INFECTION AND IMMUNITY. 567 



prepare food) in designation of the element in the blood 

 having that property. Prior to the observations of Wright 

 and his associates it had been known that if white blood 

 cells be washed free of all adhering serum they are 

 incapable of taking up bacteria, but the interpreta- 

 tion, in the light of Wright's work, seems to be incor- 

 rect. It was believed that a something in the blood, a 

 " stimulin " as it was called by some, acted not on the 

 bacteria but on the leucocytes, stimulating them to ac- 

 tivity. Wright and his colleagues have clearly shown the 

 error of this view and have convincingly demonstrated 

 that it is the action of their a opsonin " on the bacteria 

 that makes phagocytosis possible. Thus, for instance, 

 if bacteria and washed leucocytes be brought together 

 the bacteria are not taken up by the cells ; if on the 

 other hand a drop of normal serum be added, phago- 

 cytosis begins. Or, if bacteria be immersed in normal 

 serum and then carefully cleaned of all adherent serum 

 by washing they will readily be taken up by leucocytes, 

 even those also freed of all serum by careful washing. 

 In short the action of the serum on the bacteria, through 

 its "opsonin," has been to make them ingestible or 

 digestible for the leucocytes. 



This opsonizing property of the blood varies. Under 

 conditions depressing general health it may be dimin- 

 ished ; while in the course of infective diseases it is 

 sometimes lessened, sometimes increased. It may be 

 increased by immunization. 



The nature of opsonin (or opsonins) is not known. It 

 has been suggested that they are allied to the enzymes. 

 They are destroyed by heat. They may be absorbed en- 

 tirely from the blood by bacteria with which they com- 



