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 disease

• providing physical barriers to stop a pathogen

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 T-lymphocytes.

• 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 state.

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.

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General Cell Communication

Propose a cell communication mechanism that ancestral amoeba may have used to distinguish between foreign cells and other amoeba cells.

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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 communication.

•List all the observations you can make about this diagram.

Evolution and Immunity

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.

Toll-like receptor(TLR) pathways comparisonfor plants, invertebrates and vertebrates

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 penetrate

- 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 another.

• 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 quickly.

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 phagocytosis.

Innate Immunity

• Some hemocytes have Toll receptors that produce antimicrobial peptides which inactivate or kill fungal or bacteria by disrupting plasma membranes.

• 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.

Evidence

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 LeiblScience Content DirectorNational Math and Science