HISTORIC DEVELOPMENT OF IMMUNOLOGY 47 



years the system will be protected against small- 

 pox. Even if vaccination has been delayed un- 

 til smallpox has actually been contracted, inocu- 

 lation with cowpox virus early in the period of 



Carl Ludwig (1816-95). 



incubation so stimulates the tissue-cells that suf- 

 ficient antibodies are produced to modify and 

 considerably lessen the virulence of the infec- 

 tion, or to entirely prevent the development of 



