ON IMMUNITY TO BACTERIA 333 



temporarily, and are rapidly destroyed : the reasons being, firstly, 

 that they are never present in large numbers, as they would be if 

 they lived and multiplied in the circulation, and, secondly, that 

 they do not as a rule form secondary lesions, as we might expect 

 them to do, when deposited in the tissues. We may fairly assume, 

 therefore, that in a disease like typhoid fever bacteria are constantly 

 passing from the lesions into the blood, but are rapidly destroyed, 

 so that the defensive mechanism of the fluid is sufficient to prevent 

 the occurrence of a true septicaemia, in which bacterial growth 

 continues in the blood. The same holds in pneumonia and in 

 ulcerative endocarditis when not accompanied by secondary 

 infective lesions, and probably most other diseases, and does not 

 militate against the view that the blood is a sufficient defence in 

 the majority of the cases in which the infective material gains 

 access to the interior of the vessels. Excluding these and similar 

 examples, septicaemia is a rare disease, and is commoner in the 

 case of the protozoal than in bacterial infections. 



As regards the failure of the process of phagocytosis, which 

 must occur when an organism which is combated by phagocytosis 

 does gain a foothold in the blood, we may point out that we have 

 yet but little knowledge of the exact state of the bacteriotropic 

 substances in the living plasma. Wright, it is true, has shown 

 that citrated plasma contains the same amount of opsonin as 

 serum ; but the two fluids are not absolutely identical, and the fact 

 that opsonin increases in amount in the successive fractions of 

 serum squeezed out from a clot leads us to believe it probable 

 that it does not exist as such in the plasma ; and if, as we have 

 shown is likely, the naturally existent thermolabile opsonin is the 

 same as alexin, it is, on the whole, probable that there is none in 

 the living blood. 



Briscoe's experiments reproduce the natural conditions more 

 closely. He injected emulsion of bacteria into the ventricle of a 

 heart after excision and clamping of the auriculo-ventricular 

 groove. After a time some of the blood was removed and 

 examined, and practically no phagocytosis was found to have 

 taken place. But many bacteria were taken up when the same 

 animal's leucocytes and serum and the emulsion of bacteria were 

 incubated outside the body in the ordinary way. Further, in 

 some cases a little clotting occurred in the heart, and in these it 

 was found that the leucocytes which were included within the 

 clot took up many more bacilli than those which remained free. 



