CHAPTER VII 
PREVENTIVE INOCULATION 
WuEeN resistance to an infection is a natural inborn 
quality, we speak of it as natural immunity; but it is 
possible to evoke an acquired immunity by means of 
inoculation with an organism or its derivatives, just as 
an ordinary attack of infectious disease confers a certain 
degree of acquired immunity. Active immunity may be 
set up by one or more of the organisms concerned in 
attenuated condition, or by sub-lethal doses of virulent 
organisms or their toxins, or by inoculations of dead 
microbes and their products, or even of their products 
alone. As gradually increasing amounts are injected, 
the animal will develop a high degree of resistance. 
This procedure constitutes preventive inoculation or — 
vaccination. 
Immunity is, however, not immediately created, the 
time elapsing being governed by the susceptibility of 
the animal and the dose and virulence of the vaccine. 
Though it is comparatively a slow process, the active 
immunity evoked lasts a considerable time. 
In passive tmmunity protection depends upon the intro- 
duction of substances developed in some other animal, 
and it has a rapidly produced but transient effect. 
When an animal has been hyper-immunised with 
increasing doses of vaccine as just described, its blood 
serum will contain substances which have an antago- 
nistic or neutralising action on the specific bacteria or 
their toxins, and such a serum will be of value for the 
prevention or cure of certain infections in another 
animal. A notable example is anti-tetanus serum. 
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