The body of any animal can be viewed as a society or ecosystem
whose individual members are cells, reproducing by cell division
and organized into collaborative assemblies or tissues. In this
ecosystem, the cells are born, live and die under various forms of
selection pressure such as territorial limitation, population size,
source of nutrients provided, infectious agents, etc. The body is a
highly organized society of cells whose main task is the
maintenance of homeostasis of the whole organism. The failure of
control mechanisms which make the cell the unit of society, marking
the beginning of its asocial behaviour, is most frequently a
malignant alteration. This process is not abrupt, nor is it based
on a single event. It is, rather, a long-term process characterized
mainly by mutation, competition and natural selection operating
within the population of cells. The basic mechanisms controlling
the cell sociability represent the first defence line against the
altered cells, while the second line of defence is supposed to be
made up of the immune system cells.Speaking in Darwinian terms,
within the ecosystem of an organism, cells of the immune system
operate as predators of the altered and mutated cells or cells
infected by the intracellular parasites. The biological phenomena
whose mechanisms are, at present, explored and largely understood,
certainly had their own evolution. Searching for the origin and
details of the evolution of advanced solutions as well as selection
pressures that might justify their emergence and existence, we
often fail to see that many such phenomena are, in fact,
co-evolutionary by-products of evolutionary innovations. In other
words, the evolutionary emergence of advanced solutions is
sometimes, if not always, accompanied by certain by-products and by
the co-evolution of compensatory mechanisms acting as a
counterbalance to these. An example of the evolution of advanced
solutions is the evolution of adoptive immunity, and co-evolution
of auto-immunity and alloimmunity. Alongside the diversification of
the mechanisms of adoptive immunity, auto-immunity and alloimmunity
gain attributes of the evolutionary by-products and become sources
of selection pressure.To that effect, alloimmunity could be a
source of very strong selection pressure in mammals, simply because
it is directly connected with the reproductive efficacy. At the
same time, new forms of selection pressure that are connected with
adoptive immunity gave rise to new mechanisms controlling killer
machinery of the immune system. Finally, the last in a line of
by-products in the processes of evolutionary modelling and
re-modelling of vertebrate immune systems can be regarded as the
failure of anti-tumor immunity. There is now much evidence that
tumors can be immunogenic. Tumor cells very often express antigens
in a form recognizable by the host immune system, but most
frequently without consequences on tumor progression. This has been
shown in many experimental models and different experimental
conditions. Immediate mechanisms for the escape of tumors from the
immune response are very similar to mechanisms for the escape of
the fetoplacental unit (as allograft) from the maternal immune
response. The similarity between these two mechanisms is so
significant that any randomness must be banished.Mechanisms of
anti-tumor immunity in mammals are probably substantially different
from mechanisms of anti-tumor immunity in other classes of
vertebrates. Moreover, the type of most frequent tumors in
non-mammalian vertebrates is also significantly different. Finally,
the incidence of malignant tumors in non-mammalian vertebrates is
significantly lower than the incidence of malignant tumors in
mammals. These facts indicate that the mammalian immune system
during the anti-tumor immune response is tricked by the similarity
between tumor cells and trophoblast or other placental cells. From
this aspect, anti-tumor immunity failure in mammals can be defined
as an immunoreproductive phenomenon, which is developed under the
evolutionary pressure of auto-immunity and
alloimmunity/reproductive effectiveness. It may be a specific
evolutionary approach in the rendering of anti-tumor immunity
failure in mammals, and a new possibility for anti-tumor
immunotherapy.
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