122 IMMUNE SERA 



toxic for sheep, as well as hasmolytic for sheep 

 red cells. 



Common Receptors. At first it was thought that 

 the hasmolysin so produced was due to the presence 

 of small quantities of blood injected with the sper- 

 matozoa. The same result however was obtained 

 when all traces of blood could be excluded j 1 further- 

 more a number of investigators produced hsemoly- 

 sins by the injection of fluids entirely free from red 

 corpuscles, such as serum and urine. The produc- 

 tion of this haemolysin is not hard to explain if we 

 hold fast to the side-chain theory. We have 

 merely to assume that the spermatozoa or these 

 other substances possess certain receptors in com- 

 mon with the red blood cells of the same animal. 

 Ehrlich and Morgenroth 2 have repeatedly pointed 

 out that specificity is a matter not of cells, but of 

 receptors. Despite these very conclusive demon- 

 strations later investigators, who attempted to 

 produce antisera for the cells of various organs, 

 continued to use emulsions of unwashed organs, in 

 utter disregard of the presence of free receptors in 

 the organ juices and also without consideration of 

 the antibodies certain to be produced by the red 

 cells normally present. 



Cytotoxin for Epithelium. As far back as 1899, 



1 Von Dungern. See Collected Studies on Immunity, p. 47. 

 Wiley and Sons, New York, 1906. 



2 Ehrlich and Morgenroth, Ibid., p. IPO. 



