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Page 36
The host possesses the passive defences of the surfaces which have
already been considered. The first theories advanced in explanation of
immunity were influenced by what was known of fermentation. One, the
exhaustion theory, assumed that in the course of disease substances
contained in the body and necessary for the growth of the bacteria
became exhausted and the bacteria died in consequence. Another, the
theory of addition, assumed that in the course of the disease
substances inimical to the bacteria were formed. Both these theories
were inadequate and not in accord with what was known of the
physiology of the body. The most general mode of defence is by
phagocytosis, the property which many cells have of devouring and
digesting solid substances (Fig. 16-p). Although this had been known
to take place in the amoeb� and other unicellular organisms, the wide
extent of the process and its importance in immunity was first
recognized by Metschnikoff in 1884 and the phagocytic theory of
immunity advanced and defended by a brilliant series of experiments by
Metschnikoff and his pupils conducted in the Pasteur Institute.
Metschnikoff's first observations were made on the daphnea, a small
animalcule just visible to the naked eye which lives in fresh water.
The structure of the organism is simple, consisting of an external and
internal surface between which there is a space, the body cavity;
daphne� are transparent and can be studied under the microscope while
living. Metschnikoff observed that certain of them in the aquarium
gradually lost their transparency and died, and examining these he
found they were attacked by a species of fungus having long, thin
spores. These spores were taken into the intestine with other food;
they penetrated the thin wall of the intestine, passed into the body
cavity, multiplied there, and in consequence the animal died. In many
cases, however, those penetrating became enclosed in cells which the
body cavity contains and which correspond with the leucocytes of the
blood; in these the spores were digested and destroyed. The daphne� in
which this took place recovered from the infection. Here was a case in
which all the stages of an infectious disease could be directly
followed under the microscope, and the whole process was simple in
comparison with infections in the higher animals. The pathogenic
organism was known, the manner and site of invasion was clear, it was
also evident that if the multiplication of the parasite was unchecked
the animal died, but if the parasite was opposed by the body cells and
destroyed the animal recovered. The studies were carried further into
the diseases of the higher animals, and it was found the leucocytes in
these played the same part as did the cells in the body cavity of the
daphnea. The introduction of bacteria into certain animals was
followed by their destruction within cells and no disease resulted; if
this did not take place, the bacteria multiplied and produced disease.
Support also was given the theory by the demonstration at about the
same time that in most of the infectious diseases the leucocytes of
the blood became increased in number,--that in pneumonia, for
instance, instead of the usual number of eight thousand in a cubic
millimeter of blood, there were often thirty thousand or even fifty
thousand. At about the same time also chemotaxis, or the action of
chemical substances in attracting or repelling organisms, excited
attention, and all these facts together became woven into the theory.
It was soon seen, however, that this theory, based as it was on
observation and supported by the facts observed, was not, at least in
its first crude form, capable of general application. Many animals
have natural immunity to certain diseases; they do not have the
disease under natural conditions, nor do they acquire the disease when
the organisms causing it are artificially introduced into their
tissues by inoculation. Such natural immunity seemed to be unconnected
with defence by phagocytosis, for the leucocytes of the animal might
or might not have phagocytic reaction to the particular organisms to
which the animal was immune. It was also seen that recovery from
infection in certain diseases was unconnected with phagocytosis. It
had also been demonstrated, by German observers chiefly, that the
serum of the blood, the colorless fluid in which the corpuscles float,
was itself destructive, and that in an animal rendered immune to a
special bacterium the destructive action of the serum on that organism
was greatly increased. In this hostile serum the bacteria often became
clumped together in masses, the bodies became swollen, broken up, and
finally disintegrated. This property of the serum was described as due
to a substance in the serum called _alexine_, which in the immune
animal became greatly increased in amount. It was even denied by some
that phagocytosis of living bacteria took place, and that all those
included in the cells were dead, having been destroyed in the first
instance by the serum. The strife became a national one between the
French and Germans,--on the one side in France the phagocytic theory
was defended, and in Germany, on the other, the theory of serum
immunity. The mass of experimental work which poured from the
laboratories of the two countries in attack and defence became so
great that it could not easily be followed. It had a good influence
because, without the stimulation of this national rivalry, the
knowledge which gradually arose from this work would not have been so
quickly acquired. It is interesting that the mode of action of the
serum in destroying bacteria was demonstrated not by a German but by
Bordet, a French observer and a pupil of Metschnikoff. He showed that
the serum contained two distinct substances, each necessary for the
destructive action. 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. It has been shown as a
result of the work of many investigators that the shield has two
faces,--there is destruction both by cells and fluids and there is
interaction by both. The amboceptors so necessary for the destructive
action of the serum are produced by the body cells, particularly the
leucocytes. The serum assists in pagocytosis by the action on bacteria
of substances called _opsonins_ which are contained in it, and
the formation of which can be very greatly stimulated. Again, not all
inclusion of bacteria within leucocytes is indicative of phagocytosis;
in many cases the bacteria seem to find the best conditions for
existence within the leucocytes, and these and not the bacteria are
destroyed.
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