A human IgG1-K type M-protein with antibody activity against horse and rabbit alpha 2-macroglobulin

A human IgG1-K type M-protein which specifically precipitates with alpha 2-macroglobulin (alpha 2-M) from horse and rabbit is described. This protein did not combine with human alpha 2-M nor with alpha 2-M from various other species. The binding activity was exclusively located in the Fab region of the molecule. This patient had never received injection of horse anti-tetanus serum. The present case seems to be the second report of an M-protein with antibody activity against horse and rabbit alpha 2-M.