88 VACCINE AND SERUM THERAPY. 



theria immune guinea pigs or rabbits possesses the ability, when 

 injected intraperitoneally, to immunize normal guinea pigs against 

 diphtheria and a diphtheria infection already in progress. 



In 1892 these investigators further found that it is possible 

 to immunize larger animals to diphtheria, and demonstrated in 

 the blood of these animals protective and curative substances 

 which can be transferred by the serum to other animals. The 

 doses necessary to cure the disease were found to be larger than 

 those required to immunize against the same. The amount of 

 protective and curative substance produced was found to vary, 

 to some extent at least, with the degree of active immunization 

 of the animal from which the serum was obtained. It was fur- 

 ther found by Behring that serum from animals immunized to 

 diphtheria will protect against the toxin of Roux and Yersin. 



Based on these observations, Behring has formulated a law 

 which is generally known as "Behring's Law." According to 

 this dictum, the blood or blood serum from an animal possessing 

 acquired immunity is able to transmit this immunity to a sus- 

 ceptible individual or animal when this blood or blood serum is 

 injected in the right amounts into the susceptible animal or in- 

 dividual. Individuals naturally immune to a particular infec- 

 tious organism, do not possess in their blood or blood serum im- 

 munizing substances which can be transmitted to another in- 

 dividual. Immune substances which can be transmitted thus 

 are not present naturally but must be produced by a process 

 of immunization which consists either of undergoing the 

 natural course of the disease or submitting to some method 

 of artifical immunization. As a result of these investigations, 

 there has arisen the practice of passive immunization, or, as 

 it is more frequently called, serum therapy. The development 

 of methods and success obtained by passive immunization 

 against the action of tetanus and diphtheria bacilli, led to 

 the belief that it would be a relatively easy matter to make 

 specific sera against all disease-producing bacteria. It was 

 soon found, however, that this is not possible. Acquired 

 anti -toxic immunity has only a relatively limited application. 

 Most pathogenic bacteria hold their special poison so firmly 

 within the cell that the poison is only freed when the cell 

 disintegrates, or when certain stimulating conditions as are found 



