160 THE PRINCIPLES OF IMMUNOLOGY 



actual difference in the nature of normal and immune opsonin, but 

 this view has now been almost entirely abandoned. In the discussion 

 of this change of view it is essential to present first the development of 

 work in regard to the normal opsonin. Conservative workers were 

 not disposed to accept the opsonin as a new form of antibody and 

 from the ease of deterioration of the normal opsonin thought that it 

 was identical with complement. Furthermore, it was shown that fixa- 

 tion of complement by a hemolytic system or sensitized bacteria re- 

 moves the opsonin, that yeast cells, cell detritus and bacteria will absorb 

 both opsonin and complement, that blood serum and edema fluids 

 contain parallel amounts of opsonin and complement, that certain body 

 fluids, such as the aqueous humor of the eye, contain neither complement 

 nor opsonin. Nevertheless, the removal of complement, as by heating, 

 does not, as Hektoen has shown, remove all the normal opsonic power 

 of the serum; and the fixation of complement by a hemolytic system 

 or by sensitized bacteria still leaves slight opsonic power in the serum. 

 The addition of fresh serum to a slightly active heated serum restores 

 the activity practically to normal in much the same manner as a 

 hemolytic amboceptor may be reactivated by complement. The fol- 

 lowing example taken from Cowie and Chapman and slightly modified 

 serves to illustrate this reactivation. The substances indicated in the 

 protocol are added to leucocyte and bacterial emulsions and the figures 

 given are for the bacterial count per leucocyte: 



1. Unheated (normal) serum 1544 



2. Salt solution 0.18 



3. Heated serum 57 C 1.08 



4. Normal unheated serum, diluted i to 15 1.56 



5. Heated serum -f- normal serum diluted i to 15 12.40 



6. Two volumes unheated normal serum 16.08 



Thus it will be seen that heating the serum reduces the phagocytic 

 index from 15.44 to 1.08; that normal serum, diluted so that its 

 phagocytic index is reduced to 1.56, added to heated serum, raises 

 the index to 12.40, much higher than can be accounted for by the 

 total indices of the two components. It can then be concluded that 

 the normal opsonic power of serum depends upon two factors, a weakly 

 acting thermostable element and a thermolabile element which markedly 

 adds to the combined power of the mixture. Cowie and Chapman 

 have shown that at o C. the thermostable element of opsonin is ab- 

 sorbed by the bacteria, but that the thermolabile element remains in 

 the supernatant fluid and is capable of reactivating a heated serum. 

 It has also been demonstrated by absorption experiments that the ther- 

 mostable element is specific. Numerous continental workers contradict 

 this statement, but their studies have, for the most part, ignored the 

 existence of the thermostable element of normal opsonins. Hektoen 

 has shown that saturation of the bacteria with opsonin and heating so 

 as to destroy the thermolabile part leaves the bacteria in such condi- 

 tion that they cannot absorb any more opsonin from another serum. 



