168 VETERINARY BACTERIOLOGY 



cytes. Evidently the presence of the serum, or rather the op- 

 sonin which it contains, induces phagocytosis. Opsonins are 

 believed so to change or alter the bacteria as to make them posi- 

 tively chemotactic for the phagocytes. This does not mean that 

 the bacteria are destroyed, for there is no evidence that they are 

 in any way injured. That it is an alteration of the organisms, 

 and not a mere stimulation of the phagocytic cell, may be shown as 

 follows: A suspension of washed corpuscles prepared as before 

 is treated with immune serum, allowed to stand, and then washed. 

 When mixed with the bacterial suspension, no phagocytosis occurs; 

 evidently the opsonin is not bound to the phagocytes, nor does it 

 stimulate them when they are brought in contact with the bacte- 

 rial cells. The converse of this experiment may be tried, the bac- 

 teria added to the immune serum, and then centrifuged out and 

 washed free from all the serum with salt solution. When these 

 organisms are added to a suspension of the washed leukocytes, 

 active phagocytosis occurs. The opsonin is bound to the bacterial 

 cell, and makes it in a sense attractive to the leukocytes. The 

 negative chemotaxis is converted by this union into a positive 

 chemotaxis. The action of the opsonin has been likened to that 

 of an amboceptor, for it links up the bacteria and the white blood- 

 cells. The opsonins are, however, not identical with bacteriolytic 

 amboceptors. 



Opsonins for some organisms are quite constantly present in 

 the blood. For example, human blood contains opsonins for the 

 organisms producing bubonic plague, Malta fever, pneumonia, 

 dysentery, Asiatic cholera, and for the pyogenic organisms. 

 These are called normal opsonins. Immune opsonins are those 

 produced as a result of infection or immunization. It is also 

 probable that some species of bacteria may be taken up by phago- 

 cytes in total absence of opsonins. On the other hand, it is be- 

 lieved that certain bacteria, when they invade the body, may 

 develop a resistance to phagocytosis. The appearance of the 

 capsule about the anthrax bacillus in the blood has been thought 

 to be a protective agency of this character. Whether the im- 

 mune and the normal opsonins are identical is a moot question. 

 Probably they may differ, it being contended that the normal 

 opsonin is simply the normal complement of the serum, for it has 



