224 IMMUNOLOGY 



Nuttall's Conclusions. — He concludes that the rabbit is per- 

 haps the most satisfactory animal for use in obtaining potent 

 immune serum. He used the intraperitoneal method for antigen 

 inoculation and, beginning with a small dose, administered a 

 series of 5 or 6 graded doses, gradually increasing the amounts 

 at three to six day intervals and bled for antibody titration seven 

 to twelve days after the la.st injection of antigen. He noted a 

 fluctuation in precipitin content during immunization and observed 

 that precipitin persisted in the blood stream in apparently 

 undiminished titer for about one month. (The titer frequently 

 drops much eariier.) He confirmed the observation of Tchisto- 

 vltcli (1899) that precipitins disappear from the blood stream if 

 injections are administered over too long a period. In his opinion 

 antigen and antibody may both be present in the blood without 

 precipitation occurring. (ThLs has since been confirmed by Weil 

 and others and a fairly logical explanation has been offered by 

 Dean.) In regard to measuring the strength of an immune serum, 

 he suggests that the titer may be expressed either in terms of the 

 volume of precipitate formed or "by giving the highest dilu- 

 tion of blood (antigen) with which it reacts, the quantities of 

 interacting substances being stated." In his opinion quantitative 

 determinations are best recorded in terms of the volume of pre- 

 cipitate formed under standard conditions and procedure. He 

 thinks there is evidence indicating the exi.stence of iso-precipi- 

 tins, auto-precipitins and anti-precipitins being artificially formed 

 in the bodies of treated animals. In regard to normal precipitins 

 he says certain sera may contain them but they are not specific. 



He considers that precipitins are antibodies intimately asso- 

 ciated with serum globulin. They are more resistant to heat than 

 complement and do not require the latter for their activity. Bac- 

 terio-precipitins are inactivated at 58-60° C, while precipitins 

 for animal proteins are inactivated at 68-70° C. As the source of 

 the precipitate he reasons that since the amount formed is fre- 

 quently out of all proportion to the amount of antigen present, 

 the precipitate very likely comes from the immune serum. Subse- 

 quent work has definitely established the truth of Nuttall's as- 

 sumption. In his conclusions attention is also called to the 

 observation that the presence of heated precipitin serum (pre- 

 cipitoid) interferes with the reaction between fresh, unheated 



