A J chain is a protein component of the antibodies IgM

IGM

IGM as an acronym or abbreviation can refer to:* Immunoglobulin M , the primary antibody against A and B antigens on red blood cells* International Grandmaster, a chess ranking* intergalactic medium* Intragroup medium - see: Intracluster medium...

 and IgA

IGA

Iga or IGA may stand for:-Given name:* a female given name of Polish origin. The name originates from the female given name Jadwiga and stands for gia,or gina in the USA....

. It is a 137 residue

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 polypeptide.

Structure

The J Chain's molecular weight is approximately 15 kDa. It exhibits a standard immunoglobulin folding structure of two β-pleated sheets of four ribbons folded against one another. It has 8 cystine

Cystine

Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulfide bond. This organosulfur compound has the formula 2. It is a white solid, and melts at 247-249 °C...

 residues. Two of these residues link the α chains of IgA

IGA

Iga or IGA may stand for:-Given name:* a female given name of Polish origin. The name originates from the female given name Jadwiga and stands for gia,or gina in the USA....

 or the μ chains of IgM

IGM

IGM as an acronym or abbreviation can refer to:* Immunoglobulin M , the primary antibody against A and B antigens on red blood cells* International Grandmaster, a chess ranking* intergalactic medium* Intragroup medium - see: Intracluster medium...

 via disulfide bridges, effectively serving as the "glue" between two Fc regions of the antibody.

The J-chain shows a large degree of homology between avian and human species, suggesting that it serves an important function.

Function

The J Chain is required for IgM or IgA to be secreted into mucosa.

Because IgM and IgA are the only two types of antibody that polymerize, initial hypotheses stated that J chain was required for polymerization. However, it was subsequently found that IgM is able to polymerize in the absence of J chain as both a pentamer

Pentamer

A pentamer is a thing composed out of five sub-units.In chemistry, it applies to molecules made of five monomers.In biochemistry, it applies to macromolecules, in particular to pentameric proteins, made of five proteic sub-units....

 and a hexamer

Hexamer

A hexamer is a thing composed out of six sub-units.In microbiology, a hexamer is one of the proteins composing the polyhedral protein shell that encloses the bacterial micro-compartments known as carboxysomes....

, however, both of these exist to lesser numbers in organisms lacking J chains. In such case, there are also fewer IgA dimers.