110 BACTERIOLOGY. 
of the organism, in ways unknown, give out, in response 
to the bacterial stimulus, inhibitory or antitoxic sub- 
stances, or combine with the bacterial poisons to produce 
them. Here immunity reaches its height a week or ten 
days after the injection, and then continues for a week 
or two, when it slowly declines again. The serum im- 
munity is frequently called passive immunity and the 
bacterial immunity active immunity. 
If a greater quantity of protective substance is de- 
sired in the blood than occurs after one infection, re- 
peated injections of living or dead bacteria and their 
products are given, the doses being administered at 
short intervals and in sufficient amount to produce a 
slight elevation of temperature and malaise. Then, as — 
soon as the animal returns toa normal condition, another 
injection of slightly greater quantity is given. After 
several months of such treatment the blood is withdrawn, 
allowed to clot, and the serum then siphoned off asep- 
tically and stored either with or without the addition of 
preservatives. The serum is tested by mixing it with a 
certain number of times the fatal dose of a culture or its 
toxins whose virulence or toxicity is known, and then 
injecting this under the skin, in the vein, or into the 
peritoneum, according to the nature of the bacteria to 
be tested. The main point is that some definite method 
be carried out by which the relative value of the serum 
can be judged in comparison with other serums. As a 
rule, the value is stated in the number of fatal doses of 
culture or toxin which a fraction of a cubic centimetre 
of serum will prevent from destroying the animal. It 
is well to remember, that with a living germ a mul- 
tiple of a fatal dose is not as much more severe than a 
single dose as the figure would suggest. One thousand 
