OPSONINS 209 



is the danger to life. It follows from this that a large opsonin blood 

 content is more protective than a powerful bactericidal blood. 



Certain bacteria are readily devoured by phagocytes in normal 

 salt solution or in inactivated normal serum. In other words, with 

 these bacteria opsonins are not essential to phagocytosis. Naturally 

 these are avirulent organisms. It must not be inferred, however, that 

 all avirulent bacteria are phagocitized without the aid of opsonins. The 

 phagocytic-favoring substances are of two kinds. The first, known as 

 tropins, are evidently of relatively simple structure, are not inactivated 

 by heat, and consequently are present in heated serum. The true 

 opsonins are more complicated, consisting of a thermolabile part, known 

 as alexin or complement, and a thermostabile part, known as fixator 

 or amboceptor. Since both parts are essential to complete action, 

 serum is inactivated by heat. Neither tropins or opsonins induce recog- 

 nizable alterations in the bacteria which they prepare for phagocyto- 

 sis. In the study of phagocytosis in vitro, MetschnikofFs finding that 

 bacteria are devoured by the polynuclear leukocytes has been confirmed. 

 The large mononuclear cells are not wholly indifferent in the disposal 

 of bacteria but in this they play a very subordinate role. On the 

 other hand, the macrophages are the chief consumers of animal cells, 

 such as red corpuscles, spermatozoa, etc. 



It was shown early in these studies that dead, highly virulent bac- 

 teria are not readily taken up by leukocytes except in the presence of 

 immune serum. This indicates that dead, as well as living bacteria, 

 especially if of virulent strains, are prepared by the opsonins as food 

 for the leukocytes. From this we infer that in whatever reaction there 

 may be between the opsonins and the bacterial cells, the later show 

 no vital activity. 



It was shown by Neufeld and Handel that when rabbits have been 

 treated with injections of milk or egg white, their serum causes leu- 

 kocytes to engulf milk globules and albumin emulsified with oil. The 

 engulfment is not due to the oil but to the casein or albuminous 

 envelopes. 



It is well to show that the opsonins do not act by stimulating the 

 leukocytes. When leukocytes are kept in contact with immune serum 

 for twenty minutes at 37 C, then removed and washed wholly free 

 from the serum and then brought into contact with the bacteria there 



