Attempt to protect rabbits against experimental syphilis by passive immunization.

The role of humoural immunity, effected by circu- lating antibodies, in defence against a syphilitic infection is still uncertain. Turner (1939) showed that antibodies present in the serum of syphilitic patients neutralize virulent treponemes in vitro. This observa- tion was the forerunner of the discovery by Nelson and Mayer (1949) of the antibodies that immobilize treponemes in vitro. Turner and Nelson (1950) reported that there was a direct relation between the titre of immobilizing antibodies and the resistance to re-infection. However, several authors