158 PRINCIPLES OF BACTERIOLOGY 



anthrax bacillus and the use of vaccination in man are examples of 

 this method. 



3. By the injection of the organisms into tissues where development 

 will not take place, as the injection of typhoid bacilli or cholera spirilla 

 into the subcutaneous tissues. Here the solution of the bacteria with 

 the absorption of their products causes a mild chemical poisoning, with 

 considerable resulting immunity. 



4. By the injection of the chemical constituents of the dead bodies 

 of bacteria and of the chemical products which they elaborate and 

 discharge into the surrounding culture media during their life. Men 

 as well as animals have been immunized by the injections of dead 

 cultures of typhoid and anthrax bacilli and cholera spirilla, etc. A 

 few days after infection with most parasitic bacteria the body resistance 

 to the growth of the same organism is greatly increased; in other 

 infections, however, it is but slightly augmented. This increased 

 resistance is readily shown to be partly due to protective substances 

 held in solution in the blood-serum and partly to the leukocytes. 



5. By the injection of the blood serum of animals which have pre- 

 viously passed through a specific disease or have been inoculated with 

 the bacterial products. The first, probably, to think of the possibility 

 of effecting this was Raynaud, who in 1877 showed that the injection 

 of large quantities of serum derived from a vaccinated calf into an 

 animal prevented its successful vaccination. Hericourt, Richet, and 

 others demonstrated the same thing for other diseases. The results 

 obtained by Behring and Kitasato upon diphtheria and tetanus, where, 

 indeed, the serum prevented the action of the poisons rather than the 

 direct development of the bacteria, gave a still greater impetus to these 

 investigations. 



Suitable animals after repeated infections gradually accumulate in 

 their blood considerable amounts of these protective substances, so 

 that very small amounts of serum inserted in another animal will inhibit 

 the growth of the bacteria or neutralize their products. Thus, 0.1 c.c. 

 of a serum from a horse frequently infected by the pneumococcus will 

 prevent the development in the body of a rabbit of one hundred times 

 the fatal dose of very virulent pneumococci, and a few times a fatal 

 dose of less virulent ones, the actual number as well as the virulence 

 of the bacteria affecting the protective value of the serum. 



These protective substances are found also in other fluids of the body 

 than in the blood; they occur, indeed, in the substance of many cells 

 to a greater or less extent. 



The immunity produced by these five methods affects the entire body, 

 as is natural, since the blood into which they are absorbed is dis- 

 tributed everywhere. When the immunity is but slight, infection may 

 take place in the more sensitive regions, and still be impossible in those 

 tissues having more natural resistance. 



Passive as Contrasted with Active Immunity. If the serum is injected 

 into animals or man the immunity is greatest immediately after ab- 

 sorption, and then declines, being rather quickly (in several weeks or 



