THE WHITE BLOOD CORPUSCLES 205 



medium containing no germs. Under this condition the leukocytes 

 do not enter the tube. 



Opsonins. It was observed at an early date that the leukocytes 

 behave at times in a very indifferent manner toward certain types 

 of bacteria, and hence, it was thought likely that these germs must 

 first be killed before the phagocytosis can take its regular course. 

 Metchnikoff then expressed the view that the leukocytes are capable 

 of surrounding living material under ordinary conditions, but that 

 the complete destruction of the latter necessitates the presence of a 

 specific intermediary agent. It was assumed, therefore, that the 

 fluids of the body contain special activators which stimulate the leuko- 

 cytes to greater activity. 



Leishman 1 and Wright and Douglass 2 showed later on that the 

 phagocytosis may be greatly augmented by blood plasma or serum 

 which has been treated in a particular way. 

 It could be proved by the centrifugalization 

 of bacterial mixtures that this process tends 

 to diminish the reinforcing power of the 

 serum, while the bacteria are "sensitized 

 thereby," i.e., they are rendered especially 

 vulnerable to the leukocytes. In this way, 'j^.".;.'^ -YHiT 

 it has been established that the bacteria in- &v;.' '.=\"i .svfr. 

 teract with certain specific bodies of the blood. 

 These bodies which, so to speak, render the 

 bacteria palatable to the leukocytes, are called 

 opsonins (prepare for a meal). 



The function of the opsonins, therefore, is 

 to produce certain physicochemical changes ,,. .... 



in the substance of the bacteria so that the _ 

 leukocytic material is able to react with it. 



Paint applied tO window glass will SOOn O , indicates part played by 



crumble off, but will stick to it for an indefi- opsonin. 



nite period if the glass is first eroded with a 



fluoride. The opsonins are comparable to the eroding fluid. They 

 attack the bacterial substance and lessen its power of resistance so 

 that the leukocytic material is able to combine with it. 



The resistance and immunity of an animal against microbic in- 

 fections depends in a measure upon the phagocytic properties of its 

 leukocytes. But in order to attain this power it is necessary to have 

 at hand not only a sufficient number of leukocytes, but also leukocytes 

 of the proper quality. In addition, it is essential that it be in posses- 

 sion of opsonins, because without these bodies a reaction between the 

 protecting cells and the invading bacteria cannot always be brought 

 about. Conversely, it is true that a large content in opsonins cannot 



1 British Med. Jour., Ixxiii, 1902. 



2 Proc. Royal Soc., London. Ixxii, 1904. A brief discussion of opsonins has 

 been given by Hektoen, in Science, February 12, 1909. 



