FIXATION OF COMPLEMENT 127 



By this means we can readily see whether the usual 

 amount of the serum used is capable of fixing one, two, 

 three, or more units of complement. It is rarely neces- 

 sary to carry this quantitative determination beyond 

 two units of complement, though some sera will fix five 

 or six units. By means of a series of tests at intervals 

 on a case under treatment, we can observe that the 

 serum gradually loses its power to fix complement, ab- 

 sorbing perhaps two units the first time, then one unit, 

 then allowing hsemolysis of half the corpuscles, then 

 only a few corpuscles being left unhaemolyzed, and 

 finally complete haemolysis showing that the serum has 

 become negative. Some workers use signs to desig- 

 nate the strength of the Wassermann reaction, a weak 

 reaction being indicated by + or 1 plus, meaning that 

 there was only 25 to 50 per cent, inhibition of haemol- 

 ysis, between 50 and 75 per cent, inhibition being indi- 

 cated by + + or 2 plus, between 75 and 100 per cent, 

 inhibition as + + + or 3 plus, and complete inhibition 

 as + + + + or 4 plus. The percentage of haemolysis 

 cannot be thus accurately guessed at by the eye, and 

 moreover this method of notation does not give the 

 strength of the reaction beyond specific absorption of 

 one hasmolytic dose of complement. We prefer to 

 designate reactions up to one unit of fixation as nega- 

 tive, weakly positive, medium 'positive, and strongly 

 positive, giving the number of units of complement ab- 



