The occurrence of secretory IgM in the bile of rats

The concentration of IgM in the bile of normal rats is extremely small, too low to be detected by conventional immunodiffusion methods. However, IgM is easily detected in bile samples from rats bearing the IgM secreting IR202 plasmacytoma, at concentrations ranging from 0.07 to 1.2 mg/ml. The IgA concentration in the same bile samples is about half the normal level, whereas in corresponding samples of serum, IgA concentrations are elevated by a factor of three. Most of the IgM in the bile is complexed with secretory component. Thus, when IgM concentrations in the blood are sufficiently high relative to those of IgA, IgM in rats is capable of undergoing transhepatic transport by the same route as polymeric IgA. This situation probably corresponds to that in IgA deficient humans where IgM replaces polymeric IgA in the secretions.