and under precisely the same conditions. The
average number of bacteria contained within leucocytes in the case tested,
divided by the number given by the normal serum, is called the _phagocytic
index_. Wright and Douglas showed that under these conditions phagocytosis
might occur when a small quantity of normal serum was present, whereas it
was absent when normal salt solution was substituted for the serum; the
latter thus contained substances which made the organisms susceptible to
the action of the phagocytosis. They further showed that this substance
acted by combining with the organisms and apparently producing some
alteration in them; on the other hand it had no direct action on the
leucocytes. This opsonin of normal serum is very labile, being rapidly
destroyed at 55deg C.; that is, a serum heated at this temperature has
practically no greater effect in aiding phagocytosis than normal salt
solution has. Various observers had previously found that the serum of an
animal immunized against [v.03 p.0179] a particular bacterium had a special
action in bringing about phagocytosis of that organism, and it had been
found that this property was retained when the serum was heated at 55deg C.
It is now generally admitted that at least two distinct classes of
substances are concerned in opsonic action, that thermostable immune
opsonins are developed as a result of active immunization and these possess
the specific properties of anti-substances in general, that is, act only on
the corresponding bacterium. On the contrary the labile opsonins of normal
serum have a comparatively general action on different organisms. It is
quite evident that the specific immune-opsonins may play a very important
part in the phenomena of immunity, as by their means the organisms are
taken up more actively by the phagocytic cells, and thereafter may undergo
rapid disintegration.

The opsonic action of the serum has been employed by Sir A. Wright and his
co-workers to control the treatment of bacterial infections by vaccines;
that is, by injections of varying amounts of a dead culture of the
corresponding bacterium. The object in such treatment is to raise the
opsonic index of the serum, this being taken as an indication of increased
immunity. The effect of the injection of a small quantity of vaccine is
usually to produce an increase in the opsonic index within a few days. If
then an additional quantity of vaccine be injected there occurs a fall i