Abstract
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.