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.