Symbiosis, Disease and Host Defenses

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Symbiosis, Disease and Host Defenses

• An evolutionary arms race

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Symbiosis Organisms living together

• Three types based on effects
• Commensalism- positive for one organism and
  neutral for the other
• Mutualism-positive for both organisms
• Parasitism-positive for one negative for the
  other
• Parasite (Greek parasitos, to dine along side
  of)
• As a matter of convention, parasitology usually
  covers protist and animals even though bacteria
  and viruses can be parasites . Microbes are often
  referred to as pathogens even though they are
  parasitic.

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Parasitic Organisms

• Overarching questions
• How do parasites affect their hosts?
• What makes an efficient parasite?
• Why are some organisms parasitic and others are
  not?
• Why is a particular organisms pathogenic
  sometimes but not always?
• Obligate parasite- can not live without host
• Opportunistic parasite/pathogen- can be parasitic
  under certain circumstances
• Virulence Factor-structural or physiological
  characteristics that contribute to the effect
  that we call disease

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Endosymbiosis
Ancestral symbiotic event between primitive cells
What are the possible costs or benefits to each
organism involved? How does that affect the
evolutionary process?
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Selective pressure
Host fitness Symbiont fitness
No change up
up up
down up
Relationship is commensal
Relationship is mutual
Relationship is parasitic
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A biological arms race?

• Parasite increases fitness if
• Continues to benefit from the host by resisting
  defenses
• Host increases fitness if
• Excludes parasite
• Destroys parasite outside
• Destroys/digests parasite inside

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All living things have some defense mechanisms

• Restriction enzymes in bacteria
• Vesicles with chemical defenses in single-celled
  eukaryotes
• Specialized phagocytic cells in most animals,
  including arthropods and helminthes
• Pinnacle of host defenses are found in
  vertebrates (fish, amphibians, reptiles, birds,
  mammals)

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Non-Specific Host Defenses of Vertebrates

• Defense strategies include
• External (Exclusion)
• chemical-sweat, tears
• physical-keratinized structures
• Internal
• cellular-leukocytes other than lymphocytes
• chemical and molecular- complement, interferon
• inflammation, fever etc..
• Note regulation is important for all internal
  defense mechanisms

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Physical barriers the human integument
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Pathogens subvert physical barriers
Portals of entry include Ears Nose Mouth Urogenit
al tract Gastrointestinal tract Wounds
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Leukocytes in non-specific defenses
Cytoplasm Cell name Function
Granular Neutrophils Phagocytic
Eosinophils Phagocytic, antiworm chemicals released
Basophils Release histamine
Agranular Monocytes Fixed macrophage Phagocytic, guard tissues
Wandering macrophages Phagocytic, Circulate in blood and enter tissues upon infection
Dendritic (Langerhanz) cells Phagocytic, reside in tissues like epidermis
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Phagocytic cells are non-specific defenses that
destroy invaders
Lysosome containing digestive chemicals
Phagocytosis of invader
Phagolysosome formed
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Mechanisms for subverting non-specific defenses

• Structural adaptation-formation of special cell
  walls, capsules, endospores, cysts
• Physiological adaptations- enzymes that
  neutralize host chemicals, digest host physical
  barriers or destroy phagocytes
• Vectors, vehicles, fomites aid in gaining entry
  into host

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Specific Immunity defensive cells get smarter

• Remembering invaders allows for faster response
  upon subsequent encounters of that specific
  invader
• Specific immunity can be divided into two
  branches
• Humoral immunity- antibodies produced by B cells
• Cell-mediated immunity- T cells recognized
  invaders and stimulate defense mechanisms
• Antigen any molecule that generates an immune
  response (antibody generating)
• Antibody- a protein that can bind to specific
  antigens and help with the immune response in
  various ways

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Types of Specific Immunity

• Active- antibodies formed by the host
• Natural- host forms antibodies naturally upon
  natural exposure to antigen
• Artificial- host is artificially exposed to
  antigen i.e. immunization, vaccination
• Passive-antibodies not made by host
• Natural-Maternal antibodies
• Artificial-antibodies from other source (horse,
  cow etc..), for example antivenin administered
  after snake bite

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Cells of the Specific Immune System

• B lymphocytes produce antibodies which are
  special proteins that bind to foreign molecules
  (antigens) and facilitate an immune response
• Plasma cells
• Memory B cells
• Natural Killer cells Kill virus- infected cells,
  bacteria and cancer cells extracellularly
• T lymphocytes differentiate into several types
  with different immunological functions
• Helper T cells ( TH), CD4 (T4)
• Cytotoxic cells (TC), CD8 (T8)
• Memory T cells

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B lymphocytes produce antibodies (immunoglubulins)

• Antibodies are proteins and therefore are
  synthesized from the instructions in DNA
• Antibodies can be constructed to fit a variety of
  immunological functions from cell surface
  receptors to circulation, or excretion
• The sections of DNA that encode the part the
  antibody that binds to antigen is highly variable
  allowing the formation of millions of different
  antigens so antibodies may be formed to any
  foreign molecule that is complex enough

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Antibodies
Binds to antigen here
Y
Cell with antibody as cell-surface receptor
Basic structure of and antibody
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Antibodies (immunoglobulins)
IgG-circulating in blood serum, can cross
placenta IgM-first to blood serum, on B cell
membrane IgA-Body secretions, epithelium of GI,
Resp, UG tracts IgD-B cell membrane IgE-body
fluids and skin, affinity for mast cells and
basophils, main mediator for type 1
hypersensitivity
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B Cells and Humoral Immunity
Whichever B-cell binds to a specific antigen will
be produced in greater numbers to mount an immune
response
Some of the newly created B cells will become
memory cells and the others will become plasma
cells produce and secrete various forms of that
specific antibody
Countless B cells are produced expressing a
variety of antibodies on the cell surface
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Overview of Cell-mediated Immunity

• Antigen is processed by non-specific immune cells
  (e.g. macrophages) which present the antigen at
  the cell surface
• T cells that have a matching receptor bind to the
  antigen
• If the MHC protein is correct, the T cell is
  activated and it divides and differentiates
• MHC proteins
• made from highly variable gene which makes self
  different for almost all individuals
• class I MHC on all body cells
• class II MHC only on some immune cells

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Cell-mediated Immunity
Helper T cell
Antibody in the form of T-cell surface receptor
Y
antigen

MHC class II
Helper T cells are activated. They differentiate
into Th1 or Th2 cells which stimulate
intracellular destruction of antigen (Th1) or
stimulate B cells (Th2)
Macrophage presenting antigen in MHC II protein
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Cell-mediated Immunity
Cytotoxic T cell
Antibody in the form of T-cell surface receptor
Y
antigen

MHC class I
Cytotoxic T cell are stimulated to attack
infected cells
Infected cell presenting antigen in MHC I protein
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Phases of Infection
incubation
acme
Prodromal period
decline
invasive
Convalescence period
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Immune System Disordershypersensitivities

• Hypersensitivities
• Type 1 - anaphylactic, immediate, IgE mediated
• Type 2- cytotoxic, reaction to cell surface
  antigens
• Type 3- immune complex, persistent
  antibody-antigen complexes formed
• Type 4 cell (T cell) mediated, delayed
  hypersensitivity

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Immune System Disorders Autoimmunity

• Caused by lack of self recognition-T-cells and/or
  antibodies act against self tissues
• Triggers of autoimmunity include
• failure of lymphocyte programming process
• new self antigens
• mutation in genes
• Haptens (incomplete antigen)
• foreign antigens stimulate cross-reactive
  antibody production

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Immune System Disorders Immunodeficiency and
Immunosupression

• Causes may be
• Genetic (SCIDS)
• Environmental, Chemical (pollution etc..)
• Infections (AIDS)
• Psychoneuroimmunological (Severe stress)

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Herd Immunity

• Populations of hosts in close proximity may be
  similarly exposed to diseases (especially crowd
  diseases)
• The more host there are in the population that
  are immune to a specific disease, the harder it
  is for the disease to spread
• Herd immunity can occur naturally as host are
  exposed over their lifetimes and nature selects
  for the fittest individuals
• Vaccines can confer artificial herd immunity to
  limit the spread of disease