ON IMMUNITY TO BACTERIA 335 



The role of the spleen in facilitating phagocytosis is well seen 

 in the spirilloses. Metchnikoff showed many years ago that after 

 death from relapsing fever the cells of this organ are packed with 

 spirilla, such cells being rarely, if ever, seen in the circulating 

 blood. When monkeys are inoculated with blood containing the 

 parasite, a febrile attack occurs, but recovery soon takes place. 

 During the disease free spirilla occur in the blood, but they soon 

 disappear ; but if the animals are killed after this, the organisms 

 may be found within the splenic leucocytes. Further, Souda- 

 kewitch showed that recovery often failed to occur in animals 

 inoculated after splenectomy, and in this case the spirillum 

 appeared in large and increasing numbers in the blood, and no 

 phagocytosis was observed. Very similar facts have been 

 observed by Levaditi and Manouelian in tick-fever. During the 

 height of the disease the spirillum occurs in the blood, and is 

 exclusively extracellular. After the crisis it is found only in the 

 phagocytes of the spleen and liver (Kupffer's cells). 



Besides the spleen, the bone-marrow is also a region where 

 abundant phagocytosis occurs, and the functions of the two tissues, 

 like their structure, are similar. In each case there is a slow flow^ 

 of blood in the pulp, close proximity of the blood to the tissue 

 cells of peculiar type, and an abundant supply of leucocytes. 



The function of these organs in phagocytosis is probably two- 

 fold. It provides a region where the blood-stream is comparatively 

 tranquil, so that there is no mechanical obstacle to the process : 

 this is probably comparatively unimportant, since it can and does 

 take place under certain circumstances in the full torrent of the 

 circulation. And, secondly, if, as seems probable, thermolabile 

 opsonin is really complement, and if complement is formed by the 

 leucocytes, it is readily conceivable that it may be present in the 

 leucocytic organs when only present in small amount in the rest 

 of the blood. It may actually be called into existence by the action 

 of the toxin on the spleen cells or leucocytes, and be immediately 

 absorbed by the bacteria, so that it never occurs in appreciable 

 amount in the plasma. 



This action will not come into play in the case of the bacterio- 

 tropic substances which are true antibodies, for we have every 

 reason to believe that these are present in the plasma as such, and 

 are not set free in the process of clotting ; yet even here we have 

 seen some evidence which suggests that they, in common with 

 other true antibodies, are formed inlymphoid tissue or from lymphoid 



