Secretory component-binding properties of normal serum IgM.

Our aim was to investigate why serum IgM is poorly transferred into secretions in normal subjects. Indeed, the low IgM level in secretions contrasts with the capacity of monoclonal IgM to bind to secretory component (SC), but it is not well established to what extent normal serum IgM can do so. The mean SC affinity was studied with a polyclonal IgM preparation from 250 normal subjects and with a representative pool of 100 different monoclonal IgM. The SC-binding percentages varied as a function of the IgM/SC molar ratio according to a common hyperbolic curve, with similar association constants: Ka = 4.19 +/- 2.61 x 10(7) M-1 (polyclonal pool) and Ka = 5.80 +/- 2.73 x 10(7) (monoclonal pool). It thus appears that the large difference in IgM concentrations between blood and secretions cannot be due to an SC-binding defect of serum IgM, but is probably explained by its low diffusion from blood to the extravascular compartment.