The object in such treatment is to raise the opsonic index of the serum, this being taken as an indication of increased immunity.
The effect of the injection of a small quantity of vaccine is usually to produce an increase in the opsonic index within a few days.
If then an additional quantity of vaccine be injected there occurs a fall in the opsonic index (negative phase) which, however, is followed later by a rise to a higher level than before.
If the amounts of vaccine used and the times of the injection are suitably chosen, there may thus be produced by a series of steps a rise of the opsonic index to a high level.
One of the chief objects in registering the opsonic power in such cases is to avoid the introduction of additional vaccine when the opsonic index is low, that is, during the negative phase, as if this were done a further diminution of the opsonic action might result.
