Summary 545 



principal means of obtaining artificial acquired immunity consists in 

 the inoculation of viruses and of vaccines. 



Immunity is a phenomenon which has existed on this globe from [569] 

 time immemorial. Immunity must be of as ancient date as is disease. 

 The most simple and the most primitive organisms have constantly 

 to struggle for their existence ; they give chase to living organisms 

 in order to obtain food, and they defend themselves against other 

 organisms in order that they may not become their prey. When the 

 aggressor in this struggle is much smaller than its adversary the 

 result is that the former introduces itself into the body of the latter 

 and destroys it by means of infection. In this case it takes up its 

 abode in its adversary in order to absorb the contents of its host 

 and to produce within it one or more generations. The natural 

 history of unicellular organisms, both vegetable and animal, often 

 presents to us these examples of primitive infection. 



But infection also has its counter. The attacked organism defends 

 itself against the little aggressor. It protects itself by interposing a 

 resistant membrane, or it uses all the means at its disposal to 

 destroy the invader. As a very large number of organisms, in 

 order to obtain nourishment, are obliged to submit their food to 

 digestion by various chemical substances, they utilise these sub- 

 stances in the struggle against the infective agents. They digest 

 them whenever they are able to do so. 



One of the most primitive of organisms, the plasmodium of the 

 Myxomycetes, which is composed of formless protoplasmic masses 

 intermediate between lower animals and plants, ingests foreign 

 bodies of various kinds. It often happens that it incorporates 

 numerous bacteria which are growing alongside it on rotten wood 

 or elsewhere. The plasmodium allows them to live for some time 

 within its digestive vacuoles. But in the end it digests them by 

 means of its soluble ferments, substances intermediate between 

 pepsin and trypsin. Owing to this digestive power the plasmodia 

 are not attacked by bacterial infections. 



This example, taken from amongst the most simple organisms, 

 may serve as a prototype for the phenomena of immunity in general. 

 At the commencement of the study of this remarkable property of so 

 many living organisms it was thought that the pathogenic micro- 

 organisms encountered, within the refractory organism, a medium 

 which did not allow them to live, either because of the absence 

 of certain nutritive substances indispensable for their existence or 



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