s all the difference between "poison"
and "meati."

If the phagocytes and similar cells in the blood of a man or animal
exposed to the poison produced by localised microbes (such as those of
tetanus, diphtheria and septic growths) cannot produce enough
antitoxin so as to quickly destroy the poison, we can, and do,
nowadays, save his life, by injecting into his blood the required
antitoxin, obtained from another animal which we have caused (by
injection of the toxin) to produce the antitoxin in excess. That is
one sort of "immunity" or "resistance" which we can confer, and is
largely in use at the present day--the "antitoxin" treatment.

The second poison-repelling chemical activity of the blood, produced
by the living cells in and about it, consists in the blood becoming
directly poisonous to injurious microbes. It becomes "bactericidal,"
produces a bactericidal poison (called an alexin) which is usually
present in normal blood, but is greatly increased when large numbers
of certain poisonous microbes (_e.g._ those of typhoid fever) get into
the blood. Again, by other chemical substances produced in it, the
blood may, without actually killing the invading bacteria, only
paralyse them, and cause them to "agglutinate" (that is, to adhere to
one another as an inactive "clot" or "lump"). As the "agglutinating"
poison is peculiar (or nearly so) for each kind of microbe, we can
tell whether a patient has typhoid by drawing a drop of his
blood into a tube, and adding some fresh living typhoid bacilli
to it. If the patient had typhoid he will have begun to form the
"typhoid-agglutinating" or "typhoid-paralysing" poison in his blood,
and the experiment will result in the "agglutination" (sticking
together in a lump) of the typhoid bacilli. And so we prove, in a
doubtful case, that the patient has typhoid.

The third chemical activity of the blood in dealing with poisonous
microbes is also one which is conferred upon it by its living cells
when excited by the presence of those microbes. It is the production
of a "relish" (for so it must be called) which attaches itself to the
microbes and renders them attractive to the eater-cells (the
phagocytes), so that those swarming amoeba-like floating particles
at once proceed to engulf the microbes with avidity. In the absence of
the relish (the Greek word for it used by Sir Almroth Wright, its
discoverer, is "opsonin"), the eater-cells are sluggish--too
sluggish--in their work. They