more IgM blots are positive in chronic borreliosis than IgG. Every time Borrelia burgdorferi reproduces itself, it may stimulate the immune system to form new IgM antibodies.

http://www.medhelp.org/posts/Lyme-Disease/If-I-really-do-have-Lymes-Disease/show/973861?personal_page_id=5046#post_4559109