5 6 IMMUNE SERA 



inactive serum again capable of dissolving bacteria 

 in vitro by adding a little fresh alexin, according 

 to the suggestion of Metchnikoff. In other words, 

 it is thus reactivated. Another obscure point was 

 cleared up by these studies: why a specific bac- 

 tericidal serum which is inactive in vitro should 

 be intensely active in the living body. This is 

 because in the living body the serum finds the alexin 

 necessary for its working, which is not the case in 

 the test-tube unless fresh normal serum be added. 

 We see from all this that even the first experiments 

 in haemolysis have served to clear up a number of 

 practical points in an important branch of bacteri- 

 ology. 



Ehrlich and Morgenroth on the Nature of Haemo- 

 lysis. In continuing the study of haemolysins we 

 must note particularly the researches of Ehrlich 

 and Morgenroth. 1 These authors asked themselves 

 the following questions: (i) What relation does 

 the haemolytic serum or its two active components 

 bear to the cell to be dissolved? (2) On what 

 does the specificity of this haemolytic process 

 depend ? Ehrlich was led to these researches partic- 

 ularly by his so-called Side-chain Theory, which 

 we shall examine in a moment. 



He made his experiments with a haemolytic 

 serum that had been derived from a goat treated 



1 Ehrlich and Morgenroth. See the various papers in "Col- 

 lected Studies on Immunity," Wiley and Sons, New York, 1906. 



