Differentiation of Human IgM Subclasses by the Ability to Interact with a Factor Resembling the First Component of Complement

Two groups of human IgM proteins are described. One group interacts with a heat-labile component of rabbit or human serum which has characteristics similar to C′1q. The second group fails to interact with this factor. This grouping is not related to κ or λ chains or to previously described differences in human IgM molecules. Such a division may provide a tool for the identification of µ chain genetic markers.