PREVENTIVE INOCULATION 59 
Theory of Immunity.—Wright’s “opsonic” theory of 
immunity, briefly outlined, is to the effect that there is 
a substance (opsonin) present in the serum of an immune 
animal which, when introduced into another animal 
affected or likely to be affected with the disease against 
which the first animal is protected, renders the corre- 
sponding germ sensitive to phagocytosis—z.e., peculiarly 
liable to attack and destruction by the white blood-cells. 
Thus the opsonic qualities of the serum constitute the 
means by which the body frees itself from the invading 
organisms. The occurrence of a low opsonic index is 
due to the using up of available opsonin, and the object 
of a vaccination is to supply this deficiency by stimu- 
lating the mechanism for its production, and this is the 
immediate effect of the first injection; but a reaction 
then occurs, normally lasting a day or two, in which the 
amount of opsonin is actually lower than it was in the 
beginning. This is called the megative phase, during 
which the system becomes more susceptible to disease. 
When the vaccination is successful, this phase is followed 
by a rise in the opsonic value to a higher level than that 
existent at the time of infection. This is known as the 
positive phase, and the body has become more resistant to 
microbial invasion. Sometimes the positive phase is 
quickly and easily obtained, but at others there is a pro- 
longation of the negative phase, and should a second 
dose of vaccine be then administered, there is a danger 
of inducing symptoms of the disease to manifest 
themselves. 
In the earlier days it was the practice to inject attenu- 
ated living cultures of organisms into the animal to be 
protected, but now it has been found safer and more 
convenient to use dead cultures, at least at the com- 
mencement; in both the principle is the same, and both 
cultures are spoken of as vaccines. 
