CHAPTER XX. 



Resume of the Hypothesis and Summary of the 

 Evidence For and Against. 



The hypothesis is that the subepitheUal collections of 

 lymphoid tissue play an important function in im- 

 munising the body against pathogenic bacteria in 

 proximity to the tissues. These organs secure this 

 general immunity by ingesting the bactieria upon or 

 near the surface of the covering epithelium. The 

 mechanism of ingestion may be called phagotaxis — the 

 lymphocytes leaving the gland-tissue wander to the 

 surfa?ce of the epithelium or beyond into the lumen, 

 engulf bacteria and return with them into the paren- 

 chyme of the gland where the bacteria are slowly over- 

 come or sometimes passed on to a further relay of 

 lymphatic glands for destruction or even via the blood 

 to the spleen and bone marrow. Bacteria are being 

 constantly invited into places which are nearly always 

 impregnable. The bacteria are not only overcome, but 

 a great number of lymphocytes capable from experience 

 — if one may fancifully so put it — of dealing with that 

 kind of bacterium are scattered through the blood 

 stream ready to resist invasion elsewhere. The sub- 

 epithelial lymphatic glands are not only breeding 

 grounds* but also training schools for lymphocytes. 

 The opsonic index against the micro-organisms is in- 



* The researches of Goodall, GuUand and Paton appear to show 

 that very many of the lymphocytes in the blood are formed in the 

 bone marrow. The whole question of the origin of the white cells 

 in the blood is not yet settled. It is usually assumed that the lympho- 

 cytes do not develop into polymorphonuclear cells, but that the 

 former arise in lymph glands, haemolymph glands and marrow, whilst 

 the latter are formed in the red bone marrow only. 



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