CELLULAR RESISTANCE 161 



Moore has found that in guinea-pigs " the complement titer varies 

 with the opsonic index and in the same direction." These facts, to- 

 gether with the fact that vaccination with bacteria will increase specifi- 

 cally the opsonic content of the blood suggest a close similarity of 

 opsonins to agglutinins and amboceptors. The resemblance to agglu- 

 tinins is only relative for as we have seen the thermostable element of 

 opsonin is markedly augmented in activity by the addition of fresh 

 serum, whereas agglutinins are not affected in any way by the addition 

 of complementary substance. Hektoen has shown that in the process 

 of immunization the curves of opsonin and agglutinin production are 

 nearly parallel, but that heating does not influence the agglutinin and 

 markedly depresses the opsonic action, the latter being restored by the 

 addition of fresh normal serum. Levaditi, in a study of the site of 

 formation of opsonins, showed that certain organs rich in agglutinin 

 contain no opsonins. The thermostability and specific absorption of 

 opsonins suggest similarity to amboceptors, but the amboceptors are 

 not capable of acting without complement whilst the opsonin is capable 

 of acting independently of fresh serum. The fresh serum augments 

 the activity of the thermostable element of opsonins but is not an 

 absolute essential for activity. That the opsonin is not identical 

 with hemolytic and bactericidal amboceptors is indicated by the fact 

 that there are such amboceptors in sera which have no opsonic power ; 

 that in sera which show both amboceptors and opsonins there is no 

 parallelism between the activity of the two. Sera may be strongly 

 opsonic for certain bacteria and yet contain no bactericidal amboceptor. 

 Much of the material quoted above has been worked out iri connection 

 with immune opsonins, but nevertheless it is safe to conclude that the 

 opsonic action of normal serum depends upon the operation of two 

 elements, a thermostable element which behaves as a " facultative " 

 amboceptor and a thermolabile element which, if not identical with, 

 resembles complement most closely. 



Immune Opsonins. As has been indicated, it is possible by im- 

 munization to increase to a very considerable degree the opsonic activity 

 of serum. The immune opsonins were considered as of a constitution 

 different from the normal opsonin because of the claim that the appli- 

 cation of heat did not alter their activity. Dean showed, however, that 

 this assumption is not true for he found that heating to 60 C. definitely 

 though not very markedly reduces the opsonic activity of immune serum, 

 and that reactivation takes place on the addition of a fresh normal 

 serum. The following protocol shows the phagocytic index as deter- 

 mined by the use of various sera and mixtures : 



Normal serum 1 1.9 



Heated immune serum 7.1 



Heated immune serum + normal serum 33.0 



Hektoen reached the same conclusion with the hemopsonic power of 

 rabbits immunized to goat erythrocytes, diluting the serum so that it 

 ii 



