188 CHEMISTRY OF THE IMMUNITY REACTIONS 



may enter the fetal blood from the blood of pregnant female animals. 

 The presence of precipitins in the blood does not seem to prevent 

 the excretion of the foreign protein in the urine, nor are the animals 

 less susceptible to the toxic action of the foreign protein; indeed, the 

 reaction is even stronger in the immunized animals, and sometimes 

 the ordinary dose becomes fatal. Precipitin and antigen may coexist 

 ununited in the circulating blood under certain conditions. "^^ Certain 

 antibodies are carried down with the precipitates formed when the 

 serum containing them reacts under proper conditions with an anti- 

 serum; e. g., diphtheria antitoxin is precipitated when added to the 

 serum of a rabbit immunized to horse serum. This is not true of all 

 antibodies, however, '^^ As the precipitates formed in the precipitin 

 reaction, when injected into a guinea-pig make it passively hj'persensi- 

 tive to the protein used as antigen in the precipitin reaction, it would 

 seem that the precipitin and the anaphylactin are identical (Weil),®^ 

 or at least closely associated. 



Chemical Properties. — In its chemical nature precipitin reseml^les the "anti- 

 bodies" generallj', being precipitated in the euglobulin fraction of the serum, ^' 

 and slowly destroyed by trypsin, rapidly by pepsin. It cannot be separated 

 from the serum proteins. The precipitation by precipitins is not an enzjmie 

 action, for the precipitins are used up in the process. It apparently' does not 

 differ from precipitations of colloids by other colloids of opposite electrical charges, 

 except in that the reaction is specific. 



OPSONINS^s 



The correlation of phagocytic and serum immunity was accomplished when 

 A. E. Wright showed that, before any considcrat)le phagocA^tosis of bacteria 

 can take place, the bacteria must first be acted upon by serum, which in some way 

 prepares them to be ingested by the leucocj^tes. The hj-pothetical substances 

 accomplishing this sensitization of the bacteria were called opsonins by Wright 

 and they exist to a certain extent in normal serum, being increased bj- immuniza- 

 tion. Not only bacteria, but cellular elements in general, including especially 

 red corpuscles, and even unorganized particles (such as melanin), "^^ are sensitized 

 for phagocytosis by opsonins. Probably phagocytosis by endothelial"" and other 

 cells also requires sensitization of the bacteria by opsonins. Although there have 

 been many expressions of the opinion that the opsonins are not distinct antibodies, 

 but are identical with agglutinins, bacteriolytic amboceptors, or other antibodies, 

 there is much evidence to the contrary."' However, the union of opsonin and 

 bacteria seems to follow the same quantitative laws as otlior antigen-antibody 

 reactions (Amato)." 



There are two opsonizing elements in serum, one thermostable and one thermola- 

 bile, it being the former which is increased during immunization; the thermostable 



" Bayne-Jones, Jour. Exp. Med., 1917 (25), 837. 



^^ 8ee Gay and Stone, Jour. Immunol., 191(5 (1), 83. 



" Jour. Immunol., 1910 (1), 1. 



6' Funck (Cent. f. Bakt. (Kef.), 1905 (30), 744) states that if the precipitin 

 serum is very strong, part of the precipitin conies down in the pscudoglobulin. 



** Bibliography given by Neuield, Kolle and Wasscrmann's Handhucii, 1913 

 (2), 440. 



«» Shattock and Dudgeon, Proc. Royal Soc. (B), 1908 (80), 165. 



70 Briscoe, .lour. Patli. and Bact., 1907 (12), GO. iSee also Manwaring and Coe, 

 who found tliat tlie Kupffer cells can take up only opsonized pneumococci (Proc. 

 See. Exp. Biol., 191G (13), 171). 



" See Ilcktoen, Jour. Infec. Dis., 1909 (0), 78; 1913 (12), 1. 



" Sporimentale, 1918 (71), 459. 



