Anti-J-chain antibodies are protein reagents that detect specific antigens. The J-chain antigen is another term for the human protein joining chain of multimeric IgA and IgM, encoded by the JCHAIN gene. The protein is known to function in adaptive immune responses, among other biological roles. Canonically, it has an amino acid length of 159 residues and a mass of 18.1 kilodaltons. It is a secreted protein and it is expressed in many tissues, such as the appendix and breast.