Study of sera from 69 patients with untreated or inadequately treated latent syphilis revealed that immunoglobulin (Ig)G antibodies made up the bulk of the fluorescent treponemal antibody-absorption (FTA-ABS)-test reactivity found in the sera. IgM and IgA antibodies also contributed in some cases. Venereal Disease Research Laboratory slide-test reactivity was found in both 19 and 7S serum fractions, whereas Treponema pallidum immobilization-test reactivity was found mainly in the 7S fraction.

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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, 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.

Summary Natural human 19S (IgM) and 7S (IgG) antibodies reactive with Neisseria gonorrhoeae were assayed by agglutination, bactericidal and indirect fluorescent antibody (IFA) procedures. Fluorescent antibody reagents specific for IgG and IgM were used in the IFA procedure. It was found that purified 19S serum fractions contained antibodies which were readily detected by agglutination, bactericidal and IFA techniques using an anti-IgM reagent. However, bactericidal and agglutination tests appeared to be relatively insensitive to natural IgG antibodies in purified 7S fractions. In contrast, the indirect fluorescent antibody technique using an anti-IgG reagent could detect high titers of IgG antibodies within the 7S fractions. A prozone phenomenon was observed in the IFA reactivity of IgM antibodies in whole serum. It was found that the addition of purified 7S antibodies to purified 19S antibody fractions decreased the ability of the 19S fraction to react with the anti-IgM reagent. This suggested that IgG antibodies inhibited IgM antibody reactivity in the IFA procedure. A partial inhibition of 19S bactericidal activity by 7S fractions was also found. This study indicates that the unequal capacity of different antibody classes to produce reactions, and the interactions between antibodies themselves in various test systems, must be taken into account in the interpretation of the results of immunologic tests.

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