PROPERTIES AND NATURE OF OPSONINS 191 



power of the blood, these observers first determined the direct depend- 

 ence of phagocytosis upon some ingredient of the blood-serum, and fur- 

 ther proved that this substance acts directly upon bacteria, is bound by 

 the bacteria, and renders them more easily ingested by the leukocytes, 

 i. e., more readily phagocytable. To this substance they gave the name 

 opsonin (from opsono, I prepare food for). At the same time, and in- 

 dependently of Wright, Neufeld and Rimpau conducted similar inves- 

 tigations with immune serum and reached similar conclusions, but called 

 the substance bacteriotropin. Since then both terms have been used, 

 the former more frequently in English literature, and this is permissible, 

 providing that it is understood that both are practically the same anti- 

 body, and not distinct and separate from each other. 



As will readily be understood, the bacterial opsonins have been 

 studied most extensively, but opsonins may be present in normal and 

 immune serums for other cells, such as erythrocytes, and these hemopso- 

 nins are regarded as separate antibodies, distinct from hemagglutinins 

 and hemolysins. 



Definition. Opsonins are substances in normal and immune serums 

 which act upon bacteria and other cells in such a manner as to prepare them 

 for more ready ingestion by the phagocytes. 



Properties and Nature of Opsonins. There is considerable differ- 

 ence of opinion regarding the identity and probable structure of opsonins 

 in normal and immune serums. Just as agglutinin for a bacterium, 

 such as Bacillus typhosus, may be found in varying amounts in normal 

 serum, so various opsonins for different bacteria may be found in normal 

 serums. These normal opsonins appear more or less specific for a given 

 bacterium, and in immune serum the specific opsonic substance for the 

 particular bacterium or cell with which immunization has been pro- 

 duced is developed to a high degree. Both owe their full effect to the 

 interaction of two substances. One of these, the common substance, 

 is thermolabile, and destroyed by heating the serum to from 56 to 58 C. 

 for half an hour, whereas the other more specific substance remains un- 

 affected. The latter, in both normal and immune serums, is opsonic 

 by itself, although in the absence of the common thermolabile substance, 

 to a less degree, and is produced anew and specifically by artificial im- 

 munization or as the outcome of spontaneous infections. 



The true nature of opsonins is, therefore, difficult to understand. 

 They have been compared to receptors of the second order, with a hap- 

 tophore and a toxophore or opsoniferous group. Receptors of this 

 order, however, are active and independent of the presence or absence 



