An analysis of the preceding experiments discloses that antiviral bodies are demonstrable not at all or in small amounts in the sera of guinea pigs injected with a quantity of active virus not sufficient to induce immunity against the described intracerebral test for induced resistance. However, neutralizing bodies are found in immune animals, although in low concentration, and are regularly manifested when serum is added to low multiples of infective doses of virus under optimal conditions of time and temperature. Hyperimmune serum, on the other hand, reveals a distinct increase in the amount of antiviral bodies present.
Irrespective of the mode of procedure for revealing neutralizing bodies, there does not appear to be any notable difference in the content of such bodies in the serum of animals immunized with active virus or with formolized vaccine in which active virus could not be demonstrated. In other words, the antigenic complexes in active as well as in inactive virus produce similar degrees of antibody reaction. The formolization of virus tissue suspensions, therefore, can be considered as a process whereby the virus is inactivated but the antigenicity of the suspensions is preserved, as is also shown in the preceding paper of this series in tests on tissue immunity. In that article is described the remarkably high degree of tissue immunity which results from injections of inactive virus; now we demonstrate that this resistance is associated with a minimal degree of serum antibody.
Finally, the question may well be asked, if practically no antiviral bodies are demonstrable immediately or soon after mixing immune serum and virus, and are recognizable in a tenfold increase when functions of time and temperature are brought into play, whether the bodies are "neutralizing" or the phenomenon is due merely to aggregation of virus particles by the serum. From the recent work on the same virus and immune serum (9) by Merrill, there appears to be warrant for the belief in aggregation of virus particles which in turn diminishes the virus activity to the indicated degree.