IMMUNOLOGICAL BEHAVIOR 97 



ot nciitiali/ation w itii this dilution may l)e explained by assinning 

 that the mixture contained a small amomit ot" free reacting factors 

 or that there ^vas partial neutralization of these factors so that 

 only the rabbits possessing a high degree of reactivity were able 

 to respond with reactions. 



As may be also seen from the experiments of Table ni, it was 

 foiuid that normal and heterologous sera (therapeutic menin- 

 gococcus and erysipelas horse sera) free of normal agglutinins or 

 possessing normal aggkuinins of a low titer (1:16) for B. ty- 

 IjIiosus were not able to neutralize the reacting factors. There 

 was obtained a questionable neutralization Avith the seriun pos- 

 sessing normal B. typJiosus aggkuinins in dilution 1:64. 



After establishing the fact that B. typhosus reacting factors 

 can be neutralized specifically by immtme serum, it was important 

 to determine further whether the neutralizing potency of the sera 

 cotdd be quantitati\ely measured. In the experiments summarized 

 in Table iv, a constant amount of serimi was titrated against in- 

 creasing quantities of reacting factors. 



As is seen from Table iv, a given amoiuit of serinu ^vas able to 

 neutralize increasing amounts of the filtrate. The neutralization 

 was considered '"consistent" if obtained in all rabbits tested (a 

 minimtnn of 5 animals^) . As the amounts of reacting factors were 

 increased, neutralization was obtained irregularly, i.e., in some 

 of the rabbits only. The maximimi amount of reacting factors 

 "consistently" neutralized by 0.25 c.c. of a given serimi was tenta- 

 tixely used to compute the neutralizing titer of a given serum. 

 Thus, if a 0.25 c.c. of a given serimi is able to neutralize con- 

 sistently 0.5 c.c. of "A7" (R.201, Table iv) and irregularly neu- 

 tralize 0.75 c.c, its titer must be between these two figines. Since 

 0.01 c.c. per kilo of reacting factors is the minimum dose, the serinn 

 has been able to neutralize befvveen 50 and 75 mimimal reacting 

 doses; the titer, is therefore, approximately 60. 



In these titrations 0.25 c.c. of serum was used instead of 1 c.c. 

 in order to keep do^vn the vokune to be injected. The titrations 

 in terms of 1.0 c.c. must be worked out experimentally (p. 117) . 



It ^vill also be seen in Table iv (bleedings 3 and 62, Horse 4) 

 that the titer of the neutralizing antibodies in the serum increased 

 in the course of immunization. 



In addition to the clear-cut neutralization obtained, there was 



^ In later work, after several years of experience it was found iliai wiili ihc use 

 of only three rabbits accurate determinations could be made. 



