334 FUNCTION OF THE SPLEEN IN PHAGOCYTOSIS 



These results argue strongly in favour of the fact that opsonin 

 does not occur in the circulating plasma as such, and is only set 

 free during the process of coagulation or of phagolysis. This 

 conclusion was corroborated by the fact that opsonized bacteria 

 were taken up freely, showing that the leucocytes were not at 

 fault. 



We can get no information on this point from an examination 

 of the blood in these infections, since a small number of non- 

 virulent bacteria will be digested and completely disappear in a 

 few minutes if taken up by a leucocyte ; but as a matter of fact, it 

 is very uncommon to find ingested bacteria in the leucocytes of 

 the circulating blood in natural diseases. 



It might appear that these explanations go too far, and would 

 lead us to the conclusion that phagocytosis is without value as a 

 defensive process when bacteria reach the blood-stream. This is 

 not the case, and it is probable that the great factor to consider is 

 the spleen. This acts as a sort of filter, and in its pulp the flow 

 of blood is slow, so that there is abundant opportunity for phago- 

 cytosis to occur. The same is true, though in a lesser degree, of 

 all regions of the body in which the circulation is tardy. 1 Hence, 

 when bacteria gain access to the blood they will always be found 

 in considerable numbers in this organ, which Kanthack called 

 years ago " the graveyard of the bacteria." This is especially the 

 case in typhoid fever, and the method formerly in use in the 

 diagnosis of the disease, by making cultures from traces of spleen 

 pulp removed by puncture, almost invariably gave positive results. 

 Films or sections of the spleen pulp will often show bacteria 

 ingested by the leucocytes in many diseases, even although none 

 can be detected in the blood. The role of the spleen in immunity 

 is probably a complex one, and this may explain the divergent 

 views held as to its importance in this respect. Thus it was 

 found long ago by Tizzoni and Cattani that it was impossible to 

 immunize animals to tetanus after splenectomy ; but this was not 

 corroborated by other observers, who found the animals equally 

 immunizable after the results of the operation had completely 

 passed away. But in this connection we must remember that the 

 splenic tissue is very rapidly regenerated, and that in all prob- 

 ability the haemolymph glands and other structures take on its 

 functions vicariously after its ablation. 



1 Leucocytes which have taken up bacteria whilst in the circulation 

 probably accumulate in the lungs also. 



