28 INFECTIOUS AND PARASITIC DISEASES. 



importance. This is a bactericidal antibody, opsonin 

 (I prepare a meal for), that acts in conjunction with the 

 leucocytes to rid the body of invading bacteria. 



Opsonin is normally found in the blood, and is 

 increased in immunity. It acts in such a way upon 

 infectious agents that they not only strongly attract the 

 phagocytic leucocytes (positive chemotaxis), but makes 

 them easier of digestion by the latter. It is well that 

 the associated action of opsonin and phagocytosis 

 has been discovered, for bacteriologists had well-nigh 

 despaired of producing bactericidal serums comparable 

 in potency to diphtheria and tetanus antitoxins; the 

 latter, once their toxins were separated from the bac- 

 teria, were comparatively easy to prepare; but in the 

 case of those bacteria in which little or no diffusible 

 poison is secreted, the problem of making immune 

 serums presented insurmountable difficulties. This 

 is the reason that in a long list of diseases, such as 

 typhoid fever, bubonic plague, Asiatic cholera, bacillary 

 dysentery, etc., immune serums have not been forth- 

 coming. In opsonin, however, we have a force to 

 work with which, it would appear, can be increased 

 at will, so that if the expectations of workers with 

 opsonins are realized, it will not be long before the 

 immunization and cure of the above mentioned mala- 

 dies will be in our hands. 



It is now almost twenty years since Metchnikoff 

 first called attention to the part that certain white 



