Bicarbonate umbrella is a cell protective function from cytotoxic molecules by extracellular secretion of bicarbonate. Small bile ducts use this function to protect from hydrophobic bile acids, damage-associated molecular patterns (DAMPs), and pathogen-associated molecular patterns (PAMPs). On the other hand, primary biliary cholangitis (PBC) whose head of issue is characterized by the destruction of small bile ducts with the dysfunction of the bicarbonate umbrella as Cl-/HCO3- exchanger anion exchanger 2 (AE2) activation. PBC has two features; an organ-specific autoimmune disease and a cytopathic biliary disease due to bile exposure that has strong cytotoxicity. Thus, bile ducts of PBC are exposed to two types of bile acids; hydrophilic and cytoprotective ursodeoxycholic acid (UDCA) and hyrophobic and cytotoxic bile acids. UDCA is effective to protect bile exposure in the early phase of the disease, and UDCA is not so effective in the advanced phase where abnormal acquired immunity has central roles for destruction of the small bile ducts.
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