The separate action of these substances can be studied since one is _thermolabile_, or destroyed by heating the serum to one hundred and thirty-three degrees; the other _thermostabile_, or capable of withstanding a greater degree of heat.

These substances are known only by their effect, they have never been separated from the serum.

The thermostabile substance, or _amboceptor_, as it is generally called, has in itself no destructive action on the bacteria; but in some way so alters them that they can be acted on by the thermolabile substance called _complement_ whose action is destructive.

The amount of amboceptor may increase in the course of infection and its formation stimulated, the amount of complement remains unchanged.

The action of 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.