In this case a general reaction is stimulated by the vaccine which may aid in the destruction of the invading organisms. In regulating the administration of such vaccines he has introduced the method of observing the opsonic index, to which reference is made below.
By opsonic action is meant the effect which a serum has on bacteria in making them more susceptible to phagocytosis by the white corpuscles of the blood.
In estimating the opsonic power of the serum in cases of disease a control with normal serum is made at the same time and under precisely the same conditions.
A particular bacterium had a special action in bringing about phagocytosis of that organism, and it had been found that this property was retained when the serum was heated at 55° C. It is now generally admitted that at least two distinct classes of substances are concerned in opsonic action, that thermostable immune opsonins are developed as a result of active immunization and these possess the specific properties of anti-substances in general, that is, act only on the corresponding bacterium.
The opsonic action of the serum has been employed by Sir A.
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.
A large number of favourable results obtained by such treatment controlled by the observation of the opsonic index have already been published, but it would be unwise at present to offer a decided opinion as to the ultimate value of the method.
Variations in chemiotaxis towards different organisms probably depend in natural conditions, as well as in active immunity, upon the opsonic content of the serum.
Variations in the bactericidal action of the serum in vivo, variations in the chemiotactic or opsonic properties of the serum in vivo, and variations in the digestive properties of the leucocytes of the particular animal.