680 LECTURE XXX. 



pig has now obtained an entirely new property, which sharply differen- 

 tiates it from the other animals of the same class, which have not been 

 treated in the same manner. We usually speak of this as an acquired 

 immunity. This is evident from the following. The immunized animal can 

 then be infected with an amount of cholera bacteria which would formerly 

 have produced death, and it now does not even cause illness. If we 

 inject these bacteria into the abdominal cavity, and after a time withdraw 

 some of the fluid, we shall observe a very characteristic picture under the 

 microscope. The cholera bacteria have lost their activity, and form small 

 balls, which are sometimes collected into " clumps." A control animal, inoc- 

 ulated for the first time with these cholera bacteria, will show an entirely 

 different picture. The organisms in this case are endowed with great 

 activity. We find it necessary to assume that the immunized animal 

 possesses substances in its organism which injure the cholera bacteria, 

 and restrict their activity. This belief is strengthened by the investigations 

 of R. Pfeiffer. 1 If, together with the cholera bacteria, serum from an 

 immunized animal is injected into the abdominal cavity of another animal 

 which has not been previously treated, we shall observe the same phenom- 

 ena which were characteristic of an animal which had already been infected 

 with cholera bacteria. The latter become non-motile, and form balls or 

 clumps. M. Gruber 2 has shown that this phenomenon may be verified 

 in a test-tube, by bringing cholera bacteria into contact with serum from 

 an animal infected with cholera. We immediately observe under the 

 microscope, that the cholera bacteria lose their mobility, and unite in 

 clumps. This is spoken of as an "agglutination." Its appearance is indi- 

 cated by the turbid liquid becoming clear, and a precipitate forming on 

 the bottom of the vessel. It is interesting to find that a specific reaction 

 is taking place here, for it is not possible to detect an influence upon the 

 cholera bacteria by the serum of an animal which has withstood some other 

 infection. Thus an animal which has become immune against typhoid 

 bacteria does not possess a serum which acts upon cholera bacteria, and 

 conversely the serum of an animal which has been immunized against 

 cholera does not have the slightest effect upon typhoid bacteria. 



The animal organism not only produces a specific protecting material 

 against bacteria, but also against their poisons. Thus, the medium on 

 which diphtheria bacteria have been cultivated shows toxic properties 

 after the bacteria have been filtered off. Diphtheria toxine acts even in 



1 R. Pfeiffer: Z. Hygiene, 15, 268 (1894); 20, 217 (1895); Deut. med. Wochsch. Nos. 

 7 and 8, pp. 97, 119 (1896). R. Pfeiffer and Kolle: Z. Hygiene, 21, 203 (1896). R. 

 Pfeiffer and Wassermann: Ibid. 14, 46 (1893). R. Pfeiffer and Marx: Deut. med. 

 Wochschr. 1898, 47, 489; Z. Hygiene, 27, 272 (1898). 



2 M. Gruber: Miinchener med. Wochschr. 1896, 206. M. Gruber and H. E. Durham: 

 Ibid. 1896, 285. M. Gruber: Ibid. 1899, 1329. Cf. R. Kraus: Wiener klin. Wochsch. 

 1897, 32. 



