Facts bearing on acquired immunity 241 



In spite, however, of the absence of any increase in the bacteri- 

 cidal property of the blood serum and of the subcutaneous exudation 

 in vaccinated rats, the cell reaction obtained in them is very 

 different from that met with in normal, susceptible rats. In a very 

 short time (3 to 5 hours) after the subcutaneous injection of anthrax 

 bacilli into the control rats (susceptible), an evident oedema is 

 produced; in the vaccinated rat there is none. The exudation, 

 not very abundant in the latter, already contains a number of leuco- 

 cytes which are actively phagocytic, whilst in the control animal, 

 examined simultaneously, "leucocytes are rarely met with, and few 

 of them contain bacilli." Later, the difference becomes still more 

 marked. Pronounced oedema occurs in the control animal, it is poor 

 in leucocytes but rich in bacilli, which continue to multiply; but 

 " in the immunised rat, we find not a clear exudation but a thick and 

 purulent fluid, full of leucocytes." These cells devour all the bacilli ; 

 not a single one remains free. "Even after 14 hours bacilli ingested 

 by the leucocytes are present and a culture of anthrax bacilli may be 

 obtained from fluid taken from the seat of inoculation. Further, 

 guinea-pigs or rats, when inoculated with a drop of this exudation 

 (which contains no anthrax spores), succumb to anthrax." 



Even before these researches on the immunity of rats had been 

 carried out, an attempt had been made to gain some idea of the 

 differences presented by the vaccinated fluids of animals as compared 

 with those presented by the fluids of control animals susceptible to 

 anthrax. In 1886 I was able to demonstrate 1 that the anthrax [25<t] 

 bacillus develops abundantly in the defibrinated blood of sheep that 

 had acquired immunity as the result of vaccination by Pasteur's 

 method. When these bacilli contain spores and are inoculated into 

 rabbits they rapidly produce a fatal anthrax; but when no spores 

 are present the injection of bacilli does not produce a fatal disease, 

 and such infection is well supported by the rabbits. From this I 

 concluded at that time that the anthrax bacillus must, in the blood 

 of the vaccinated sheep, undergo a real attenuation in virulence, an 

 interpretation which, as will be seen in the next chapter, was found 

 to be erroneous. 



Nuttall 2 showed that the defibrinated blood of refractory sheep 

 acted as a nutrient medium for the anthrax bacillus. Making com- 

 parative investigations, by the plate method, on the bactericidal 



1 Ann. de FInst. Pasteur, Paris, 1887, 1. 1, p. 42. 



2 Ztschr.f. Hyg., Leipzig, 1888, Bd. iv, S. 353. 



B. 16 



