resemble a child who will not eat dry
toast, or, at best, only slowly, but will devour rapidly many pieces
when the toast is buttered. It is of the utmost importance to us that
our white corpuscles, or eater-cells, should not be sluggish but
greedy.

There are some microbes which will produce deadly poison if grown in
the clear fluid (serum) of the blood of an animal (as, for instance,
the cholera-microbe when grown in the serum of the frog's blood), yet
when inoculated living into the blood of that animal never cause the
slightest illness! Why? Because they are at once eaten by the vigilant
phagocytes of the blood before they can produce any appreciable amount
of poison. That is easily demonstrated by experiment. Our main means
of defence against microbial disease, says Metchnikoff--though
cleanliness and precaution against access of microbes are all very
well in their way--is the activity of our phagocytes. Now it appears
that just as in the other cases I have been considering, so in the
production of "relish," the power to produce it resides in the blood
(and perhaps the cells of its vessels), but is not set at work until
the enemy is in the blood. Suppose there is an infection, an invasion
of the blood and tissues by one or other disease-causing microbe.
Gradually if the body is healthy the "relish" is produced and becomes
attached to the invading microbes. The phagocytes swallow them
greedily and make an end of the invasion.

It is proved that this aroused avidity of the phagocytes is due to no
change in the phagocytes themselves; since if they are transferred to
the serum of a normal man they show no such predilection for the
special invading microbe. The "opsonin," or "relish," is something
exuded into or produced in the blood fluid when the attacking microbe
arrives. It attaches itself to them: that is the essential fact. In
many of us the phagocytes are not at a given moment so "avid" of this
or that disease-microbe as they should be in order to protect us from
its multiplication and poison production. But it is found that by
injecting boiled and cooled (therefore dead) microbes of a particular
kind into the blood of a man, you can start the production of the
"relish" appropriate to that kind. The dead microbes answer this
purpose; they excite the production of the opsonin appropriate to them
and yet are not themselves dangerous, since they are dead. When
subsequently (or possibly concurrently in small quanti