THEORIES OF IMMUNITY 131 
cytes, while strychnine seems to increase the so-called opsonic power of 
the blood serum. 
Engulf ment.— The earlier view associated the protrusion of pseudo- 
podia and the subsequent engulfment of bacteria or other cells as an 
autovoluntary act of the leukocyte. The inclusion of inert particles, 
as dust or other minutely comminuted granules, would appear to 
discredit this hypothesis. The engulfment of living or inert bacteria 
or other minute bodies is, as Wells aptly expresses it, "but an exten- 
sion of the phenomena of chemotaxis. When the substance toward 
which the leukocyte is drawn is small enough, the leukocyte simply 
continues its motion until it has flowed entirely about the particle." 
Digestion.— The ultimate solution of engulfed substances other 
than purely inert particles is by intracellular enzymes contained 
within the phagocytic cells. These enzymes are of two kinds: inacro- 
cytase, present in the macrophages, and microcytase, found in the 
microphages.^ Van de Velde,- Buchner,'' Hahn^ and Bordet^ have 
demonstrated such endo-enzymes. The solution of bacteria engulfed 
in leukocytes can be shown by appropriate staining methods; the 
organisms gradually lose their ability to take up stain and eventually 
disappear. 
At this stage in the development of the phagocytic theory of immu- 
nity, the important part played by the blood serum in preparing 
bacteria for phagocytosis was prominently set forth in the investiga- 
tions of Wright and Douglas,*^ although foreshadowed by the excellent 
and comprehensive observations of Denys and LeClef^ and Xeufeld 
and Rimpau.^ Wright and Douglas showed that leukocytes, freed 
carefully from adherent serum by washing with salt solution, would 
not engulf bacteria, or, at least, but slowly. The addition of serum 
from a normal or immunized 'animal caused active phagocytosis to 
take place. The substances in the blood which prepare bacteria for 
engulfment by leukocytes were called "opsonins" by Wright and 
Douglas: the immune opsonins— which are specifically increased in 
immunized animals— are almost certainly identical with the substances 
called bacterial tropins by Xeufeld and Rimpau. That the opsonic 
substances of the serum act primarily upon the bacteria rather than 
upon the leukocytes was clearly shown by the observations of Hektoen 
and Reudiger.^ Streptococci suspended in plasma, blood serum or 
defibrinated blood were engulfed by leukocytes. Leukocytes, washed 
free from seriun or plasma, were without phagocytic action upon the 
same bacteria. If the streptococci, however, were allowed to stand 
in contact with serum, plasma, or defibrinated blood for a short time 
' For a detailed discussion of leukocytic enzymes, see Opie: Jour. Exper Med., 1906, 
8. 410. 
2 Centralbl. f. Bakt., 1898, 23, 692. ' Miinch. med. Wchnschr., 1894, 41, 717. 
* Arch. f. Hyg., 1895, 25, 105. ^ Ann. Inst. Pasteur, 1895, 9, 462. 
« See Wright: Studies in Immunization, 1909, Constable. 
' La Cellule, 1895, 11, 175. 
« Deutsch. med. Wchnschr., 1904, 30, 1458; Ztschr. f. Hyg., 1905, 51, 28.3, 
9 Jour. Infec. Dis., January, 1905. 2, 128. 
