276 RECEPTORS OF THE THIRD ORDER 



Incubate one-half hour in a water-bath or one hour in an 

 incubator. 

 Then add the indicator which is— 



Antigen + Amboceptor 



(red blood corpuscles) (for corpuscles, serum of 



a rabbit immunized against 



the red corpuscles) 



Incubate as above. 



In practice all the different ingredients must be accurately 

 tested, standardized and used in exact quantities, and 

 tests must also be run as controls with a known normal 

 blood of an animal of the same species as the one examined 

 and with a known positive blood. 



It should be stated that in one variety of the Complement-fixation Test, 

 namely, the "Wassermann Test for Syphilis" in human beings, an antigen 

 is used which is not derived from the specific organism {Treponema pallidum) 

 which causes the disease nor even from sjT)hilitic tissue. It has been deter- 

 mined that alcohol will extract from certain tissues, human or aiiimal, sub- 

 stances which act specifically in combining with the syphilitic amboceptor 

 present in the blood. Alcoholic extracts of beef heart are most commonly 

 used. Details of this test may be learned in the advanced course in Immunity 

 and Serum Therapy. 



The actual making of a complement-fixation test can be 

 learned only by repeated practise in the laboratory. It 

 should not be difficult for the student to understand the 

 theory of the test without actually doing it. It is the 

 application of principles he should have learned in his 

 freshman chemistry. It is a test to find an unhiown chemical 

 substance. The substance sought is an amboceptor. The 

 student knows that in testing for an unknown chemical 

 in solution one adds to the solution other chemicals with 

 which the unknown will react. Amboceptors react with 

 their specific antigens and with complement. Hence to 

 the patient's serum, which is being tested for the unknown 

 amboceptor, are added a solution of the specific antigen 

 and some guinea-pig's serum which contains complement. 

 These three substances will give no reaction which can he 

 detected by any known means. It is another principle known 

 to the student that in such cases it is customary to add some 

 other substance which will show a reaction with some of the 



