ed out by M. Verworn to
depend upon stimulation to contraction or the reverse. Metchnikoff showed
that in animals immune to a given organism phagocytosis is present, whereas
in susceptible animals it is deficient or absent. He also showed that the
development of artificial immunity is attended by the appearance of
phagocytosis; also, when an anti-serum is injected into an animal, the
phagocytes which formerly were indifferent might move towards and destroy
the bacteria. In the light of all the facts, however, especially those with
regard to anti-bacterial sera, the presence of phagocytosis cannot be
regarded as the essence of immunity, but rather the evidence of its
existence. The increased ingestion of bacteria in active immunity would
seem to depend upon the presence of immune opsonins in the serum. These, as
already explained, are true anti-substances. Thus the apparent increased
activity of the leucocytes is due to a preliminary effect of the opsonins
on the bacteria. We have no distinct proof that there occurs in active
immunity any education of the phagocytes, in Metchnikoff's sense, that is,
any increase of the inherent ingestive or digestive activity of these
cells. There is some evidence that in certain cases anti-substances may act
upon the leucocytes, and to these the name of "stimulins" has been given.
We cannot, however, say that these play an important part in immunity, and
even if it were so, the essential factor would be the development of the
substances which act in this way. While in immunity there probably occurs
no marked change in the leucocytes themselves, it must be admitted that the
increased destruction of bacteria by these cells is of the highest
importance. This, as already pointed out, depends upon the increase of
opsonins, though it is also to be noted that in many infective conditions
there is another factor present, namely a leucocytosis, that is, an
increase of the leucocytes in the blood, and the defensive powers of the
body are thereby increased. Evidence has been brought forward within recent
years that the leucocytes may constitute an important source of the
antagonistic substances which appear in the serum. Much of such evidence
possesses considerable weight, and seeing that these cells possess active
digestive powers it is by no means improbable that substances with
corresponding properties may be set free by them. To ascribe such powers to
them exclusively is, however, not justifiable