IMMUNITY 67 



the diphtheria poison and rendered it harmless. It 

 was natural to see whether this blood-serum could be 

 used to treat other animals which had previously been 

 injected with diphtheria poison, and on doing this von 

 Behring found that the serum thus used was able to save 

 the animals from death. The action of the substance in 

 the serum which counteracted the effect of the poison 

 was found to be exactly like that of an alkali on an acid, 

 i. e., it neutralized the poison. It was, therefore, called 

 an antitoxin, meaning against toxin. The antitoxic 

 serum from an animal treated with toxin does not differ 

 in appearance from that of a normal untreated animal. 

 And even when tested chemically, but little difference 

 can be discovered between the two. In order, therefore, 

 to recognize the presence of this antitoxin in the serum, 

 and especially in order to measure its amount, we must 

 test it in animals, and see how small a quantity of anti- 

 toxic serum will save an animal after injection with a 

 certain amount of diphtheria toxin. When guinea-pigs 

 are used for the test, it may be found that T ^ nf cm. 

 of the antitoxic serum will often be sufficient to 

 save the animal from death, even after it has been 

 injected with ten fatal doses of the diphtheria toxin! 

 Sometimes, in fact, as little as -g-^Vu- cm. suffices. The 

 strength of the antitoxic serum is, therefore, expressed 

 in units. In the examples just cited the serum would 

 be said to have a strength of 100 or of 500 units 

 respectively. 



Bacteriolysins. Just as when an animal injected 

 with gradually increasing doses of toxin produces an 

 antitoxin in its blood, so also, when injected with bac- 

 teria, it produces substances which kill and dissolve 



