COMPLEMENT FIXATION 395 



haemolytic system ; on the other hand the presence of 

 haemolysis would show that no appreciable amount of 

 antibody has yet been formed in response to the inocu- 

 lations. In other words, there is an absence of infec- 

 tion, since the complement remained unfixed at the 

 time of the addition of the erythrocyte solution and 

 haemolytic serum, and was ready to combine with those 

 reagents to complete the haemolytic system. 



The method may be shown diagramatically as under 

 using the symbols already indicated 



Test-tubes. 



NOTE. It is sometimes more convenient to sensitise the erythro- 

 cytes just before they are needed. This is done forty-five minutes 

 after the experiment has been started (page 394, step 6), that is to 

 say, before the completion of the first period of incubation, thus : 



1. Measure out into a sterile test-tube (or flask) five c.c. of 

 erythrocyte solution. 



2. Measure out twenty minimal haemolytic doses of haemolysin, 

 add to the erythrocyte solution on the test-tube. 



3. Allow the erythrocyte and hsemolysin to remain in contact for 

 fifteen minutes at room temperature. The red cells are then sensi- 

 tised and ready for use. 



4. When the tubes are removed from the incubator at the end of 

 the first hour (i.e., step 7) add i c.c. sensitised red cells to each 

 tube by means of a graduated pipette. 



5. Mix thoroughly, return the tubes to the incubator at 37 C. 

 and complete the experiment as previously described (steps 8 

 onward) . 



