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Page 37
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. Certain of the infectious diseases are,
as has been said, essentially toxic in their nature, and in cultures
the organisms produce poisonous substances. By the injection into the
tissues of such substances the same disturbances are produced as when
the bacteria are injected. Such a disease is diphtheria. In this there
is only a superficial invasion of the tissues. The diphtheria bacilli
are located on the surface of the tonsils or pharynx or windpipe,
where, as a result of their action, the membrane so characteristic of
the disease is produced. The membrane may be the cause of death when
it is so extensively formed as to occlude the air passages, but the
prominent symptoms of the disease, the fever, the weakness of the
heart and the great prostration are due not to the presence of the
membrane, but to the action of toxic substances which are formed by
the bacteria growing in the superficial lesions and absorbed. Tetanus,
or lockjaw, is another example of these essentially toxic diseases.
The body must find some means of counteracting or destroying these
injurious toxic substances. It does this by forming antagonistic
substances called antitoxines, which act not by destroying the
toxines, but by uniting with them, the compound substance being
harmless. It has been found that the production of antitoxine can be
so stimulated by the injection of toxine that the blood of the animal
used for the purpose contains large amounts of antitoxine. The horse
is used in this way to manufacture antitoxine, and the serum injected
into a patient with diphtheria has a curative action, a greater amount
being thus introduced than the patient can manufacture.
[Illustration: FIG. 18.--DIAGRAM TO ILLUSTRATE EHRLICH'S THEORY OF
ANTITOXINE FORMATION. The surface of the cell (_n_) is covered with
receptors some of which (_b_) fit the toxine molecule, (_a_) allowing
the toxine to act upon the cell. Under the stimulus of this the cell
produces these receptors in excess which enter into the blood and
there combine with the toxine as in _a^1 b^1_, thus anchoring it and
preventing it from acting upon the cells. The receptors _c_ and _d_ do
not fit the toxine molecule.]
A very ingenious theory which well accords with the facts has been
given by Ehrlich in explanation of the production of antitoxine and of
the reaction between toxine and antitoxine (Fig. 18). This is based on
the hypothesis, which is in accord with all facts and generally
accepted, that the molecules which enter into the structure of any
chemical substance have in each particular substance a definite
arrangement, and that in a compound substance each elementary
substance entering into the compound molecule has chemical affinities,
most of which may be satisfied by finding a suitable mate. Ehrlich
assumes that the very complex chemical substances which form the
living cells have many unsatisfied chemical affinities, and that it is
due to this that molecules of substances adapted for food can enter
the cells and unite with them; but there must be some coincidence of
molecular structure to enable the union to take place, the comparison
being made of the fitting of a key into a lock. The toxines--that
produced by the diphtheria bacillus being the best example--are
substances whose molecular structure enables them to combine with the
cells of the body, the combination being effected through certain
chemical affinities belonging to the cells termed _receptors_.
Unless the living cells have receptors which will enable the
combination with the toxine to take place, no effect can be produced
by the toxine and the cells are not injured. This is the case in an
animal naturally immune to the action of the diphtheria bacillus or
its toxines. In the case of the susceptible animal the receptors of
the cells of the different organs combine with the toxine to a greater
or less extent, which explains the fact that different degrees of
injury are produced in the different tissues; the toxine of tetanus,
or lockjaw, for example, combines by preference with the nervous
tissue, that of diphtheria with the lymphatic tissue. It is known that
in accordance with the general law of injury and repair, a loss in any
part of the body stimulates the tissue of the same kind to new growth
and the loss is thus repaired; it is assumed that the cell receptors
which combine with the toxine are lost for the cell which then
produces them in excess. The receptors so produced pass into the
blood, where they combine with the toxine which has been absorbed; the
combination is a stable one, and the toxine is thus prevented from
combining with the tissue cells. The antitoxine which is formed during
the disease, and the production of which in the horse can be
enormously stimulated by the injection of toxine, represents merely
the excess of cell receptors, and when the serum of the horse
containing them is injected in a case of diphtheria the same
combination takes place as in the case of receptors provided by the
patient. In the case of the destruction of bacteria in the blood by
the action of amboceptor and complement, the amboceptor must be able
to combine with both the bacterial cell and the complement which
brings about its destruction, and just as antitoxine is formed so new
amboceptors may be formed.
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