HO 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 to a 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 several months of such treatment the blood is withdrawn, 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 



