IMMUNIZATION 151 



looked for the first appearance of the correspond- 

 ing antibody, a haemolysin against ox erythrocytes. 

 He demonstrated the presence of this antibody just 

 after the disappearance of the erythrocytes or 

 perhaps a little before. The time of incubation had 

 therefore a mean value of 72 hours, which agrees 

 completely with an observation of Bulloch. If 

 the erythrocytes were injected subcutaneously the 

 time of incubation was much longer, as we might 

 expect, namely 7 days* An analogous case is found 

 in infectious diseases, which may be regarded as 

 a special case of active immunization. In small- 

 pox the infection generally comes through the 

 respiratory organs, and the time of incubation lasts 

 no less than from 10 to 14 days, whereas after 

 inoculation of genuine small-pox (variolation) or of 

 weakened virus from cow-pox (vaccination) the time 

 of incubation is only 3 to 5 days. Still shorter some- 

 times is the time of incubation after repeated vaccina- 

 tion. This circumstance makes it possible for a 

 man, freshly infected with small-pox, to be (partially) 

 protected by vaccination. The antibodies appear 

 after the time of incubation, and this is after vac- 

 cination so short that it may be at an end before 

 the incubation time of the genuine small-pox is 

 completed. In this case this latter time of incuba- 

 tion is shortened, and the patient gets an easy form 

 of small-pox, the so-called varioloid, as is generally 

 the case with vaccinated people who are attacked 

 by the genuine small-pox. Still longer is the time 

 of incubation in hydrophobia ; in this case it depends 



