ANTIBODIES TO PNEUMOCOCCUS 393 



mococcal infection, the cocci were not taken up by the leucocytes 

 of either resistant or susceptible animals. The serum of all the re- 

 sistant species tested — dog, cat, sheep, pig, and horse — possessed 

 marked opsonic properties not found in the serum of animals of 

 susceptible species, such as the rabbit, guinea pig, and man. There 

 appeared, however, to be no essential difference in the phagocytic 

 activity of the leucocytes from the various animals. Heating the 

 serum, according to Robertson and Sia, abolished the destructive 

 power of serum-leucocyte mixtures for pneumococci. The validity 

 of the authors' classification of resistant and susceptible animals 

 is open to question. To group the horse with such highly resistant 

 animals as the dog, cat, sheep, and pig, and to include species of 

 such diverse susceptibility in the non-resistant class, may not be 

 wholly justified. In another portion of the report, Robertson and 

 Sia drew attention to other conditions which may influence the 

 opsonizing property of serum on pneumococci subjected to the ac- 

 tion of leucocytes. The strain and type of the culture employed, 

 the age of the animals, and other factors discussed in the chapter 

 on Pathogenicity must be taken into account in evaluating the 

 results. 



From the conclusions in an extensive report on an investigation 

 of experimental pneumococcal septicemia and antipneumococcal 

 immunity published by Wright in 1927 , 1647 certain passages may 

 be cited. Avirulent pneumococci of Type I inoculated intrave- 

 nously into rabbits are rapidly removed and do not reappear in 

 the blood; virulent strains of the same serological type similarly 

 inoculated disappear for a short period and then subsequently in- 

 crease in number. Previous active immunization enhances the ca- 

 pacity of the rabbit to remove virulent organisms and to prevent 

 their reappearance, and in the immunity so established the out- 

 standing effect is the increased activity of the body fluids favor- 

 ing phagocytosis, although the existence of a slight degree of 

 residual and purely cellular immunity cannot be excluded. Because 

 leucocytes could be considerably decreased in number without in- 



