396 BIOLOGY OF PNEUMOCOCCUS 



develop this power in vitro in the presence of leucocytes. Woo 1541 

 later (1926) found, in testing the pneumococcidal activity of nor- 

 mal serum-leucocyte mixtures, that virulence of the cultures and 

 the age of the test rabbit influenced the outcome of the test. 

 Avirulent strains were readily destroyed in a mixture of normal 

 rabbit serum and leucocytes, whereas virulent cultures resisted the 

 destructive action of the mixture. The absence of pneumococcidal 

 properties in the blood of very young rabbits agreed with the ex- 

 treme susceptibility to pneumococcal infection of immature ani- 

 mals of the species. 



Strouse, 1346 by both in vitro and in vivo tests, demonstrated the 

 same quality in the peritoneal fluid of pigeons. Wright 1547 noted 

 the rapid disappearance of virulent pneumococci injected directly 

 into the blood stream of rabbits and the still more rapid removal 

 of avirulent forms. Specific immunization enhanced the activity of 

 body fluids favoring phagocytosis. Robertson and Sia, 1144 and 

 later Sia, 1268 not only proved the existence of normal opsonins for 

 pneumococci in the blood of cats, dogs, sheep, and pigs but, by ab- 

 sorption experiments, exhausted the serum of opsoninizing sub- 

 stance for the strain used as absorbent without removing opsonins 

 for organisms of other types. The results suggest that there are 

 separate type-specific opsonins for pneumococci in the serum of 

 animals naturally resistant to pneumococcal infection. 



In 1933, Ward and Enders 1484 published a communication, based 

 on experiments with Pneumococcus, dealing with a serological 

 analysis of the opsonic or tropic action of normal and immune se- 

 rum. The authors studied the action of the anticarbohydrate sub- 

 stance in normal serum as an opsonic or tropic agent, and also the 

 properties of normal and immune serum which promote phago- 

 cytosis after the anticarbohydrate substance or antibody had been 

 removed from the serum by appropriate amounts of type-specific 

 polysaccharide. The authors analyzed the mechanism of the 

 phagocytosis of Pneumococcus as follows: In normal, unheated 

 human serum virulent pneumococci may be prepared for phago- 



