the amboceptor is specific, that is, directed against a single species
of bacterium only; the destructive power of the blood may be very
great against a single bacterium species and have no effect on others.
There seem naturally to be many different amboceptors in the blood,
and the number may be very greatly increased. It has been shown as a
result of the work of many investigators that the shield has two
faces,--there is destruction both by cells and fluids and there is
interaction by both. The amboceptors so necessary for the destructive
action of the serum are produced by the body cells, particularly the
leucocytes. The serum assists in pagocytosis by the action on bacteria
of substances called _opsonins_ which are contained in it, and
the formation of which can be very greatly stimulated. Again, not all
inclusion of bacteria within leucocytes is indicative of phagocytosis;
in many cases the bacteria seem to find the best conditions for
existence within the leucocytes, and these and not the bacteria are
destroyed.

So far it has been shown that the best defence of the body is, as is
the best defence in war, by offensive measures, as illustrated by
phagocytosis and destruction by the serum. Both of these actions can
be increased by their exercise just as the strength of muscular
contraction can be increased by exercise, and the facility for doing
everything increased by habit. Certain of the infectious diseases are,
as has been said, essentially toxic in their nature, and in cultures
the organisms produce poisonous substances. By the injection into the
tissues of such substances the same disturbances are produced as when
the bacteria are injected. Such a disease is diphtheria. In this there
is only a superficial invasion of the tissues. The diphtheria bacilli
are located on the surface of the tonsils or pharynx or windpipe,
where, as a result of their action, the membrane so characteristic of