Patients with Borrelia-caused relapsing fever produce cross-reacting antibodies to Borrelia burgdorferi, the anti-genetically related causative agent of
Lyme borreliosis. The antibody response of the serum of a patient (acute and convalescent) with relapsing fever was analysed by the immunoblot technique using Borrelia hermsii and B. burgdorferi as antigens. The diagnosis was established by microscopic detection of spirochetes in the patient’s blood. The patient’s serum showed significantly elevated titers of IgG and IgM in a B. burgdorferi indirect immunofluorescence assay. Immunoblot analysis indicated the presence of cross-reacting antibodies directed to B. burgdorferi antigens with apparent molecular weights of 60, 41, 40, 36, 30 and 20 kDa.