Serum IgM-bound secretory component (sIgM) in liver diseases: comparative molecular state of the secretory component in serum and bile.

The sera of 15 patients with liver disease and high serum concentrations of secretory component (SC) were analyzed by density gradient ultracentrifugation and radioimmunoassays for SC, IgA, and IgM to determine the molecular state of SC and IgA. Results in serum were compared to those obtained by simultaneous analysis of bile in five of the patients, and to those in serum and bile of a case of total IgA deficiency. Free SC was virtually not found in sera, although it was well represented in all bile, up to 88 and 97% of total bile SC in a case of complete biliary obstruction and in IgA deficiency, respectively. IgM-bound SC was found in all sera, amounting to 91% and 100% of total serum SC, respectively, in a case of acute hepatitis with a very high serum IgM concentration and in IgA deficiency. The proportions of SC bound to IgM and to polymeric IgA (p-IgA) in the sera correlated with their IgM/p-IgA molar ratio. This suggests that during liver disease, the hepatobiliary tissues could release free SC into the circulation, where it binds to p-IgA and IgM according to their respective concentrations and affinities for SC. The proportion of p-IgA in serum was not increased in four cases of biliary obstruction, in contrast to our cases of cirrhosis and acute hepatitis, indirectly supporting a minor transfer of p-IgA from blood to bile in humans, in contrast to rats and rabbits.