HEALTHILY ACQUIRED IMMUNITY 15 



demic cerebro-spinal meningitis they may be ten times 

 as numerous (Ledingham, 1912). So that of every 

 eleven persons whose throats are reached by the menin- 

 gococcus ten are enabled to resist any general invasion 

 of their bodies. In what way have they become im- 

 mune ? At first one might imagine that they had all 

 suffered from very mild infections of the blood stream. 

 But when their blood has, at times, been examined, 

 no agglutinins have been found such as are present in 

 the blood stream of convalescents. One is tempted, 

 then, to look elsewhere, and reasons will be given for 

 saying that i7n7nunity has been acquired through the 

 agency of the subepithelial lymphatic glands, often, 

 perhaps, without any invasion of the blood stream at 

 all. 



It is perhaps the general view of bacteriologists that 

 the immunity acquired against an infectious disease by 

 contacts who have not obviously suffered from the ill- 

 ness is due, nevertheless, to an invasion of the blood 

 stream by the causal organism. There is evidence that 

 this is sometimes the case. But whether the infecting 

 organisms reach the blood or not, there is, in the light 

 of evidence adduced in subsequent sections, good reason 

 for supposing that the infecting organisms are first 

 drawn into the subepithelial lymphatic glands. Anti- 

 bacterial substances and specially provided lympho- 

 cytes are formed in the subepithelial lymphatic glands 

 and pass into the blood stream. It is true that the 

 lymphocytes do not provide the chief reaction to all 

 acute infections. Nor is it usually considered likely 

 that the lymphocytes are transformed later into poly- 

 morphonuclear leucocytes.* Yet the ingestion of 

 bacteria in enormous numbers by the subepithelial 



* Some observers, however, have considered that the natural fate 

 of Ivmphocytes is conversion into polynuclear leucocytes (see Gruner, 

 1913). 



