Evolution Of Immunity And The Invertebrates Essay, Research Paper

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 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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