126 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



cells or leucocytes. Nearly forty years ago Metchni- 

 koff, the Russian biologist, saw for the first time that 

 some of the white blood cells are able to pick up, 

 surround by ameboid movement, and finally to 

 destructively digest animalcules, bacteria, etc. This 

 remarkable observation has been variously inter- 

 preted, but there has emerged from it one fact deeply 

 significant for the bodily defenses. These primitive, 

 unspecialized cells, the leucocytes, are able to sur- 

 round and destructively digest bacteria which have 

 entered the human blood stream, provided these 

 bacteria have first been given the coup de grdce by 

 other agencies. Fortunately the blood possesses 

 in health a small but potent supply of highly organ- 

 ized substances capable of injuring various types of 

 bacterial enemies of man in such a way that the 

 leucocytes, attached to the invaded spot, can finally 

 dispose of them. These substances were called 

 opsonins by Sir A. E. Wright, who first discovered 

 that the ability of the leucocytes to incorporate pus- 

 making bacteria or tubercle bacilli or typhoid bacilli 

 depends upon the existence of these opsonins. It is 

 fair to think of these opsonins as highly complex 

 labile materials which act as preparative sauces to 

 bacteria, rendering them attractive and accessible 

 to the finally destructive leucocytes. Some kinds 

 of immunity or protection against special bacteria 

 seem to depend upon this combined action of fluids 

 and cells ; and, on the other hand, a decline in 

 opsonins causes a falling off in the capacity of the 



