B. recurrentis is an invasive bacterium. It enters the skin andcauses disease.
The ability of B. recurrentis to undergo antigenic shift and escape from immune clearance of the host is the main virulence factor attributed to the bacteria. The antigenic shift or antigenic varia-tions exhibited by the bacteria are primarily responsible for peri-odic febrile and afebrile stages observed in the relapsing fever.
B. recurrentis penetrates the skin, reaches the blood circulationand invades various organs in the body and cause the disease. After invasion of blood by bacteria, Borrelia-specific IgM antibodies agglutinate Borrelia and cause complement-mediated lysis. This leads to rapid clearing of Borrelia from the blood stream. Borrelia present in the internal tissue alters their serotype-specific outer proteins through gene rearrangement and exhibits a new antigen.
The IgM antibodies produced against old antigens are not effective against the Borrelia pathogens exhibiting new antigenic variation. The host immunity responds again by producing specific antibodies against these new antigens and clears the organism from the circulation. Patients recover after a number of relapses due to development of humoral immunity.
Host immunity is characterized by the development of specific humoral immunity against Borrelia. The humoral immunity contributes to recovery in a patient after a number of relapses.