IMMUNITY IN PNEUMOCOCCAL INFECTIONS 17 



COCCUS albus), and the changes in the opsonic content in a particular 

 infection concern only the opsonic content with reference to the infecting 

 agent. 



In the course of active immunisation of animals against the pneu- 

 mococcus, the opsonic content of the serum follows a fairly definite course. 



On inoculation of a rabbit with 1,000 millions of pneumococci killed 

 by heat (counted by Wright's method), the opsonic index fell to 7 and 

 •65 on the two succeeding days; on the third day, it rose to 1*3, and 

 remained above normal for ten days. 



A second rabbit was inoculated with 30,000 millions of pneumococci. 

 The negative phase persisted for seven days, after which a very marked 

 positive phase, giving indices of v6, r8, 1-9, followed, the serum not 

 giving the normal index for four weeks after inoculation. 



Many similar experiments with varying dosage gave results which 

 would point to the conclusion that the smaller the dose the less pro- 

 nounced and the less durable was the negative phase. The amount and 

 persistence of the positive phase was not so constant, as in many cases, 

 after a long negative phase from a large dose, the positive phase was 

 entirely absent, the opsonic index gradually rising to unity. 



The specificity of the opsonin was also apparent in these experiments 

 on animals, for in some cases the serum was tested throughout against 

 both pneumococcus and staphylococcus, and showed that while the 

 opsonic index for pneumococcus was passing through wide variations, 

 that for staphylococcus remained almost unchanged from day to day. 



In the lower animals there are great differences in opsonic index 

 observed between different species, but the individuals of any given 

 species do not appear to vary much. For pneumococcus the indices of 

 the laboratory animals are in comparison with man 0*4, 0*6, and o*8 for 

 rabbits, guinea-pigs, and rats respectively. On the other hand, their 

 opsonic index for the tubercle bacillus is found to be 0'6 and 0*4 for 

 rabbits and guinea-pigs respectively. Their opsonic index seems to 

 correspond very closely with their susceptibility to infection. 



Nature of the Opsonic Substance in the Serum. — Before making any 

 systematic enquiry into variations in the opsonic content of the serum, 

 it is necessary to enquire into the effects of preservation, effects of light, 

 of dilution, of addition of salts, etc. 



Effect of preserving the 5^^^^;;/. —Samples of blood were drawn from 



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