130 IMMUNITY AND INFECTION 
MetchnikoflF. These cytases are liberated into the blood stream 
when the leukocytes are destroyed (phagolysis) . 
The phenomenon of phagocytosis may be divided into three sepa- 
rate and distinct phases: the method of approach of the phagocytic 
cell to its prey (chemotaxis), the engiilfment, and finally the digestion 
or destruction of the latter. 
The Method of Approach. — It was a matter of observation by 
MetchnikoflF and his followers that phagocytosis was more marked in 
mild bacterial infections and during recovery than in severe infections 
and the early acute stages of the disease. The importance of chemo- 
taxis as the attractive force of leukocytes to bacteria, however, was 
not clearly realized until Massart and Bordet^ showed by ingenious 
experiments that non-virulent bacteria apparently secrete substances 
which draw phagocytic cells to them.- Virulent organisms of the 
same strain not only do not appear to attract leukocytes, but they seem 
to repel them. Bordet explained the increase of virulence of bacteria 
through passage in experimental animals on the ground that the less 
virulent individuals were engulfed and killed; the more virulent mem- 
bers survived and produced a thoroughly virulent strain. Vaillard 
and Vincent^ and Vaillard and Rouget^ showed that bacterial toxins 
may repel or paralyze leukocytic activity; if tetanus spores are bathed 
with tetanus toxin before injection into the animal body, the leukocytes 
do not collect at the point of injection, the spores germinate and the 
animal dies of tetanus. If, however, the spores are washed free from 
tetanus toxin and then injected, leukocytes appear at the site of inocu- 
lation, engulf the spores, and either destroy them or prevent their 
germination. 
The mechanism of chemotaxis has been a subject of much discus- 
sion. Evidence is accumulating which would suggest that chemo- 
tactic stimuli of bacterial origin which reach leukocytes, enter the 
phagocytic cell in greater concentration on that side which is nearer 
the source of the chemotactic substance, lowering the surface tension 
at that point. A flow of protoplasm in this direction, in obedience 
to the lowered resistance, will result in the protrusion of a pseudo- 
podium, which will continue to advance until the surface tension is 
equalized.^ This generally occurs when the leukocyte has flowed 
around or engulfed the organism. Arkin'' has studied the influence 
of certain therapeutic agents upon chemotaxis. In general substances 
which tend to inhibit oxidation, as chloroform, ether, alcohol and 
morphine decrease phagocytosis: calcium and magnesium chloride, 
colloidal metals, strychnine and arsphenamine appear to stimulate 
phagocytosis. Morphia appears to act more directly upon the leuko- 
1 Ann. Inst. Pasteur, 1891, 5, 417. 
' Inert particles, as coal dust, are engulfed by phagocytic cells; it is difficult to explain 
this phenomenon on the basis of chemotaxis. 
3 Ann. Inst. Pasteur, 1891, 5, 1. ' Ibid., 1892, 6, 385. 
6 See W^ell's Chemical Pathology, 1914, 2d ed., pp. 230-251 (W. B, Saunders Com- 
pany) , for an excellent resume of the literature. 
6 Jour. Infec. Pis., 1913, 13, 408, 
