Transcript Reasons why there is a high incidence of septic shock

Over-reactions of the immune
system
Dr Kathy Triantafilou
University of Sussex
School of Life Sciences
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

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

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
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

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)

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

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

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

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

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

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

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)

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

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

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

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

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)

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

Histamine
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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
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

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
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

Consequences of type I
Systemic anaphylaxis
 Localised anaphylaxis
 Allergic Rhinitis
 Asthma
 Food allergies
 Atopic dermatitis

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

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

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

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

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)
 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
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

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

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, IL3, IL-5

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

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
Detection of type I
Another method is to determine serum
levels of IgE
 Using the radioimmunosorbent test (RIST)
 Patient’s serum is reacting with agarose
beads or paper disks coated with rabbit antiIgE

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

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 mastcell activation

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

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)

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

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)

Delayed hemolytic reaction
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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
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

Hemolytic disease of the
newborn
During pregnancy, fetal red blood cells are
separated from the mother’s 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 mother’s circulation
 The fetal red blood cells activate the Rh-specific
B-cells of the mother

The secreted IgM antibodies clear the fetal red
blood cells from the mother’s 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

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 mother’s circulation and facilitate
their clearance before B-cell activation

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

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

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

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 nonhuman 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
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

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

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
