Studies on the Treponema pallidum immobilizing activity in normal human serum. 4. The importance for the outcome of the conventional TPI-Test.

T. pallidum immobilizing activity was demonstrated in the majority (84%) of samples of unheated sera obtained from healthy adult subjects. A similar activity was found in unheated sera obtained from some animal species such as monkey and white rat, but not in sera from rabbit or guinea pig. The results of serological studies of acquired, early syphilis in man suggested that the determination of the activity of the naturally occurring IgM immobilisins was not influenced by coexisting TPI antibodies of the IgG class. The immobilizing activity of human normal sera occurred in low titres only, rarely in dilutions of greater than 1:32. Moreover, sera from guinea pig and rabbit were found to inhibit this activity in normal sera from man. These findings were supposed to explain why the immobilizing activity of the naturally occurring IgM antibodies did not interfere with the outcome of the conventional TPI test.