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Introduction
Why explain non-human immunity? Cos I'm a zoologist that's why!
Despite working as a clinical geneticist/dermatologist/immunologist
I started out studying zoology, one aspect of which involved learning
about the evolutionary development of the immune system. I believe
it is important to show that there is more than one method of
immunological defense that mother nature has developed. The human
immune system is just one of many successful mechanisms that operate
in the animal world. The human system is also the most advanced
and complex and the more complex a system the greater the potential
for things to go wrong. To reach a level of understanding of the
complex system that will allow us to understand why malfunctions
such as autoimmunity and immune deficiency develop, we have to
strip the system down to its basic components and their functions.
The next article will address what individual cells of the immune
system do and how they do it. Before we come to that, we have
to ask why we need a defense function at all and how has it become
so complex?
Why
have an immune system in the first place?
An organism's integrity is under daily threat from external sources.
The integrity of the individual must be protected from challenges
which can be grouped into five main categories.
1) Competition for survival. We live on an over populated world.
Not just humans but many species within their own spatial niches
are competing for limited space and food.
2) To protect against assimilation. The constant battle involves
avoiding cell fusion. Single cell organisms can be readily fused
together. A more aggressive species or subspecies may attempt to
"assimilate" non-aggressive or weaker cells into the more
aggressive population. In this way the aggressive cell population
reduces the competition for food space and light and may obtain
added advantages from receiving some of the weaker population's
DNA. At the single cell level, fusion can be accomplished between
widely different species and the remnants of this cell fusion acceptance
can be observed in humans. Siamese twins are the result of partial
fusion of embryos. Frequently, one twin becomes dominant over the
other. This twin will try to assimilate the other (unconsciously
of course) and will thrive while the other becomes steadily weaker.
Extreme examples of this dominance of one Siamese twin over another
may be observed in the total assimilation of one twin while still
within the womb. The only clue to this assimilation may be the Fossilized
remnants of weaker twin revealed during X-ray of the survivor.
3) To protect against organ damage and aid repair. Penetration
of the body by a sharp rock. An extremity lost during a fight. A
tissue succumbing to attack by bacteria. These, and other destroyers
of tissue, can endanger the welfare of an individual.
4) To protect against Parasitism. A parasite is an organism that
lives on another organism and depends on it entirely for nourishment
and protection. Within this definition we can include macroscopic
parasites such as tape worms and microscopic "parasites"
(pathogens). Every species is the potential host for numerous parasites.
The parasite may feed on the host to exhaustion and ultimate death,
or the parasite may remain almost unnoticed. Regardless, parasites
are a threat to the host's integrity and welfare. At the very least
they compromise the host by taking nourishment intended for the
host's cells. At worst they hijack the host's body functions for
the use of the parasite leading ultimately to death of the host.
Microorganisms are perhaps the greatest threat to humans. While
some are benign others have the ability to cause disease (dis-ease)
in other words they have pathogenicity, they are pathogenic organisms.
Invasion of the host is an infection and if transfer from host to
host is possible, then the pathogen is capable of infectious disease
and the initiation of an epidemic within a host population.
5) Regulation of integrity. Multi-celled organisms develop from
a highly regulated division of cells. Variants or mutants may occur
during duplication of cells in the development and maintenance of
the individual. These variant cells may be the result of viral contamination,
chemical modification, or even the imperfect cell division mechanisms
and duplication of DNA. At best the variant cells are benign, they
simply take up space and nutrients. At worst, the cells are a threat
to the host integrity, proliferating out of control, threatening
to use up all the nutrients and resources available. They are neoplasms
or tumors.
In this sense, the variant cells are very much parasites except
the danger comes from the individual's own cells rather than a different,
invading species. Regardless of being benign or cancerous, the variant
cells are a draw back compromising the optimum functioning of the
individual. THEY MUST BE REMOVED.
Defense
of self
The object of the individual is to survive as an individual. To
maintain integrity organisms have developed passive and aggressive
defense mechanisms. we will discuss some of the passive mechanisms
operating in mammals in the next chapter. Here we will briefly look
at passive biochemical and cellular defenses in some invertebrates
as well as examining the development of more aggressive defense
activity.
The vertebrate immune system is a highly complex and interactive
system. It was not developed overnight. While it is difficult to
recognize that specific order of immune system development, we can
suggest an overall evolutionary time line from examining other vertebrate
and invertebrate species and comparing the complexity of their defense
mechanisms to ours. There are many risks in this approach. We don't
fully understand the evolution of vertebrates or what their common
ancestor was. The immune system of all vertebrate species today
is very similar which suggests that the immune system developed
very rapidly with the evolution of invertebrates. Our understanding
of invertebrate evolution has a lot of gaps in it. With the extinction
of many species and the evolutionary orders they represent, we have
lost many valuable clues about immune system development.
Defense of self is the overriding object and there are many forms
of defense reaction operating in the invertebrate world. Some are
evolutionary dead ends but others may have implications for our
immune system. The point however is that there is more than one
method of individual defense that successfully operates on this
planet.
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