HISTORIC DEVELOPMENT OF IMMUNOLOGY 43 



responding antigen, either in vitro or when 

 transferred to the body of another animal. 



Varieties The effect of antibodies on bacteria 

 or cells is more easily observed than are their 

 reactions with soluble substances. Among the 

 immune bodies produced in response to the in- 

 jection of antigens, we identify the agglutinins, 

 the lysins, the opsonins, and the precipitins, etc. 

 The agglutinins cause the cells to clump in 

 masses, a result which does not seem especially 

 protective. The lysins disintegrate the cells and 

 bring the constituents into solution; but in or- 

 der to effect solution of the cells, they require 

 the presence of complement, a substance which 

 is present in normal serum and which is not in- 

 creased during immunization. Immune bodies 

 of this type, which are inefficient without com- 

 plement and are thought to act by uniting the 

 complement to antigen, are referred to as ambo- 

 ceptors. We recognize that serum produces an 

 invisible change in antigenic cells which enables 

 leukocytes to ingest them, and that this prop- 

 erty is increased in immunization. We attrib- 

 ute it to the presence of substances known as 

 opsonins or tropins. If a soluble antigen is 

 combined with the corresponding immune se- 



