VACCINATION. IMMUNIZATION. INOCULATION 229 



indifference of the cells toward the infectious material, in conse- 

 quence of which the cells avoid the pathogenic bacteria (negative 

 chemotropism or chemotaxis). "Living cells, governed by their 

 susceptibility, approach pathogenic microbes or flee from them; 

 take them within the cell body or allow them to lie free." The 

 Metschnikoff theory received considerable support from the dis- 

 covery by Pfeffer of the law of the chemotropism of vegetable 

 cells (attraction or repulsion by certain chemical substances). 

 The behavior of the white blood-cells in inflammation, which is 

 analogous to the chemotaxis of plant cells, was first demonstra-ted 

 by Leber. 



Other Theories.— 1. The Exhaustion Theory (Pasteur). The first at- 

 tempt to explain the cause of immunity was made by Pasteur. In his studies 

 of fermentation he observed that the ferment organisms became inefiFective 

 after a certain time, when they had used up a certain quantity of the nutritive 

 material from the nutrient solution, and he concluded that the pathogenic 

 organisms likewise withdraw from the invaded body certain substances indis- 

 pensable to their development, so that the body is in a certain sense exhausted 

 and microorganisms invading the body later fail to find suitable nutritive 

 material. Similarly, the so-called soil exhaustion is observed after long- 

 continued planting of the same fodder crop. 



2. The Aggressin Theory (Bail). Aggressins is the term used by Bail 

 to describe the bacterial products which are formed, especially at the point 

 of infection, to assist the bacteria in the combat between them and the leuco- 

 cytes. The aggressins possess negative chemotactic properties, as a result 

 of which the leucocytes are repelled. They are found especially in the oedema 

 at the point of infection (in anthrax) and in peritonitic and pleuritic exu- 

 dates. The injection of such "aggressive exudates" produces active im- 

 munity. The blood-serum of an actively immunized animal contains 

 passively immunizing "anti-aggressins." 



3. The Opsonic Theory (Wright). According to Wright, the specific 

 action of serum is due to opsonins, in addition to antitoxic and bactericidal 

 substances. Opsonins are protective substances which promote phagocytosis 

 by making the bacteria more readily digestible by the leucocytes (bacterio- 

 tropic substances). 



4. The Assimilation Theory (Baumgarten). The blood of immune animals 

 contains no antitoxins (no one has demonstrated them!), but is so changed 

 chemically that the pathogenic organisms can no longer develop in it. (See 

 the colyseptic action of the disinfectants.) 



