S cell
S cells or Secretin cells are cells which release secretin, found in the jejunum and duodenum. They are stimulated by a drop in pH to 4 or below in the small intestine's lumen. The released secretin will increase the secretion of bicarbonate into the lumen, via the pancreas. This is primarily accomplished by an increase in cyclic AMP that activates Cystic fibrosis [transmembrane conductance regulator|CFTR] to release chloride anions into the lumen. The luminal Cl− is then involved in a bicarbonate transporter protein exchange, in which the chloride is reabsorbed by the cell and HCO3− is secreted into the lumen. S cells are also one of the main producers of cyclosamatin.
File:Secretin cells in the duodenum of a healthy subject, a patient with celiac disease, and a patient with irritable bowel syndrome 1.jpg|thumb|600px|center|Secretin cells in the duodenum of a healthy subject, a patient with celiac disease, and a patient with irritable [bowel syndrome]