"immunity" are incorrect.
Nature provides at least three varieties of defence within the blood
of higher animals against disease-producing microbes which have broken
through the outer line of fortification, the skin. These three methods
are effective in different cases (one in this disease, the other in
that), and, on the whole, are sufficient to preserve the races of
animals (including man) from complete destruction. These are (1) the
production in the blood of an antidote to the toxin or poison
elaborated by the invading microbe--an antitoxin, which chemically
neutralises the toxin; (2) the production in the blood of the attacked
animal of a "germicidal" poison which repels and kills the attacking
microbes themselves (not merely neutralising their poisonous
products); (3) the extermination of the intrusive, disease-producing
microbes by a kind of police, which scour the blood channels and
tissues and "eat up"--actually engulf and digest--the hostile
intruders. These latter agents, actual particles of the living animal
in which they exist, are the "eater-cells," or "phagocytes"--minute,
viscid, actively moving cells, resembling the animalcules called
"amoeba." They are only the one two-thousandth of an inch in
diameter, and are known as the white or colourless corpuscles of the
blood. They are far less numerous than the red blood-corpuscles, which
are the agents for carrying oxygen, but there are eight thousand
million of them in a large spoonful of blood. They are the really
important agents in protecting us from microbes, since they not only
engulf and digest and so destroy those intruders, but it is probable
(not certain) that they also are the manufacturers of the antitoxins
and of the germicidal poisons.

* * * * *

If these three defensive processes given us by Nature are in working
order, that is to say, if we are "healthy," they should secure to us a
sufficient "immunity"--at at any rate, "recovery"--from any attack of
disease-producing microbes. But they are not in "unselected," widely
ranging mankind always equal (in their unaided natural state) to their
task.

The attempts to produce immunity by vaccination with weakened or
localised disease germs is really an attempt to train and develop to a
high point the activities of the phagocytes or eater-cells of the
blood.

The introduction of antitoxins by injection of them into the blood (as
in the treatment of diphtheria, loc