CD40LG and infection: The inevitably observed IgM-only seropositive results may be explained by (i) early infection at the time of sampling, (ii) abrogated isotype-switching due to spirochetal clearance from effective antibiotic treatment [26,27], (iii) persistent IgM response [28] or (iv) false-positive reaction due to cross-reacting IgM antibodies [23,29].