Cholecystokinin (CCK) is a neuropeptide that affects growth rate in chickens by regulating appetite. CCK peptides exert their function by binding to two identified receptors, CCKAR and CCKBR in the GI tract and the brain, respectively, as well as in other organs. In mammals, CCK/CCKAR interactions affect a number of immunological parameters, including regulation of lymphocytes and functioning of monocytes.

CCK, also known as pancreozymin, is synthesized and secreted by enteroendocrine cells in the duodenum. The main function of CCK is to cause the release of digestive enzymes and bile from the pancreas and gallbladder, respectively. It also induces drug tolerance to opioids like morphine and heroin. Cholecystokinin (CCK) has strong bioactivity in the regulation of a number of cell activities.