Assays have been developed that detect M. leprae-specific IgM antibodies against PGL-I [5, 6], which are able to identify multibacillary (MB) leprosy patients (with strong humoral immunity to M. leprae), but these fail to detect most paucibacillary (PB) leprosy patients and leprosy patients' contacts as these typically develop strong cellular but not humoral immunity. This evidence concerns the gene CD40LG and leprosy.