206 INFECTION AND RESISTANCE 



This serum is inactivated at 56 C. for 20 minutes. 



In this case 0.001 c. c. still causes complete hemolysis of 1 c. c. 

 of a 5 per cent, emulsion of sheep cells (volumetric measurement of 

 cells sedimented in the centrifuge), and this amount (yinnr c. c.) 

 is called the "hemolytic unit" of sensitizer ; two units are then used 

 in the reactions. 



Against these cells alexin can, in each case, be titrated as follows : 



Alexin Titration: 



Fresh Guinea Pig Serum Pipetted from Clot 



The smallest amount of alexin which completely hemolyzes the 

 red cells (0.05 in this case) is the amount used. Since it is easier 

 to measure larger volumes with accuracy, the alexin is diluted 1 to 

 10 in salt solution before use. A typical Wassermann reaction can 

 then be carried out as follows : 



done blood serum or plasma will be injected with them and the treated animal 

 will respond by the formation not only of hemolysin but of precipitins for 

 the serum proteins as well. When a subsequent hemolytic test is carried out, 

 a precipitin reaction between the precipitin in the antiserum and serum ad- 

 hering to the corpuscles will follow, and this, as we have seen, will fix alexin, 

 obscuring other reactions which may be under observation. 



