430 BIOLOGY OF PNEUMOCOCCUS 



— showed marked opsonic properties which were absent from the 

 serum of animals of such susceptible species as the rabbit, guinea 

 pig, and man. In contrast to the activity of serum there appeared 

 to be no essential difference in the phagocytic power of the leuco- 

 cytes from the various animals. The influence of the age of the ani- 

 mals on the pneumococcidal properties of the blood was shown by 

 the fact that mixtures of adult rabbit serum with either adult or 

 young hare leucocytes exerted pronounced growth-inhibitory and 

 coccidal action, whereas mixtures of serum from immature hares 

 with leucocytes from either adult or young rabbits completely 

 lacked any similar action. In the natural defense mechanism 

 against pneumococcal infection, it appears that the serum con- 

 tains the potential elements and that in a susceptible animal like 

 the rabbit the elements develop with the growth of the animal. 



The results reported by Robertson and Sia were practically 

 duplicated by those of Woo, 1541 who found that rabbit serum-leu- 

 cocyte mixtures possessed the power to kill avirulent pneumococci 

 in relatively large numbers, but failed to inhibit growth of even 

 minute quantities of virulent organisms, an observation also re- 

 ported by Wright (1927). 154T Woo found further that the serum 

 of very young animals when mixed with leucocytes was powerless 

 to affect cultures which were without virulence for mature rab- 

 bits. Bull and Tao (1927) 182 introduced citration for determining 

 the antipneumococcic properties of whole blood. One per cent by 

 volume of saturated, neutral sodium citrate delayed coagulation 

 of the blood for twenty-four hours and did not inhibit the growth 

 of pneumococci. The killing action of citrated blood was more po- 

 tent than that of serum-leucocyte mixtures. When the method was 

 applied to the blood of normal rabbits and chickens, Bull noted 

 that it required one million times as many pneumococci to infect a 

 given quantity of chicken blood as it did to infect the same volume 

 of rabbit blood. 



The hypothesis that it was the natural, humoral antibodies 



