Immunoglobulin Class of Natural Human Antibodies Reactive with Treponema pallidum

To aid the development of serological tests for incubating syphilis, a characterization was made of the background of natural anti-Treponema pallidum antibodies against which the initial immune response to syphilis takes place. Sera from presumed normal persons were studied with monospecific antisera in an indirect fluorescent-antibody procedure. Of 36 sera tested at a 1:5 dilution, all showed IgG reactivity with T. pallidum (Nichols strain), 58% showed IgM reactivity, and 20% showed IgA reactivity. The titer of IgG reactivity was considerably higher than that of the other two immunoglobulins. Heating the sera for 1 hr at 65 C abolished IgA and IgM anti-T. pallidum reactivity, but one-third of the sera retained IgG reactivity. Human Cohn fractions II and III(1) from three commercial sources contained mostly IgG antibodies reactive with T. pallidum, but IgM and IgA antibodies were also present. IgG reactivity was found in a pool of presumably normal maternal sera from 20 mothers and in a corresponding pool of umbilical cord serum from their infants. IgM and IgA reactivities were present only in the maternal serum pool.