INFECTION AND RESISTANCE 



complex, and this, even when the reacting quantities, antigen and 

 antibody, are so small that visible precipitation will not take place. 

 For this reason, it is plain, it should be possible by means of com- 

 plement fixation to detect amounts of a foreign protein too small to 

 be demonstrable by direct precipitation with an antiserum. 



The method has, therefore, been suggested chiefly by Neisser and 

 Sachs 36 for the forensic determination of unknown proteins, as an 

 adjuvant to, and improvement upon, the forensic precipitin test. Our 

 discussion of the principles involved in the introductory paragraphs 

 of this chapter will render unnecessary an extensive discussion of 

 the reasoning upon which this reaction is based. It is well to remind 

 the reader, however, of the facts which we have discussed regarding 

 the quantitative proportions which govern the occurrence of precipi- 

 tation when an antigen, say human serum, is mixed with its antibody, 

 in this case antihuman rabbit serum. The actual precipitation may 

 be absent either when an excess of the antigen is used or when the 

 antigen is present in too small a quantity. Thus a given quantity 

 of the antiserum may precipitate strongly dilutions of the antigen 

 ranging from 1-50 to 1-10,000. No precipitation or, at least, a very 

 slight one only may occur when concentrations stronger than 1-50 

 are used and when the dilution is greater than 1-10,000. Neverthe- 

 less, in both cases, alexin fixation may be exerted by the complex 

 although no precipitation takes place. As Gay 37 has shown, com- 

 plement fixation may be exerted even when a formed precipitate has 

 been redispersed by the subsequent addition of more antigen. The 

 importance of the forensic reaction of Neisser and Sachs, however, 

 lies chiefly in its application to the detection of quantities of un- 

 known protein too small to be detected by precipitin reactions. 



The tests are carried out by mixing a dilution of unknown pro- 

 tein with given quantities of antiserum, adding small quantities of 

 alexin (quantities determined best by previous alexin titration as 

 indicated in our section on the Wassermann reaction) ; these reagents 

 are left together for a given time at 37.5 C., and then sensitized cells 

 are added to determine whether or not the alexin has been bound. 



The table on the following page, taken directly from the article 

 of Neisser and Sachs, loc. cit, will not only illustrate the method of 

 carrying out the reactions but will also give an indication of their 

 extreme delicacy. 



It will be seen that 0.00001 c. c. of the normal human serum still 

 gave almost complete complement fixation of 0.05 c. c. of comple- 

 ment in the presence of 0.1 c. c. of the antihuman serum. The table 

 also shows that this reaction follows a general law of relative specific- 

 ity so often noted in other reactions, namely that, of all the ani- 

 mals tested, the serum of monkeys alone gave reactions with the 



36 Neisser and Sachs. Berl. kl Woch., Vol. 42, No. 44, 1905, p. 1388. 



37 Gay. Univ. of Cal., "Publications in Pathology," 1912. 



