400 COLLECTED STUDIES JN IMMUNITY. 



of blood-cell possesses, it is not surprising that certain types are 

 common to the majority if not to all the vertebrate species. In this 

 connection I shall only point out the fact that receptors for ricin, 

 abrin, ichthyotoxin (which injure a large number of different erythro- 

 cytes) are widely distributed in the animal kingdom. Side by side 

 with such generally distributed groups, however, there are types 

 which are limited to a comparatively small group of animal species. 

 Thus by means of cross immunization we have demonstrated that the 

 blood-cells of goat and sheep possess several special receptors in 

 common. This was shown by the fact that the isolysins obtained 

 by injecting goats with goat blood usually effected solution of sheep 

 blood-cells, although to a less degree. In making the counter ex- 

 periments, immunizing goats with sheep blood-cells, we obtained 

 in addition to sheep lysin the isolysin acting on goats. 



Besides this there are groups of receptors which are specific for 

 each animal species. This is best shown by the normal course of 

 the Belfanti-Bordet experiments. In these as a rule only specific 

 hsemolysins are formed, i.e., hsemolysins directed against the erythro- 

 cytes exciting the immunization. 1 



Such variations in the zoological distribution of certain recep- 

 tors (also of the complements, etc.) is readily explained by the very 

 natural assumption that the metabolic processes, whose indicator 

 the receptors really are, show corresponding variations. It is just 

 as little to be doubted that certain assimilative processes are specific 

 for only one species of animal as that others occur in exactly the 

 same manner in man and in the frog. 



It is also of considerable importance that in any given animal 

 species a considerable individual variation of the receptors may occur, 

 a fact first observed in experiments with crotin on rabbits. The 

 strongest confirmation of this point is the result of our experiments 

 on goat isolysins. As already stated, out of the goats we used there 

 were always only a few which reacted to one of the thirteen different 

 isolysins. 



Through the opportunity so offered we convinced ourselves of 

 another important fact, namely, that the susceptibility of a given 

 individual can change in a comparatively short time. We found 

 that a goat which reacted to a certain isolysin became unsuscep- 



1 We have obtained entirely analogous results also with other constituents 

 of blood serum, e.g., with complements. 



