IMMUNITY AGAINST BACTERIAL CELLS 147 



nuclein does not remove the immune bodies from the serum. 

 Immune serum kept three months in alcohol yielded an extract 

 with distilled water that was rich in immune bodies, but almost 

 free from proteid. Pick, Rhodain, and Fuhrmann found that 

 immune bodies are precipitated entirely in the euglobulin frac- 

 tion of the serum proteids. From these experiments it seems 

 probable that the immune body is not itself a proteid, although 

 closely associated with the serum globulins. 1 



Opsonin. 2 Bactericidal substances are not so readily pro- 

 duced for all bacterial organisms as they are for typhoid bacilli, 

 cholera spirilla, etc., particularly not for the pus cocci, B. anthra- 

 cis, and B. tuberculosis. In defending the body against these 

 organisms it would seem that phagocytosis by leucocytes is 

 an important process, but in just what the difference lies 

 between immunized and normal animals was formerly not clear. 

 It now seems to have been established, particularly by the 

 work of Wright and Douglas, 3 that phagocytosis depends 

 upon the presence of certain substances in the plasma, which 

 they call opsonins. Not until bacteria have been acted upon 

 by the opsonin can they be taken up by the phagocytes. 

 Opsonin exists in the normal blood of many animals, and can 

 be increased by immunization, and the opsonin of one species 

 of animal can sensitize bacteria for the phagocytes of another 

 species. It resembles toxin and complement in having a hapto- 

 phore group to combine with the bacteria, and an opsoniferous 

 group susceptible to heat of 60-65 ; when thus heated, the 

 opsonin is converted into an opsonoid. Nothing is yet known 

 concerning the change brought about in the bacteria by the 

 opsonin, although it has been established that it is the bacteria 

 that are modified and not the leucocytes. The chemical nature 

 of the opsonins is likewise unknown, except that they may com- 

 bine with certain inorganic ions and are then inert (Hektoen 

 and Ruediger 4 ). This topic is discussed further in connection 

 with phagocytosis. 



Antienzymes. The development of substances inhibiting the action 

 of bacterial enzymes during the course of immunization has been dis- 

 cussed in a preceding chapter (under " Enzymes"). Their importance 



1 Ascoli found that the active substance of anthracidal serum, which is not 

 an amboceptor, is contained in the pseudo-globulin fraction of asses' serum, 

 but in goat's serum part is in the euglobulin fraction. (Biochem. Centr., 1906 

 (5), 458.) 



2 Ke"sum6 and literature by Hektoen, Jour. Amer. Med. Assoc., 1906 (46), 

 1407. 



3 Proc. of the Koyal Society, 1903 (72), 357; 1904 (73), 128. 



4 Jour. Infectious Diseases, 1905 (2), 129. 



