CHAPTER XXX. 

 PHAGOCYTOSIS-OPSONIXS. 



It has been mentioned that Metchnikoff, in a pubUcation 

 in 1883, attempted to explain immunity on a purely cellular 

 basis. It has been known since Haeckel's first >observation 

 in 1858 that certain of the white corpuscles do engulf solid 

 particles that may get into the body, and among them bac- 

 teria. Metchnikoff at first thought that this engulfing and 

 subsequent intracellular digestion of the microorganisms 

 were sufficient to protect the body from infection. The 

 later discoveries (discussed in considering Ehrlich's, theory 

 of immunity) of substances present in the blood serum and 

 even in the blood plasma which either destroy the bacteria 

 or neutralize their action have caused Metchnikoff to modify 

 his theory to a great extent. He admitted the presence of 

 these substances, though giving them other names, but ascribed 

 their formation to the phagocytes or to the same organs 

 which form the leukocytes — lymphoid tissue generally, bone 

 marrow. It is not within the province of this work to 

 attempt to reconcile these theories, but it may be well to 

 point out that Ehrlich's theory is one of chemical substances 

 and that the origin of these substances is not an essential 

 part of the theory, so that the two theories, except in some 

 minor details, are not necessarily mutually exclusive. 



Sir A. E. Wright and Douglas, in 1903, showed that even in 

 those instances where immunity depends on phagocx-tosis, 

 as it certainly does in many cases, the phagocytes are more 

 or less inactive unless they are aided by chemical substances 

 present in the blood. These substances act on the bacteria, 

 not on the leukocytes, and change them in such a way that 

 they are more readily taken up by the phagocytes. Wright 

 proposed for these bodies the name opsonin, derived from a 



