CHAPTER IV 



THE MECHANISM OF NATUKAL IMMUNITY AND THE 



PHENOMENA FOLLOWING UPON ACTIVE 



IMMUNIZATION 



ANTIBODIES AND ANTIGENS. THE ORIGIN OF ANTIBODIES 



THE MECHANISM OF NATURAL IMMUNITY 



PASTEUR'S work on active immunization was carried out in the 

 later seventies and the early eighties. During and immediately after 

 this time it was very natural that the attention of investigators 

 should have concentrated upon the elucidation of the causes under- 

 lying both the natural resistance against bacteria observed 'in animals 

 and man, and the changes which during active immunization were 

 fundamentally responsible for the acquisition of resistance. 



It was easily determined that there were no anatomically and 

 physiologically determinable differences between the various mam- 

 malia which could account for the observed striking variations of 

 susceptibility, nor could gross anatomical or histological changes 

 be noted in an animal which had been artificially immunized. Mor- 

 phologically such an animal was indistinguishable both in the size 

 and appearance of its organs, and in the arrangement and structure 

 of its cells from any other individual of the same species not sub- 

 jected to treatment. 



It was a natural development of the investigations brought to 

 bear upon this problem that attention should, for a time, be concen- 

 trated upon the phenomena of inflammation, processes which were 

 regularly associated with infections of all kinds and seemed indeed 

 to represent a sort of local expression of tissue resistance to the 

 invading micro-organisms. 



It was in the course of investigations upon the nature of inflam- 

 mation that Metchnikoff first became interested in problems of re- 

 sistance. In 1883 he presented a paper at the Naturalists' Congress 

 at Odessa, in which he referred the absorption of dead or foreign 

 corpuscular elements in the bodies of invertebrates to a process of 

 intracellular digestion carried out by phagocytic cells. As early as 

 1874 Panum had suggested that possibly resistance against invading 

 micro-organisms might be due to a similar intracellular destruction, 



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