472 SEROTHERAPY 



bouillon culture of an organism and is obtained by using the nitrate of 

 these cultures. 



The precipitin is the substance formed in the animal serum by inocu- 

 lating precipitogen and is analogous to agglutinin. 



The precipitate is formed in a test tube as a consequence of the mixture 

 of precipitin and precipitogen in the proper proportions. 



Other substances than bacterial nitrates will act as precipitogen. 

 Immunization with cow's milk causes a precipitin to form which causes 

 casein of cow's milk to precipitate, but will not precipitate the casein of 

 goat's milk or any other species. Also, the egg-white of a chicken causes 

 a specific precipitin to form which will not precipitate any other kind of 

 egg-white. 



This precipitation reaction is important from a medicolegal stand- 

 point. It is used to identify blood stains found in certain murder cases; 

 also used to detect horse or other meat sold as beef. 



C. COMPLEMENT-FIXATION REACTION 



This test is used in diagnosis of certain diseases but principally syphilis. 

 In 1901, Bordet and Gengou observed that when an antigen was mixed 

 with its specific antibody in the presence of complement, the complement 

 became "fixed" and was rendered unavailable for further reactions. As 

 an indicator to show that the complement is bound, a hemolytic system 

 must be used and this system is prepared as follows: 



An animal, usually a rabbit, is immunized by injection of human red 

 blood cells (erythrocytes) and its blood serum thus comes to contain an 

 "amboceptor" which will dissolve the hemoglobin of human r.b.c. in the 

 presence of "complement." This is called an antihuman hemolytic sys- 

 tem, but other systems, as antichicken and antisheep, have been used 

 with good results. The "complement" above mentioned is obtained 

 by using fresh guinea-pig serum, and 0.2 mil of a i to 10 dilution of serum 

 in isotonic salt solution is usually considered as i unit. One unit of ambo- 

 ceptor, then, is that smallest amount which, in the presence of i unit 

 of complement, will produce complete hemolysis of i mil of a 10 per cent, 

 suspension of homologous r.b.c. in 30 minutes at a temperature of 37C. 



The test is made by placing 2 units of complement, 2 units of antigen, 

 and o.i mil of suspected serum into a test tube and incubating at 37C. 

 for 30 minutes in a water bath; then 2 units of amboceptor with o.i mil of 

 10 per cent, of r.b.c. are added, the tube shaken, and incubated again for 

 30 minutes at 37C. A positive result is indicated by absence of hemoly- 

 sis, the r.b.c. having settled to the bottom of the tube and no color above 

 them. This, as will be readily seen, is due to the fact that the complement 

 was fixed during the first incubation and thus made unavailable for further 

 use in hemolysis. A negative result is indicated by hemolysis of the red 

 blood cells. 



