Secreted proteins are proteins that are produced by cells and released into the extracellular space. They play crucial roles in various biological processes, including cell signaling, immune responses, and tissue repair. These proteins can be found in bodily fluids such as blood, saliva, and mucus, and they often act as messengers or enzymes that facilitate communication between cells. The process of secretion typically involves the synthesis of proteins in the endoplasmic reticulum and their transport through the Golgi apparatus before being released from the cell. Secreted proteins can be classified into different categories, such as hormones, antibodies, and growth factors, each serving specific functions in the body.