MECHANISM OF ACTIVE IMMUNIZATION 659 



diphtheria and tetanus toxins (Behring), and in some preparations of 

 tuberculin. 



3. By inoculating with bacterial constituents, as the soluble toxins, 

 bacterial extracts, and products of bacterial autolysis, as in the preparation 

 of Koch's tuberculin T. R., Koch's old tuberculin, mallein, diphtheria 

 and tetanus toxins, etc. 



In Chapter XIII a method is given for preparing bacterial vaccines, 

 of which typhoid vaccine is a type. Special methods of preparing cer- 

 tain bacterial vaccines and other vaccines, such as cowpox virus and 

 rabies vaccine, are given in this chapter. 



Mechanism of Active Immunization. As was stated in the chapters 

 on Immunity, in the presence of an infection the host endeavors to pro- 

 tect itself and overcome the invaders by various means, among which 

 are phagocytosis and the production of more or less specific antibodies 

 that may neutralize the poisons of the parasite (antitoxins), directly 

 kill or destroy them (bactericidans), or so lower their vitality or re- 

 sistance that they are more easily phagocyted (opsonins or bacterio- 

 tropins). 



During an infection, one or more of these protective forces, or all 

 of them, are brought into action. After the infection has been over- 

 come, the antibodies do not always disappear at once, but remain for 

 some time in the body-fluids and gradually diminish, so that if the host 

 is reinfected with the same parasite, the antibodies are at hand im- 

 mediately to overcome it and protect the host absolutely, or at least 

 so to modify the pathogenicity of the parasite or neutralize its products 

 that the host will suffer but mildly while the parasite is being finally 

 destroyed. The concentration and duration of the various defensive 

 forces or antibodies vary in different individuals and in different in- 

 fections, so that the degree and duration of an active acquired immunity 

 are variable factors. Nevertheless and this is the basis of active im- 

 munization an animal or a person may have specific antibodies for a 

 certain parasite, produced by its own cells, without actually or necessarily 

 suffering from the disease, due to the effects of inoculation with the 

 germ or virus in a modified or attenuated form. The dose of vaccine 

 may be so controlled that general symptoms the result of stimulation 

 of the body-cells are slight or not at all apparent, and, by gradually 

 increasing the dose, more and more antibodies may be produced until 

 a high degree of immunity is secured. 



Active immunization may be practised for two purposes: 



(a) For prophylaxis, or the prevention of a disease, which is accom- 



