CHAPTER XIII 



OPSONINS, AGGLUTININS, PRECIPITINS, LYSIN 



Opsonins. The process of phagocytosis has been found to 

 depend not so much on inherent properties of the phagocytes as 

 upon a certain substance present in the body fluids. Apart from 

 this substance the leukocytes do not become phagocytes. Metch- 

 nikoff suggested that the presence of this substance stimulated the 

 leukocytes to become active. Later studies, however, have shown 

 that the substance acts directly upon the bacteria and renders 

 them more attractive to and easily digested by the leukocytes. 

 These bacteriotropic substances have been named opsonins (from 

 opsono, I prepare food for). Opsonins are present normally in 

 the blood. They may, however, be greatly increased in immuni- 

 zation. 



Bacteria differ in their susceptibility to opsonins. Their resist- 

 ance may be due to capsule formation or, according to the theory 

 of Welch, to actual self-immunization of the bacteria. Opsonins 

 are much more active in some infections than in others ; they are 

 especially operative in pyogenic conditions, in which phagocytosis 

 is recognized as the chief defensive force. The relation of normal 

 and immune opsonins to the other antibodies is as yet unsettled ; 

 they may be of the same nature as amboceptor and complement 

 or they may be separate antibodies. However that may be, they 

 are an important factor in the production of immunity, since it is 

 upon their action that phagocytosis depends. 



The existence of opsonins in a given serum is easily demonstrated 

 by mixing the serum with a suspension of bacteria and adding 

 washed leukocytes; the leukocytes will in all probability take 

 up large numbers of the bacteria. If, however, leukocytes washed 



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