The neutralization of poliovirus infectivity by enzymatically derived rabbit antibody fragments has been examined. Although univalent fragments prevented the adsorption of virus on cells and inhibited the infectious capacity of cell-adsorbed virus as efficiently as native γ-globulin, the pepsin-digested (5 S) bivalent fragments and pepsin-digested reduced (3.5 S) univalent fragments were progressively less stable in the maintenance of virus neutralization.

Evidence has been presented that antibody fragments are able to abolish the infectivity of large concentrations of poliovirus prior to dilution. On the other hand, the development of neutralized virus-antibody complexes stable to dilution at neutral pH was considerably slower with univalent antibody fragments than with intact γ-globulin. Moreover, mature neutralized complexes of antibody fragments and virus were more readily reactivated at moderately acidic pH than those with intact γ-globulin. On this basis, it is proposed that the difference in the virus neutralizing activities between native antibody and antibody fragments is due to the reduced capacity of antibody fragments to develop stable unions with virus. These findings are presented as evidence that virus neutralization is not only a function of the antigen-binding sites of the antibody but is dependent on the entire structure of the γ-globulin molecule.

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