The Stoichiometry of J Chain in Human Secretory IgA

Quantitative methods were developed to measure the J chain content of the polymeric immunoglobulins. The methods involved two steps: first, isolation of a light-J chain fraction, and second, determination of the ratio of the two chains in the fraction from differences in amino acid content. When these methods were applied to human colostral IgA, the dimers were found to contain one mole of J chain for every two monomer units while the tetramers contained one J for every four monomer units. This stoichiometry supports the hypothesis that J chain plays an obligatory role in polymer assembly and suggests the function of the J chain is to initiate polymerization by linking two monomer units.