Serologic evidence of syphilis is based on the presence of what, for lack of a better term, is called "reagin," and while it is accepted that its presence is evidence of interaction between the invading spirochetes and the reacting tissues, exact information as to the nature of reagin is so far lacking.
The occurrence of negative serologic reactions in the presence of known syphilis may be accounted for by: (1) the absence of interaction between the spirochetes and the tissues, as in latent or dormant syphilis in which the spirochetes are safely enclosed by a wall of fibrous tissue and their action on the tissues is reduced to a minimum; (2) the fact that the interaction between the spirochetes and the tissues has not been exerted for a sufficient time to allow the accumulation of reagin in amounts detectable by the serologic procedure utilized.
Banciu1 suggested another possibility, namely,
KILDUFFE RA. THE HEMOCLASTIC COMPLEMENT-FIXATION TEST IN SYPHILIS. Arch Derm Syphilol. 1930;22(3):504–506. doi:10.1001/archderm.1930.01440150130013
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