Evolution of Immunity and the 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 within one animal. Immunoglobulins such