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Immune System Part I:
The Evolution of the Immune System
Immune System and Innate Immunity
• The human body has over 100 trillion
bacteria living on it or within it.
• An immune system protects an organism
from pathogens and other foreign material.
• The innate immune system provides the
first line of defense by
• activating a small variety of immune
cells to produce molecules that stop the
invading pathogens and its resulting
• providing physical barriers to stop a
Adaptive Immune System
• The innate immune system is
immediate and utilized by many
multicellular organisms.
• In contrast, an adaptive immune
system produces antibodies specific
to a pathogen or foreign particle after
the pathogen activates B- and Tlymphocytes.
• The adaptive immune response is
slower than the innate immune
response and it may take days to
become effective.
• Present only in jawed vertebrates
Homeostasis and the Immune System
• Pathogens invade organisms
because they are seeking:
- a source of food or water
- protection
- a place for reproduction
• Defense mechanisms evolved
to rid the organism of
pathogens and return the
organism back to its original
Evolution of the Immune System
• Ancestral amoebas performed
phagocytosis to obtain food.
• It would be advantageous for
these ancestral amoeba to be
able to distinguish between
self and non-self when
ingesting other cells.
• The interaction between
receptors and signature
molecules could provide this
distinction mechanism.
General Cell Communication
Propose a cell communication mechanism that ancestral
amoeba may have used to distinguish between foreign cells
and other amoeba cells.
Cell Communication and Immune System
Example of receptors and
various signature
molecules or PAMPs.
•Explain how this part of
the immune system is an
example of cell
•List all the observations
you can make about this
Evolution and Immunity
for plants,
and vertebrates
1. List any similarities that you observe among the 3 pathways.
2. If this pathway was once in an ancestral cell, explain why the
differences exist between the three organisms.
Plant Defenses
Plants, like animals, must
defend against pathogens
and herbivores with:
• physical barriers
• secondary metabolites
• immune responses
Plants’ Physical Defenses
• Pathogens can invade plants.
• Plants’physical defenses include:
- hairs that insects cannot
- thorns or spines that large
herbivores avoid
- waxy cuticles that bacteria and
fungi cannot penetrate
Plant Defenses Against Herbivory
•Herbivores harm plants by
eating them.
•Defenses against herbivory
include production of
secondary metabolites.
Defense and Secondary Metabolites
Plant Defenses Against Herbivory
Plant Defenses Against Herbivory
Coevolution of Herbivores and Plants
• Certain herbivores and plants have coevolved with one
• This butterfly can breakdown the secondary
metabolite produced by the passion vine and use a
nitrogen by-product in protein metabolism.
Induced Defense Response
•Certain pathogens will
produce molecules that
produce the hypersensitive
response (HR).
•This results in localized
death of plant cells very
Systemic Acquired Resistance
• Methyl salicylate from
the HR is spread to rest
of the plant.
• Once converted to
salicylic acid, a
transduction pathway is
activated causing
systemic acquired
resistance (SAR) for
the entire plant that
lasts for several days.
Immunity and Invertebrates -- Physical Barriers
Examples of innate immunity in Drosophila:
• First line of defense is the exoskeleton made of chitin.
• Chitin also lines the digestive tract.
• The enzyme lysozyme found in the gut breaks down the
cell walls of bacteria.
Innate Immunity
• If the pathogen escapes the
digestive tract, it will be in
a fluid called hemolymph.
• Moving throughout the
hemolymph are hemocytes.
• Some hemocytes can
destroy pathogens by
Innate Immunity
• Some hemocytes have Toll receptors that
produce antimicrobial peptides which inactivate
or kill fungal or bacteria by disrupting plasma
• Each Toll receptor identifies certain signature
molecules as a pathogen or foreign material.
Toll Receptor
The Toll receptor recognizes
signature molecules or PAMPs.
This activates signal
transduction pathways, often
producing transcription factors.
These transcription factors
activate genes needed to
synthesize anti-microbial
peptides for destruction or
deactivation of pathogens.
These flies were engineered so that
the immune cells expressed a green
fluorescent protein when the innate
immune system was activated.
The top fly was inoculated with bacteria and the one
below was “stabbed” with a sterile needle (no injection
of bacteria). The top fly’s immune system was
activated while the bottom fly’s was not.
Created by:
Carol Leibl
Science Content Director
National Math and Science