Except for the abovementioned steroid hormones, lipids have long been thought to function mainly in energy metabolism and membrane structure. The elucidation of cell signaling mechanisms and the variety of molecules that are employed in these myriad of processes is particularly well exemplified by the lipid messengers.

Dennis, in Handbook of Cell Signaling (Second Edition), 2010 Lipid Signaling

Aberrant signal transduction underlies pathological changes such as fibroblast proliferation and cancer. Secretion of mucins by bronchial epithelial cells and surfactant by type II alveolar epithelial cells, phagocytosis and the respiratory burst by alveolar macrophages, production of cytokines by various cells, replacement of various cell types by cell division, and differentiation are among processes that are highly regulated through signaling pathways in the lung. Signal transduction is critical in regulating the lung nonrespiratory functions. Such information is vital to the reawakening of ‘systems biology’. The challenge in the future will be to better understand how signal specificity is achieved. Signal transduction is now thought to occur through tightly organized networks in which protein–protein interactions and reversible assembly of signaling complexes are controlled by a small number of modular domains. During the past 50 years, intensive research uncovered the enzymes and molecules that participate in this process (i.e., receptors, second messengers, phospholipases, kinases, phosphatases, etc.) and delineated the mechanisms by which cells integrate multiple signals. Signal transduction refers to all biochemical processes by which cells translate extracellular signals originating from their environment into specific responses.

Forman, in Encyclopedia of Respiratory Medicine, 2006

Cell signal