The Evolution of Immunity in Invertebrates
"Article Summery"
Name: "Immunity and the Invertebrates"
Periodical: Scientific American Nov, 1996
Author: Gregory Beck and Gail S. Habicht
Pages: 60 - 71
Total Pages Read: 9
The complex immune systems of humans and other mammals evolved over quite a long time -
in some rather surprising ways. In 1982 a Russian zoologist named Elie Metchnikoff
noticed a unique property of starfish larva. When he inserted a foreign object through
it's membrane, tiny cells would try to ingest the invader through the process of
phagocytosis. It was already known that phagocytosis occurred in specialized mammal cells
but never in something less complex like a starfish. This discovery led him to understand
that phagocytosis played a much broader role, it was a fundamental mechanism of
protection in the animal kingdom. Metchnikoff's further studies showed that the host
defense system of all animals today were present millions of years before when hey were
just beginning to evolve. His studies opened up the new field of comparative immunology.
Comparative immunologists studied the immune defenses of past and current creatures. They
gained further insight into how immunity works.
The most basic requirement of an immune system is to distinguish between one's own cells
and "non-self" cells. The second job is to eliminate the non-self cells. When a foreign
object enters the body, several things happen. Blood stops flowing, the immunity system
begins to eliminate unwanted microbes with phagocytic white blood cells. This defensive
mechanism is possessed by all animals with an innate system of immunity. Innate cellular
immunity is believed to be the earliest form of immunity. Another form of innate immunity
is complement, composed of 30 different proteins of the blood.
If these mechanisms do not work to defeat an invader, vertebrates rely on another
response: acquired immunity. Acquired immunity is mainly dealt by specialized white blood
cells called lymphocytes. Lymphocytes travel throughout the blood and lymph glands
waiting to attack molecules called antigens. Lymphocytes are made of two classes: B and
T. B lymphocytes release antibodies while T help produce antibodies and serve to
recognize antigens. Acquired Immunity is highly effective but takes days to activate and
succeed because of it's complex nature. Despite this, acquired immunity offers one great
feature: immunological memory. Immunological Memory allows the lymphocytes to recognize
previously encountered antigens making reaction time faster. For this reason, we give
immunizations or booster shots to children.
So it has been established that current vertebrates have two defense mechanisms: innate
and acquired, but what of older organisms ? Both mechanisms surprisingly enough can be
found in almost all organisms (specifically phagocytosis). The relative similarities in
invertebrate and vertebrate immune systems seem to suggest they had common precursors.
The oldest form of life, Protozoan produce these two immune functions in just one cell.
Protozoan phagocytosis is not uncommon to that of human phagocytic cells. Another basic
function of immunity, distinguishing self from non-self, is found in protozoan who live
in large colonies and must be able to recognize each other. In the case of metazoan,
Sponges, the oldest and simplest, are able to do this as well refusing grafts from other
sponges. This process of refusing is not the same in vertebrates and invertebrates
though. Because vertebrates have acquired immunologic memory they are able to reject
things faster than invertebrates who must constantly "re-learn" what
is and is not self. Complement and lymphocytes are also missing from invertebrates, but
which offer an alternative yet similar response. In certain invertebrate phyla a response
called the prophenoloxidase (proPO) system occurs. Like the complement system it is
activated by enzymes. The proPO system has also been linked to blood coagulation and the
killing of invading microbes.
Invertebrates also have no lymphocytes, but have a system which suggests itself to be a
precursor of the lymph system. For instance, invertebrates have molecules which behaving
similarly to antibodies found in vertebrates. These lectin molecules bind to sugar
molecules causing them to clump to invading objects. Lectins have been found in plants,
bacteria, and vertebrates as well as invertebrates which seems to suggest they entered
the evolutionary process early on. This same process occurs in human innate immune
systems with collections of proteins called collectins which cover microbes n a thin
membrane to make them easier to distinguish by phagocytes. And although antibodies are
not found in invertebrates a similar and related molecule is. Antibodies are members of a
super family called immunoglobulin which is characterized by a structure called the Ig
fold. It is believed that the Ig fold developed during the evolution of metazoan animals
when it became important to distinguish different types of cells wi
thin one animal. Immunoglobulins such as Hemolin have been found in moths, grasshoppers,
and flies, as well as lower vertebrates. This suggests that antibody-based defense
systems, although only active in vertebrates, found their roots in the invertebrate
immune system.
Evolution seems to have also conserved many of the control signals for these defense
mechanisms. Work is currently being done to isolate invertebrate molecules similar to the
cytokines of vertebrates. Cytokines are proteins that either stimulate or block out other
cells of the immune system as well as affecting other organs. These proteins are critical
for the regulation of vertebrate immunity. It is suspected that invertebrates will share
common cytokines with vertebrates or at least a close replication. Proteins removed from
starfish have been found to have the same physical, chemical, and biological properties
of interleukins (IL-1, IL-6), a common cytokine of vertebrates. This research has gone
far enough to conclude that invertebrates possess similar molecules to the three major
vertebrate cytokines. In the starfish, cells called coelomocytes were found to produce
IL-1. The IL-1 stimulated these cells to engulf and destroy invaders. It is thus believed
that invertebrate cytokines regulate much of their
host's defense response, much like the cytokines of vertebrate animals in innate
immunity.
Comparative Immunology has also found defense mechanisms first in invertebrates only
later to be discovered in vertebrates. Invertebrates use key defensive molecules such as
antibacterial peptides and proteins, namely lysozyme, to expose bacterial cell walls.
Thus targeting the invader. This offers great potential for medicinal purposes, because
lysozyme is also found in the innate immunity of humans in it's defense of the oral
cavity against bacteria. Peptides of the silk moth are currently being developed as
antibacterial molecules for use in humans. Two peptides found in the skin of the African
clawed frog actively fight bacteria, fungi, and protozoa. Antibodies which bind to these
two peptides also bind to the skin and intestinal lining of humans.
The potential of these peptide antibiotics only now being discovered is a rather
considerable thing to ponder. For that reason it is surprising that such little attention
has been paid to invertebrate immune responses. In the end, the complexity of vertebrate
immune systems can only be understood by studying the less complex systems of
invertebrates. Further studies look to explain immunity evolution as well as aid in the
solving of problems of human health.
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