertoire of regulatory mechanisms. However, as increasingly larger numbers of cell signaling components and pathways are being identified and studied, it has become apparent that these linear pathways are not free-standing entities but parts of larger networks. Several articles in this review series describe in exquisite detail how individual classes of signaling pathways are organized and function. As we understand the details of such functional organization and move to the next level of analyzing integrated cellular functions, it will become increasingly important to identify and study the properties and capabilities of signaling networks as a whole.

One of the more surprising revelations that is coming from the initial studies of networks and component interactions in different cell types is that there may be a general signaling network that receives signals from cell type–specific inputs (ie, receptors) and engage cell type–specific machinery. The molecular identity of the signaling components and their interacting partners may be cell type–specific, but the overall function of these components and the logic of the circuitry is preserved from cell type to cell type. We will compare two cell types, T cells (