PRODUCTION OF ANTIPNEUMOCOCCIC SERUM 541 



the individual animal. Barach 75 studied the rate of development of 

 immunity in mice and rabbits following administration of vaccine 

 prepared from intact pneumococci of Types I and II and observed 

 that protective substances developed three days after injection. 

 The immunity increased markedly to the fifth day and remained 

 essentially stationary to the seventh day. Barnes and White, 86 in 

 studying the antibody response of rabbits to vaccines of Type I 

 Pneumococcus, determined the average protective titer in a group 

 of animals after twelve, twenty-four, thirty-six, and forty-eight in- 

 jections of vaccine. The accompanying table illustrates the results 

 obtained. 



Course of injections 

 Number of injections prior to bleeding 

 Number of weeks between start of immuniza- 

 tion and bleeding 

 Average protective titers 



It is probable that the protective potency of immune serum does 

 not develop so rapidly in horses as in rabbits, and it has been be- 

 lieved that a period of from eight or nine months to a year or two 

 of immunization might be required before a serum of satisfactory 

 potency could be obtained. For over two years at the Massachu- 

 setts laboratory, as a result of studies on the immunizing efficacy 

 of various antigens and injection schedules, all horses used in the 

 production of antipneumococcic serum have been placed on routine 

 bleedings approximately two months after the initiation of the im- 

 munization process. The plan has had no apparent harmful effect 

 on the animals and, almost without exception, the serum has been 

 found to be sufficiently high in protective antibody content to be 

 suitable for concentration. 



The interval to be allowed between the last injection of vaccine 

 and the bleeding is somewhat elastic. By regulation of the Na- 

 tional Institute of Health, at least one week must elapse in order 

 that there may be no antigen circulating in the blood stream. 

 Avery and his associates 36 stated that bleeding ordinarily took 



