PART I 



PROPERTIES OF H^EMOLYTIC SERA 

 INTRODUCTORY 



IT is now recognized that in immunity reactions the all- 

 important phenomenon, so far as the serum is concerned, 

 is the appearance of anti-substances which possess a specific 

 combining affinity for the molecules introduced into the 

 animal used for immunization. Such anti-substances may be 

 developed by means of substances in solution, toxins, fer- 

 ments, proteins, &c., or by means of formed elements, such as 

 tissue cells, bacteria, &c., the specific reaction being brought 

 about by certain of their constituent molecules. The 

 result is, however, essentially the same in the two cases. 

 The molecules which lead to the formation of anti-substances 

 are known as antigens, and it is now generally accepted 

 that they are the same molecules as those for which the 

 anti-substances show the specific affinity, as demonstrated 

 by test-tube or animal experiments. Ehrlich, in accordance 

 with his side-chain theory, has applied the term ' receptors ' 

 to molecules in the cells which have a combining affinity 

 for other molecules, and which, no doubt, functionate in 

 the processes of nutrition. These receptors are the molecules 

 which act as antigens, when tissue cells or bacteria are 

 injected into an animal. Similar molecules may, however, 

 be discharged from the cells and exist free in the body 

 fluids ; thus, for example, we may speak of serum receptors. 



The anti-substances developed in immunity reactions are 

 now generally recognized as belonging to three main classes. 

 In the first class, the anti-substance simply combines with 



MUIB T* 



