1 8 AIDS TO BACTERIOLOGY 



attack the organisms are presumed to leave behind them 

 some product of metabolism that is inimical to their 

 existence. A later form of this theory was that antitoxin 

 was a modified toxin, which supposition has been dis- 

 proved. 



The Phagocytosis Theory (Metchnikoff). The large 

 mononuclear leucocytes and the polymorphonuclear 

 leucocytes ingest and destroy such bacteria as obtain 

 access to the blood-stream. If infection occurs in one 

 locality, there is a simultaneous movement of leucocytes 

 to that spot to cope with the bacteria (positive chemiotaxis). 

 When no such attraction takes place, negative chemiotaxis is 

 said to occur. Metchnikoff ascribes the process of im- 

 munisation to the ' education ' of the phagocytes. It is 

 supposed that phagocytes contain digestive ferments 

 (cytases) which effect bacteriolysis (solution of bacteria) 

 intracellularly. Or, on the breaking up of the phagocyte 

 (phagolysis), bacteriolysis may be effected extracellularly, 

 and the cytases are then the alexines or complements of 

 Ehrlich's theory. While phagocytosis constitutes a most 

 important factor in the production of immunity, it requires 

 further development to explain passive immunity. 



Phagocytosis depends on the presence in the serum of 

 ' opsonins,' which act on the bacteria, and in some way 

 render them suitable for ingestion by phagocytes. 



Opinions are divided on the question whether normal 

 blood contains specific opsonins for different diseases, or 

 whether the opsonins are ' common ' and act irrespective 

 of the bacteria proffered. However, in the process of 

 immunisation, opsonins of specific character are developed. 



EhrlicJi's Side-Chain Theory. Ehrlich assumes that 

 protoplasm is composed of complex molecules possessing 

 very unstable ' side-chains,' ready to combine with other 

 atomic groups if suitable ones come within the sphere of 

 action. In the production of antitoxin the following is 

 assumed to take place: A group on the toxic body (or 

 toxoid), called the haptophore group, combines with a 

 group on the cell called the receptor group. If the ceil is 

 not too much damaged, it produces an excess of receptor 

 groups more than sufficient to combine with the toxin, 

 thus following Weigert's law that continued stimulation 

 is attended by overproduction hypertrophy. After re- 

 peated injections of toxin, so many receptor groups are 



