A1BG (?-1B-glycoprotein), also known as A1B, ABG, GAB or HYST2477, is a 495 amino acid secreted glycoprotein that contains five immunoglobulin (Ig)-like V-type domains and belongs to the immunoglobulin superfamily. At an average concentration of 22mg/dl, A1BG is expressed in normal adult plasma and is thought to be involved in the regulation of cell behavior and cell recognition. In plasma, A1BG specifically binds to human CRISP-3, a member of the cysteine-rich secretory protein (CRISP) family comprised of evolutionarily conserved proteins which are believed to play a role in the innate immune system. Through its association with CRISP-3, A1BG is believed to function in protecting the body from the circulation of free CRISP-3, a circumstance with potentially harmful effects.