Reasons why there is a high incidence of septic shock

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Over-reactions of the immune system
Dr Kathy Triantafilou University of Sussex School
of Life Sciences
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Reactions of the immune system

• The immune system possesses recognition events
  that distinguish molecular components of
  infectious agents from those of the human body
• Besides infectious agents, humans come into
  contact with numerous other molecules that are
  equally foreign but do not threaten health
• These molecules are derived from plants and
  animals that are present in the environment where
  we live

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Over-reactions

• In some circumstances, molecules stimulate the
  adaptive immune response and the development of
  immunological memory
• on subsequent exposures to the antigen the immune
  memory produces inflammation and tissue damage
• The person feels ill, as though fighting an
  infection, when no infection exists
• These over-reactions of the immune system to
  harmless environmental antigens are called
  hypersensitivity or allergic reactions

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Gell and Coombs classification

• P.G.H. Gell and R.R.A. Coombs proposed a
  classification system for hypersensitivity
  reactions
• Type I
• Type II
• Type III
• Type IV

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Type I hypersensitivity

• Antigens (allergens) induce a humoral immune
  response
• commonly cause by inhaled antigens (i.e. plant
  pollen)
• This immune response results in the generation of
  antibody-secreting plasma cells and memory cells
• The plasma cells secrete IgE
• this class of antibody binds with high affinity
  to Fc receptors (mast cells, basophils, etc)
• these IgE-coated cells are said to be sensitised

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Degranulation

• Exposure to the same allergen later cross-links
  the membrane bound IgE on sensitised mast cells
  and basophils
• This causes degranulation of these cells
• The pharmacologically active mediators released
  from the granules act on surrounding tissue
  causing
• vasolidation and smooth muscle contraction
• either systemic or localised (depending on the
  extent of mediator release)

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Components of Type I

• Allergens
• IgE antibodies
• mast cells and basophils
• IgE binding Fc receptors
• IgE-mediated degranulation
• receptor crosslinking
• Mediators
• histamine
• Leukotrienes, postaglandins and cytokines

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Allergens

• IgE responses are mounted against parasites
• Some persons, however have an abnormally called
  atopy
• hereditary pre-disposition to the development of
  hypersensitivity reactions
• IgE regulatory defects suffered by atopic
  individuals allow non-parasitic antigens to
  stimulate inappropriate IgE production
• Allergen refers specifically to non-parasitic
  antigens capable of stimulating type I
  hypersensitivity reactions

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Allergens

• Common allergens include rye grass pollen,
  ragweed pollen, codfish, birch pollen, timothy
  grass pollen, and bee venom
• What makes these agents allergens?
• Allergens possess diverse properties
• most are small proteins (15,000-40,000)
• no common chemical properties
• allergenicity is a consequence of a series of
  interactions involving
• dose, sensitising route, genetic condition of the
  individual

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IgE

• The existence of a human serum factor that
  reacted with allergens was first demonstrated by
  K. Prausnitz and H. Kustner in 1921
• The response that occurs when an allergen is
  injected into an individual is called a P-K
  reaction
• In the mid 1960s K. and T. Ishizaka isolated the
  new isotype of antibody, IgE

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IgE

• Serum levels in normal individuals are in the
  range of 0.1-0.4 mg/ml
• IgE was found to be composed of two heavy chains
  and two light chains with a combined molecular
  weight of 190,000
• It has an additional constant region than IgG
• This additional domain changes the conformation
  of the molecule and enables it to bind to
  receptors on mast cells and basophils
• Half-life in the serum of 2-3 days, once bound to
  receptors is stable for a number of weeks

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Mast cells and basophils

• Blood basophils and tissue mast cells can bind
  IgE
• Mast cells are found throughout the connective
  tissue, near blood and lymphatic vessels
• skin and mucous surfaces of the respiratory and
  gastrointestinal track (10,000 mast cells per mm
  of skin)
• mast cell populations in different sites differ
  in the types and amounts of allergic mediators
  they contain

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IgE-binding Fc receptors

• The activity of IgE depends on its ability to
  bind to a receptor specific for the Fc region of
  the heavy chain
• Two classes of Fc receptors
• High affinity receptor (FceRI)
• mast cells and basophils (40,000-90,000 receptors
  on a cell)
• binds with 1000 fold higher affinity
• Low affinity receptor (FceRII)

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High affinity receptor (FceRI)

• The high affinity receptor contains four
  polypeptide chains
• an a, a b chain and two identical g chains
• Displays immunoglobulin-fold structure, and thus
  belongs to the immunoglobulin superfamily
• The a chain binds the IgE molecules
• The b chain spans the membrane four times and is
  thought to link the a to the g homodimer
• The g chains contain ITAMS similar to CD3

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Low affinity receptor (FceRII)

• The low affinity receptor (CD23) is specific for
  the CH3/CH3 domain of IgE
• It has a lower affinity for IgE
• Allergen crosslinkage of IgE bound to FceRII has
  been shown to activate B cells, alveolar
  macrophages and eosinophils
• When this receptor is blocked, IgE secretion by B
  cells is diminished
• A soluble form of the receptor exists that has
  been shown to enhance IgE production by B cells
• Sensitised individuals have higher levels of CD23

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Receptor crosslinkage

• IgE-mediated degranulation begins when an
  allergen crosslinks IgE that is bound to the Fc
  receptor on a mast cell or basophil
• the binding of IgE to FceRI has no effect on the
  target cell
• It is only after the allergen crosslinks the
  fixed IgE-receptor complex that degranulation
  begins
• monovalent antigens can not crosslink and thus
  can not trigger degranulation

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Intracellular events leading to degranulation

• The cytoplasmic domains of the b and g chains of
  the FceRI are associated with protein tyrosine
  kinases (PTKs)
• Crosslinking of the receptor results in the
  phosphorylation of tyrosines within the PTKs
• Within 15 sec after crosslinking, methylation of
  various membrane phospholipids is observed,
  resulting in the formation of Ca2 channels
• An increase in Ca2 channels reaches a peak
  within 2 min

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Ca2 channels

• The Ca2 increase eventually leads to the
  formation of arachidonic acid which is converted
  into two classes of mediators
• postaglandins
• leukotrienes
• The increase of Ca2 also promotes the assembly
  of microtubules and the contraction of
  microfilaments (necessary for the movement of
  granules to the cell surface)

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Mediators

• The manifestation of the type I hypersensitivity
  reactions are related to the biological effects
  of the mediators released from the granules
• The mediators can be classified as
• primary mediators
• produced before degranulation (histamine,
  proteases, eosinophil chemotactic factor,
  neutrophil chemotactic factor and heparin)
• secondary mediators
• after degranulation (platelet activating factor,
  leukotrienes, postaglandins, cytokines

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Histamine

• Is formed by decarboxylation of the amino acid
  histidine
• Histidine is a major component of mast cell
  ganules, accounting for 10 of the granule weight
• Once released, it binds to specific receptors on
  various target cells
• Three types of histamine receptors have been
  identified H1, H2, and H3
• binding to the receptors induces contraction of
  intestinal and bronchial smooth muscles,
  increased permeability of venules, and increased
  mucus secretion

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Leukotrienes and postaglandins

• Secondary mediators which are not formed until
  the mast cell goes through degranulation, and
  enzymatic breakdown of membrane phospholipids
• Longer time for the biological effects to become
  apparent
• Their effects are more pronounced and longer
  lived than histamine

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Leukotrienes and postaglandins

• Leukotrienes
• bronchoconstriction
• increased vascular permeability
• mucus production
• 1000x more potent as bronchoconstrictors than
  histamine
• prolonged bronchospasm and buildup of mucus
  (asthmatics)

• Postaglandins
• bronchoconstriction

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Cytokines

• Cytokines released from mast cells and
  eosinophils contribute to the clinical
  manifestation of type I hypersensitivity
• Human mast cells secrete IL-4, IL-5, IL-6 and
  TNF-a
• These cytokines alter the local environment
  leading to the recruitment of inflammatory cells
• IL-4 increases IgE production by B-cells
• IL-5 is important in the recruitment of
  eosinophils
• TNF-a contribute towards the shock in anaphylaxis

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Consequences of type I

• Systemic anaphylaxis
• Localised anaphylaxis
• Allergic Rhinitis
• Asthma
• Food allergies
• Atopic dermatitis

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Systemic anaphylaxis

• A shock-like (often fatal), whose onset occurs
  within minutes of a type I hypersensitivity
  reaction
• This was the reaction observed by Portier and
  Richet
• Caused by venom from bee, wasp, hornet and ant
  stings drugs such as penicillin, insulin and
  antitoxins, seafood and nuts
• Epinephrine is the choice of drug for anaphylaxis
  (counteracts the effects of mediators by relaxing
  the smooth muscle, and reducing vascular
  permeability

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Localised anaphylaxis

• The reaction is limited to a specific target
  tissue or organ
• Often involving epithelial surfaces at the site
  of allergen entry
• The tendency to manifest localised anaphylactic
  reactions is inherited and is called atopy
• atopic allergies afflict about 20 of the
  population

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Asthma

• Common localised anaphylaxis is asthma
• There are two types of asthma
• allergic asthma
• airborne or blood-borne allergens, such as
  pollen, dust, fumes, insect products or viral
  antigens trigger an asthmatic attack
• intrinsic asthma
• induced by exercise, cold, independently of
  allergen stimulation

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Asthma

• Like hay fever, asthma is triggered by
  degranulation of mast cells with release of
  mediators
• Instead of occurring in the nasal mucosa, the
  reaction develops in the lower respiratory tract
• The resulting contraction of the bronchial smooth
  muscles leads to bronchoconstriction
• Airway edema, mucus secretion, and inflammation
  contribute to the bronchial constriction and to
  airway obstruction

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Asthmatic response

• The asthmatic response can be divided into
• early response
• occurs within minutes of allergen exposure and
  primarily involves histamine, leukotrienes and
  postaglandin
• bronchoconstriction, vasolidation, and some
  build-up of mucus
• late response
• occurs hours later
• involves IL-4, IL-5, IL-16, TNF-a, eosinophil
  chemotactic factor (ECF) and platelet activating
  factor (PAF)

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• The overall effects is to increase endothelial
  cell adhesion as well as recruit inflammatory
  cells into the bronchial tissue
• the inflammatory cells are capable of causing
  significant tissue damage
• this lead to the occlusion of the bronchial lumen
  with mucus, proteins and cellular debris,
  thickening the basement of the epithelium and
  hypertrophy of the bronchial smooth muscles

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Food allergies

• Various foods can cause localised anaphylaxis in
  allergic individuals
• allergen crosslinking of IgE on mast cells along
  the upper and lower gastrointestinal track can
  induce localised smooth muscle contractions and
  vasolidation
• this leads to symptoms such as vomiting and
  diarrhea

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Atopic dermatitis

• Atopic dermatitis (allergic eczema) is an
  inflammatory disease of skin that is frequently
  associated with a family history of atopy
• The disease is observed more frequently in young
  children
• Serum IgE levels are often elevated
• The allergic individual develops skin eruptions
  that are erythematous
• The skin lesions have Th2 cells and an increased
  number of eosinophils

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Late-Phase reaction

• As the reaction begins to subside, mediators
  released during the course of the reaction often
  induce a localised inflammatory response, called
  the late-phase reaction
• It develops 4-6 hours after the type I reaction
  and persists for 1-2 days
• Characterised by infiltration of neutrophils,
  eosinophils, macrophages, lymphocytes and
  basophils
• Mediated by cytokines such as TNF-a, IL-1, IL-3,
  IL-5

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Detection of type I

• Skin testing
• Small amounts of potential antigens are
  introduced at specific skin sites either by
  intradermal injection or by superficial
  scratching
• a number of tests can be applied to the site on
  the forearm or back
• If the person is allergic, local mast cells
  degranulate and the release of histamine produces
  a wheal and flare within 30 min

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Skin test

• Advantages
• inexpensive
• large number of allergens tested

• Disadvantages
• sometimes sensitises the allergic individual to
  new allergens
• rarely induces systemic anaphylactic shock
• a few manifest a late-phase reaction

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Detection of type I

• Another method is to determine serum levels of
  IgE
• Using the radioimmunosorbent test (RIST)
• Patients serum is reacting with agarose beads or
  paper disks coated with rabbit anti-IgE

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Therapy of type I

• Identify the offending allergen and avoid contact
  if possible
• removal of house pets, dust-control measures, or
  avoidance of offending food
• elimination of inhalant allergens (such as
  pollen) is impossible
• immunotherapy with repeated injections of
  increasing doses of allergens (hyposensitization)
  has been known to reduce the severity of type I

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Therapy of type I

• Antihistamines have been the most useful drugs
  for symptoms of allergic rhinitis
• They bind to the histamine receptor and block the
  binding of histamine
• The H1 receptors are blocked by the classical
  antihistamines, whereas the H2 receptors are
  blocked by a newer class of antihistamines
• Several drugs block release of allergic mediators
  by interfering with biochemical steps in
  mast-cell activation

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Therapy of type I

• Disodium cromoglycate prevents Ca influx in mast
  cells
• theophylline is commonly administered to
  asthmatics orally or through inhalers (blocks
  degranulation)
• Cortisone and other anti-inflammatory drugs have
  been shown to reduce type I reactions

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Type II hypersensitivity (Antibody-mediated
cytotoxic)

• Involves antibody-mediated destruction of cells
• This type is exemplified by blood transfucion
  reactions
• Host antibodies react with foreign antigens on
  the incompatible transfused blood cells and
  mediate destruction of those cells
• Antibodies mediate cell destruction by activating
  the complement system or though
  antibody-dependent cell-mediated cytotoxicity
  (ADCC) (cytotoxic cells bind to the Fc region of
  antibodies on target cells)

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Transfusion reactions

• Antibodies to the A, B, and O antigens on red
  blood cells are usually IgM class
• An individual with blood group A has antibodies
  against B in their blood
• If a type A individual is accidentally transfused
  with blood containing type B cells, the anti-B
  antibodies will bind to the B blood cells and
  mediate their destruction by means of
  complement-mediated lysis

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Transfusion reactions

• Transfusion of blood into a recipient possessing
  antibodies to one of the blood-group antigens can
  result in a transfusion reaction
• massive intravascular hemolysis (can be immediate
  or delayed)
• Reactions that begin immediately are associated
  with ABO incompatibilities, which lead to
  complement-mediated lysis
• within hours, free hemoglobin can be detected in
  the plasma, filtered through the kidneys, some of
  it gets converted into bilirubin (high levels are
  toxic)

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Delayed hemolytic reaction

• Occurs in individuals who have received repeated
  transfusions of ABO-compatible blood that is
  incompatible for other blood groups
• The reaction develops within 2-6 days after
  transfusion
• The transfused blood induces clonal selection and
  production of IgG against a variety of receptors
• Blood group antigens that cause this Rh, Kidd,
  Kell, and Duffy
• Symptoms fever, low hemoglobin, increased
  bilirubin, jaundice and anemia

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Hemolytic disease of the newborn

• Develops when maternal IgG antibodies specific
  for fetal blood-group antigens cross the placenta
  and destroy fetal red blood cells
• Severe hymolitic disease of the newborn, called
  erythroblastosis fetalis, most commonly develops
  when an Rh expressed an Rh antigen on its red
  blood cells that the Rh- mother does not express

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Hemolytic disease of the newborn

• During pregnancy, fetal red blood cells are
  separated from the mothers circulation by a
  layer of cells called the trophoblast
• During her first pregnancy with an Rh fetus, an
  Rh- mother is usually not exposed to enough
  antigen to activate her Rh-specific B-cells
• At the time of delivery separation of the
  placenta from the uterine wall allows large
  amounts of fetal blood to enter the mothers
  circulation
• The fetal red blood cells activate the
  Rh-specific B-cells of the mother

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• The secreted IgM antibodies clear the fetal red
  blood cells from the mothers circulation, but
  the memory cells remain
• A subsequent pregnancy with a Rh fetus can
  activate the memory cells, which results in
  secretion of IgG anti-Rh antibodies which cross
  the placenta and damage the fetal red blood cells
• Mild to severe anemia can develop in the fetus,
  sometimes fatal

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Prevention

• Hemolytic disease of the newborn caused by Rh
  incompatibility can be almost entirely prevented
  by administering antibodies against the Rh
  antigen to the mother within 24-48 hours after
  the first delivery
• These antibodies are called Rhogam
• They bind to fetal red blood cells that have
  entered the mothers circulation and facilitate
  their clearance before B-cell activation

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Therapy

• If hemolytic disease develops, the treatment
  depends on the severity of the reaction
• For a severe reaction, the fetus can be given an
  intrauterine blood-exchange transfusion
• This replaces the fetal Rh cells with Rh- cells
• This transfusion is given every 10-21 days until
  delivery
• In less severe cases, a blood-exhange transfusion
  is not given until after birth

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Drug-induced hemolytic anemia

• Certain antibiotics (penicillin, cephalosprin,
  and streptmycin) can absorb nonspecifically to
  proteins on RBCs
• In some patients these complexes induce formation
  of antibodies, which then bind to the cells and
  induce complement-mediated lysis and thus
  progressive anemia
• When the drug is withdrawn the hemolytic anemia
  disappears

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Type III hypersensitivity (immune-complex-mediated
)

• The reaction of antibody with antigen generates
  immune complexes
• Generally this complexing of antigen with
  antibody facilitates the clearance of antigen by
  phagocytic cells
• In some cases, large amounts of immune complexes
  can lead to tissue damaging type III
  hypersensitivity reactions

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immune-complex-mediated

• Large amounts of immune-complexes are carried and
  deposited at different sites
• The deposition of these complexes initiates a
  reaction that results in the recruitment of
  neutrophils to the site
• The tissue there gets injured as a consequence of
  the granular release by the neutrophils
• When antibodies or other proteins from non-human
  species are given therapeutically to patients,
  type III reactions are the potential side-effect

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Type IV (TDTH-mediated) Hypersensitivity

• Type IV reactions develop when antigen activates
  sensitised TDTH cells
• These cells are generally TH1, although sometimes
  Tc
• Activation of TDTH cells by antigen on
  appropriate antigen-presenting cells results in
  the secretion of various cytokines, such as IL-2,
  interferon gamma, etc)
• The overall effect is to draw macrophages into
  the area and activate them, promoting increased
  phagocytic activity and increased conc. of lytic
  enzymes

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Type IV

• As lytic enzymes leak out from the macrophages
  into the surrounding tissue, localised tissue
  destruction can ensue
• These reactions typically take 48-72 hours to
  develop, the time required for the accumulation
  of macrophages
• The hallmarks of type IV are the delay in time
  required for the reaction to develop and the
  recruitment of macrophages as opposed to
  neutrophils

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Type IV

• Many contact dermatitis reactions, including
  responses to formaldehyde, phenol, nickel,
  various cosmetics and hair dyes, poison oak and
  poison ivy are mediated by TDTH cells
• Most of these substances are small molecules that
  can complex with skin proteins
• This complex is then internalised by APCs in the
  skin, processed and presented together with an
  MHC class II molecule, causing activation of
  T-cells

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Poison oak

• A pentadecacatechol compound from the leaves of
  the plant complexes with skin proteins
• When T-cells react with this compound displayed
  by local APCs they differentiate into sensitised
  TDTH cells
• A subsequent exposure to this compound elicits
  activation of TDTH cells and cytokine production
• 48-72 hours after the second exposure,
  macrophages are recruited to the site
• Activation of the macrophages and release of
  their lytic enzymes leads to a IV reaction

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