Blocking effect of J chain and J-chain antibody on the binding of secretory component to human IgA and IgM.

Isolated released J chain showed only a small affinity for free secretory component (SC), as indicated by a marginal but reproducible blocking effect on the binding of SC to Ig polymers. The SC-binding site was completely blocked by J-chain antibody in those Ig polymers where the bound J chains were accessible to the antibody. Along with the established masking effect of SC on the antigenicity of J chains present in secretory IgA, these results are compatible with the idea that the conformation of Ig-associated J chains contributes to the SC-binding site of Ig polymers.