166 ANTIGENS AND THE TECHNIC OF SERUM REACTIONS 



their injection into susceptible animals stimulated the formation 

 of "antiaggressin," which greatly increased the resistance of the 

 animal to subsequent infection. The sera of animals immunized 

 with aggressin-containing fluids conferred a limited degree of immunity 

 to specific infections in non-immune animals (passive immunity). 

 It has been claimed by Doerr 1 and others that the aggressins are of 

 the nature of bacterial endotoxins and that the immunizing properties 

 of aggressin fluids are due to their content of specific substances 

 derived from the autolysis of bacterial cells. 



The aggressin theory must, for the present, be regarded as not 

 definitely proved. 



OPSONINS. TROPINS. BACTERIAL VACCINES. 



A most important contribution to the literature of immunity is 

 the work of Denys and his associates, 2 who showed that the sera of 

 rabbits immunized to Streptococcus pyogenes possessed two properties 

 not exhibited by the serum of a normal animal, namely, the property 

 of restricting the development of the organism, and the property of 

 stimulating phagocytosis. Their very comprehensive studies demon- 

 strated that the leukocytes of normal animals, suspended in the serum 

 of immunized animals, phagocytized streptococci energetically, but 

 the leukocytes of immunized animals suspended in normal serum 

 failed to exhibit phagocytic activity. Their conclusion was that the 

 immunity of rabbits to the streptococcus resides in the serum. These 

 observations not only added materially to the restricted field in which 

 they were cast they brought sharply into focus the interrelation of 

 the humoral and cellular aspects of immunity. 



Wright and Douglas, 3 using a modification of the technic of Leish- 

 man, 4 were able to study phagocytosis in vitro: by an ingenious series 

 of experiments they showed that normal serum contains substances 

 opsonins which prepare bacteria for phagocytosis, as described 

 in a preceding section (Cellular Immunity). The technic of meas- 

 uring the potency of opsonins in the sera of normal and infected 

 individuals, as practised by Wright and his associates, consisted 



1 Wien. klin. Woch., 1906, No. 25. 



2 Denys and Le Clef, La Cellule, 1895, xii; Bull, de 1'Acad. roy. de Belgique, 1895; 

 Denys and Marchand, Ibid., 1896; Van de Velde, Ann. Inst. Past., 1886, x; Marchand 

 Arch, de Med. exp., 1898; Denys, Cent. f. Bakt., 1898, xxiv, 685. 



3 See Studies in Immunization, Constable, 1909, for complete biography. 

 Brit. Med. Jour., 1902, i, 73. 



