Our experiments show that the albumen fraction, or end-piece, obtained by the dialysis of normal guinea pig serum possesses definite opsonic action. This action is often almost equal to that residing in the unfractionated alexin. It is evident, however, only if the reaction maintained during the experiments approximates that of the original serum. By the addition of small quantities of a weak sodium hydrate solution to the dialyzed serum we have been able to bring back opsonic action which was not evident in the same end-piece if simply rendered isotonic.

Although our attention was called to the question of reaction by the work of Bronfenbrenner and Noguchi, like Liefmann, we have been unable to reactivate the hemolytic function of end-piece by alteration of reaction. Our experiments suggest that the opsonic action of the albumen fraction is enhanced by preliminary sensitization of the bacteria with heated normal serum and by persensitization of such bacteria with the globulin fraction. However, we cannot be positive of this, since the slight differences of phagocytic counts upon which such an opinion can be based, fall within the limits of what we consider our experimental error.

The fact that the albumen fraction can exert opsonic activity upon bacteria but cannot hemolyze blood cells seems to us particularly interesting in the light of the fact that alexin can be absorbed by unsensitized bacteria but not by similarly untreated blood cells. The literature upon the relation of the alexin fractions to bacteria and the bactericidal effect is confusing in that contradictory results have been obtained by other workers. We are studying this phase of the problem with particular attention to the alkalinity or acidity under which the reactions are carried out.

We think that our experiments do not point to a differentiation of normal opsonin from alexin, but we believe they indicate that the so called end-piece can enter to a slight extent into non-specific relationship with unsensitized bacteria, and it is therefore active, whereas it cannot enter into a similar relation to unsensitized cells. This conception, however, is tentatively made, since we are studying further the non-specific absorption of alexin or complement by unsensitized bacteria.

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