ON THE NATURE OF OPSONINS. 397 



Levaditi* on bacterial sera. We have then in our hemolytic sera 

 also a thermolabile opsonin which becomes fixed to the blood cells 

 and thus causes phagocytosis. It is at once suggested that here 

 the opsonin may be regarded as identical with the specific sensi- 

 tizer. One can, as a matter of fact, accurately remove the specific 

 sensitizer from a hemolytic serum by previously treating it with 

 the corresponding blood corpuscles and then prove that it has at 

 the same time and in the same degree lost its opsonic power. Thus 

 Wakelin Barrattf in his experiments on the quantitative opsonic 

 absorption by blood corpuscles from an inactivated hemolytic 

 serum worked only with the specific sensitizers. If one adds a 

 fresh specific serum to blood corpuscles or bacteria, a combined 

 lytic and opsonic action by sensitizers and alexin is obtained, as in 

 tubes 2 and 3 of the last experiment. 



Let us return for a moment to our experiment with normal serum. 

 We have seen that the opsonin was fixed by specific sensitizers 

 with the alexin and was also destroyed by heat. There remain, 

 however, in the normal sera after fixation or destruction of the 

 opsonins (alexins) almost constantly a few opsonic effects (compare 

 page 363, tubes 2 and 5) which are much greater in the specific sera. 

 And since we do not find any reason for assuming that the alexin 

 in the normal serum becomes fixed to the bacteria without the in- 

 tervention of sensitizers we have to assume, that the opsonic 

 action either of normal or of specific sera is in the main part 

 a combined action of sensitizers and alexin. In normal serum 

 it is caused chiefly by the alexin. In the specific serum it is 

 increased and brought about chiefly by the sensitizers. I concur, 

 therefore, with Dean$ in the belief that normal as well as specific 

 opsonins have a dualistic complexity. § 



* C. R. Soc. de Biologie, April and May, 1907. 

 t Proc. Roy. Soc, 1905 and 1907. 

 t Proc. Roy. Soc, 1905 and 1907. 



§ See, further, my dissertation, "Phagocytose en Opsoninen," Amsterdam, 

 1908, where I explain more fully the nature and action of the opsonins. 



