Secreted

Secreted is a term used in biology to describe substances that are released by cells or organisms into the extracellular space or into a body lumen, outside the cell. Secretion serves physiological functions, signaling, lubrication, digestion, or defense, and is distinct from excretion, which principally removes wastes. Substances that are secreted include enzymes, hormones, mucus, antibodies, and neurotransmitters.

Most secreted proteins and some lipids follow the classical secretory pathway: synthesis in the endoplasmic reticulum, processing in the Golgi apparatus, packaging into secretory vesicles, and release by exocytosis. Some proteins use non-classical pathways that bypass the ER-Golgi route. Secreted products may be released constitutively or in a regulated manner in response to physiological stimuli.

Examples of secreted substances include digestive enzymes such as amylase and proteases released into the digestive tract; hormones like insulin secreted by pancreatic beta cells; mucus secreted by goblet cells in the respiratory and gastrointestinal tracts; antibodies secreted by plasma cells; and neurotransmitters released at synapses. Secreted factors also include growth factors and components of the extracellular matrix that influence neighboring cells and tissues.

In research and medicine, the collection of all secreted proteins produced by a cell, tissue, or organism is referred to as the secretome. Studying secretion helps illuminate physiological processes, disease mechanisms, and biomarker discovery. Defects in secretion can contribute to disorders ranging from cystic fibrosis (impaired mucus secretion) to various endocrine diseases (impaired hormone release).