Studies on the Treponema pallidum immobilizing activity in normal human serum. 5. On the protective role against syphilis.

It was shown that T. pallidum immobilized in vitro by normal serum had lost its infectivity in rabbits. This finding suggested that the immobilizing activity of normal serum might play a role in the natural resistance of man against infections with T. pallidum. However, the results of serological studies of acquired early syphilis in man and of experimental syphilis in cynomolgus monkeys did not present evidence that the immobilizing activity of normal serum protected against syphilis infection.