.\.\TA(;u\isM IU-:T\VI-:I-:\ TIII-: uvTm BODY AND BACTERIA 159 



months) almost entirely lost, so that repeated injections are required 

 to maintain the immunity. This passive immunity is distinctly in 

 contrast to the active immunity acquired after the introduction of 

 bacterial products, where the tissues of the organism, in ways to us 

 unknown, throw out, in response to the bacterial stimulus, inhibitory 

 or antitoxic substances, or combine with the bacterial poisons to 

 produce them. Here immunity is actually lessened for one or two 

 days, and then is increased, and 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 immunity produced by the transfer 

 of serum from the immunized to the non-immunized is frequently 

 called passive immunity and the immunity produced by infection 

 active immunity. 



Production of Antitoxin for Therapeutic Purposes. If a greater 

 quantity of protective substance than is required for the protection 

 of the individual is desired in the blood, repeated injections of living 

 or dead bacteria and their products are given, the doses being ad- 

 ministered at short intervals, and in sufficient amount to produce a 

 slight elevation of temperature and malaise. After several months of 

 such treatment the blood is withdrawn, allowed to clot, and the serum 

 then siphoned off aseptically and stored either with or without the addi- 

 tion of preservatives. 



Testing of Protective Power of Antibacterial and Antitoxic Sera. 

 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 multiple of a fatal dose is not as much more 

 severe than a single dose as the figure would suggest. One thousand 

 times a fatal dose of a very virulent micro-organism will be neutralized 

 by several times the amount of serum which a single fatal dose requires, 

 since in the case of very virulent living bacteria, whose virulence is due 

 to their ability to increase, it is not the organisms which are introduced 

 that kill, but the millions that develop from them. 



Limitation of Curative Power of Serums. As a rule, the serum has to 

 be given before the bacteria introduced into the body have multiplied 

 greatly. After that period has elapsed the serum usually fails to act. 

 This is due to the fact that bactericidal power in serum is due to 

 the combined effect of two substances, one only being contained in the 

 injected serum. The serum loses all appreciable protective value as 

 measured in test animals in the usual doses before the person is 

 liable to infection. Repeated injections of serum continue this con- 

 dition of immunity indefinitely. 



