Over-reactions of the immune Over-reactions of the immune systemsystem

Dr Kathy Triantafilou

University of Sussex

School of Life Sciences

Reactions of the immune systemReactions 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-reactionsOver-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 classificationGell 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 hypersensitivityType 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

DegranulationDegranulation 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 IComponents 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

AllergensAllergens 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

AllergensAllergens 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

IgEIgE 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

IgEIgE Serum levels in normal individuals are in the range of

0.1-0.4 g/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 basophilsMast 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 receptorsIgE-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 (FcRI)mast cells and basophils (40,000-90,000

receptors on a cell)binds with 1000 fold higher affinity

– Low affinity receptor (FcRII)

High affinity receptor (FcHigh affinity receptor (FcRI)RI) The high affinity receptor contains four

polypeptide chains:

– an , a chain and two identical chains Displays immunoglobulin-fold structure, and

thus belongs to the immunoglobulin superfamily The chain binds the IgE molecules The chain spans the membrane four times and

is thought to link the to the homodimer The chains contain ITAMS similar to CD3

Low affinity receptor (FcLow affinity receptor (FcRII)RII) 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 FcRII 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 crosslinkageReceptor 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 FcRI 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 Intracellular events leading to degranulationdegranulation

The cytoplasmic domains of the and chains of the FcRI 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

CaCa2+2+ channels 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)

MediatorsMediators 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

HistamineHistamine 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

Leukotrienes and postaglandinsLeukotrienes 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 postaglandinsLeukotrienes 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

CytokinesCytokines 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-

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 IConsequences of type I

Systemic anaphylaxis Localised anaphylaxis Allergic Rhinitis Asthma Food allergies Atopic dermatitis

Systemic anaphylaxisSystemic 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 anaphylaxisLocalised 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

AsthmaAsthma

Common localised anaphylaxis is asthma There are two types of asthma:

– allergic asthmaairborne or blood-borne allergens, such as

pollen, dust, fumes, insect products or viral antigens trigger an asthmatic attack

– intrinsic asthmainduced by exercise, cold, independently of

allergen stimulation

AsthmaAsthma 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 responseAsthmatic 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 allergiesFood 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 dermatitisAtopic 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 reactionLate-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

Detection of type IDetection 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 testSkin 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 IDetection 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 anti-IgE

Therapy of type ITherapy 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 ITherapy 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

Therapy of type ITherapy 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 Type II hypersensitivity (Antibody-mediated cytotoxic)(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 reactionsTransfusion 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 reactionsTransfusion 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 reactionDelayed 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

Hemolytic disease of the Hemolytic disease of the newbornnewborn

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 Hemolytic disease of the newbornnewborn

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

PreventionPrevention

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

TherapyTherapy 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 Drug-induced hemolytic anemiaanemia

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 Type III hypersensitivity (immune-complex-mediated)(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-mediatedimmune-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

Type IV (TType IV (TDTHDTH-mediated) -mediated)

HypersensitivityHypersensitivity 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 IVType 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 IVType 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 oakPoison 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