As a consequence, four criteria should be systematically sought when an IgM test for Borrelia is found positive without IgG: (i) verification of the positivity criteria for serology; (ii) high probability of tick exposure (depending on the geographic area and of the season); (iii) symptoms and clinical signs highly evocative of early Lyme borreliosis; (iv) IgG seroconversion on retesting more than 4 weeks later. This evidence concerns the gene CD40LG and Lyme disease.