CONCEPTION OF IMMUNITY. 3 



phases in the study of immunity. Unreliable and erroneous results are the 

 inevitable outcomes of such unscientific work. 



The plan followed in this book consists in taking up all of the important 

 principles and methods of immunity, even though at present some may 

 attract no direct practical attention. The principle of the now widely 

 important Wassermann reaction had been described years previously by 

 Bordet and Gengou, but merely from a purely theoretical standpoint. 

 Only with the development of the Wassermann test, did it attain its ~prac- 

 tical importance. 



To start systematically, it is necessary, primarily, to under- 

 Conception stand certain terms frequently employed. First, the word 

 of Immunity, immunity, requires explanation: 



After an individual has recovered from an infectious disease, 

 he passes into a state where he is less or even not at all susceptible to the 

 same infection, although no macroscopical, microscopical or chemical 

 change can be shown to have taken place in his system. This condition is 

 one of immunity. And as the body itself by its own struggle with the invad- 

 ing bacteria has brought about this immunity, it is known as " active immun- 

 ity." Jenner and Pasteur have employed this mode of immunity acquired 

 spontaneously with the overcoming of an infection in then* principle of 

 prophylactic vaccination. The exact nature of this active immunity is only 

 partially understood. It can be shown, however, that the individuals 

 thus actively immunized have within their organism reaction bodies of a 

 specific nature directed against the infecting elements and their poisonous 

 products. These reaction bodies which circulate mainly in the blood serum, 

 are known as Antibodies. 



The antibodies are of different classes depending entirely upon their 

 varied forms of activity. While some, such as the agglutinins and preci- 

 pitins have the property of grouping their respective invading agents 

 into small clumps or precipitates without, however, at the same time 

 embracing protective powers, there are other antibodies which act, essen- 

 tially, for the defense of the organism whether by neutralizing the poison 

 of the bacteria (antitoxin) or by destroying the bacteria (bacteriolysin) , , 

 or so altering the bacteria that the latter can be more easily destroyed 

 by the cells (bacteriotropin, opsonin). The last three types of immunity 

 can be designated respectively, as antitoxic, bactericidal, and cellular 

 immunity. Naturally there are many intermediate forms. It is very 

 probable that besides these well recognized forms of immunity there 

 may be others, still unknown. Cellular immunity must surely have a far 

 greater range of importance than is 'at present ascribed to it. There is, no 

 doubt, a distinct cell immunity which acts without the aid of any serum sub- 

 stance and is known as "Tissue Immunity" ("histogene" Immunitat). 



If the serum of an animal which has been immunized and containing 



