A comparative study was carried out with the light chains of IgA and IgM from a single patient (SC). Both light chains were of κ type and were homogeneous. They were not distinguished by urea disc electrophoresis, amino acid composition, N-terminal amino acid sequence (consequently Vκ subgroup), Inv allotype and peptide map. Both were of VκIII subgroup which is found in very low frequency among human κ chains. It appears that both light chains of SC IgA and SC IgM were synthesized by the same genes (the same variable portion gene and the same constant portion gene).


This work was supported in part by Grant AI-8472 from the National Institute of Allergy and Infectious Diseases.

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