PHAGOCYTOSIS 259 



It is clear, therefore, that serum has a great power in aiding 

 phagocytosis. Is this due to an action on the bacteria or to a 

 stimulation of the leucocytes ? Two experiments show that the 

 former occurs ; there is but little direct evidence for or against 

 the latter. 



In one experiment Wright (having shown that the power of the 

 serum is destroyed by heating it to 55 C. for thirty to sixty 

 minutes, or to 60 to 65 C. for fifteen minutes) allowed serum to act 

 on bacteria, and then heated the mixture until the activity of the 

 serum was removed. He found bacteria thus treated were taken 

 up readily. This must have been due to an action which the 

 serum had exerted on them before it was heated, and any action on 

 the leucocytes is out of question, since they were only acted on by 

 heated and inactive serum. 



Another and even better proof of the same fact may be obtained 

 by acting on bacteria with serum, centrifugalizing and removing 

 all trace of the latter by repeated washings with saline solution. 

 Bacteria thus treated are taken up with great readiness, and here 

 no free serum at all comes into contact with the leucocytes. 1 

 Opsonin, therefore, combines with bacteria, and Bulloch showed 

 that this process goes on at ordinary temperatures and at o C.' 2 

 Bacteria which have once been acted on by opsonin ("opsonized") 

 may be heated to 60 C. for five hours, and are still assimilable by 

 leucocytes ; this shows that they are profoundly affected, but they 

 may be absolutely unchanged in appearance. 



There is no method by which an absolute measurement of the 

 amount of opsonin present in a specimen of blood can be made, 

 but comparative measurements can be made easily enough by the 

 process elaborated by Sir Almroth Wright. In order to do this 

 it is necessary to have as a standard either the serum of a normal 

 person, or preferably a mixture of sera from several normal 

 persons, so that slight individual variations or abnormalities may 

 be ruled out. The emulsion of leucocytes (" cream ") is prepared 

 as described above, and the emulsion of bacteria made by stirring 

 a little of a young culture of the organism in question in some 

 saline solution, taking care to remove clumps by sedimentation or 

 centrifugalization. When tubercle bacilli are being used it is 

 most convenient to employ dead and dried bacilli, which are 



1 This experiment was performed by Markl in 1903, using plague bacilli. 

 This is not altogether confirmed by Ledingham's more recent work, 

 which is discussed subsequently. 



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