Allergy and Hypersensitivity

I. Introduction

A. Definitions

 Allergy  Immune-mediated response to innocuous environmental antigen  Can be humoral or cell-mediated reaction  Usually involves prior exposure to antigen resulting in sensitization of individual  Allergen  Innocuous antigen  Universal  Non-reactiving to most people

 Hypersensitivity reactions  Harmful IRs that cause tissue injury and may cause serious pathologies  Atopy  State of increased susceptibility to immediate hypersensitivity usually mediated by IgE Abs  Over-react to common environmental Ags

B. Four types of immune-mediated hypersensitivity reactions causing tissue damage

 Type I = Anaphylaxis hypersensitivity (TH 2 = IgE)  Type II = Cytotoxic hypersensitivity (IgG)  Type III = Immune complex hypersensitivity (IgG)  Type IV = Cell-mediated hypersensitivity (TH 1 , TH 2 , CTL)

II. Type I (Anaphylaxis) Hypersensitivity

A. Pathway

 IgE made during primary response to soluble Ag  Binds to high affinity Fc e RI on mast cells, basophils and activated eosinophils  Sensitizes individual (become allergic)   IgE aka reagin Secondary exposure  allergen binds to IgE on sensitized mast cells, basophils or eosinophils  IgE Ab crosslinking on leads to rapid release of preformed inflammatory mediators

High affinity Fc composed of

e

RI is functional on mast cells, basophils, and activated eosinophils. It is

a,b

and two

g

chains. Crosslinking of Fc degranulation.

e

RI on cells by Ag and IgE induces

 Induces degranulation  Release of inflammatory mediators [pre-formed substances including histamine, slow reacting substance of anaphylaxis (SRS-A), heparin, prostaglandins, platelet activating factor (PAF), eosinophil chemotactic factor of anaphylaxis (ECF-A), and various proteolytic enzymes]  Eosinophils release major basic protein which induces degranulation of mast cells and basophils

 Tachyphylaxis  Depletion of mast cell granules  Accounts for unresponsiveness of a patient to a skin test following an anaphylactic reaction (lasts 72-96 hours after a reaction)

B. Ig-E mediated reactions differ depending on route of administration and dose

 Connective tissue mast cells  Associated with blood vessels  IV-high dose  bloodstream  Activated by allergen in the systemic  Systemic release of histamine  Systemic anaphylaxis

 SC-low dose  subcutaneous injection  local release of histamine  Wheal and flare reaction

 Mucosal mast cells  Inhalation – low dose  Activated by inhaled allergen  Smooth muscle contraction of lower airways  Bronchoconstriction  Asthma  Allergic rhinitis (hay fever)  Increased mucosal secretions  Irritations

Fig. 10.24: Allergen-induced release of histamine by mast cells in skin causes localized swelling. Swellings (wheals) appear 20 min. after intradermal injection of ragweed pollen (R), histamine (H). Saline bleb (S) is due to volume of fluid.

Fig. 10.14

Properties of inhaled allergens that favor TH2 priming that promotes IgE isotype switching.

Fig. 10.15

Sensitization to an inhaled allergen.

Soluble allergen is processed by APC and displayed to TH T cells help B cells to produce IgE which then binds to mast cells. IL-4 promotes isotype switching to IgE.

2 T cells.

Fig. 10.21: Allergic rhinitis (hay fever) is caused by inhaled allergen entering the respiratory tract. Sneezing, runny nose – nasal discharge is full of eosinophils. Allergic conjunctivitis results if the conjunctiva of the eye is affected (itchy, watery, and swelling of eyes).

 Ingestion – Activated by ingested allergen  Food allergy  Gut epithelial cells are involved  Intestinal smooth muscle contraction  Vomiting  Diarrhea  Dissemination through bloodstream causes urticaria (hives) or anaphylaxis (rare)

Fig. 10.25: Ingested allergen can cause vomiting, diarrhea and urticaria.

Summary of Type I Hypersensitivity Reactions

Fig 10.12

C. Hereditary predisposition for IgE synthesis

 Fc e R genes  Cytokine genes involved in     Isotype switching Eosinophil survival Mast cell proliferation Example: IL-4 promoter mutation which leads to elevated IL 4 can favor IgE  MHC class II   MHC:peptide combinations may favor TH2 response Example: ragweed pollen associates with HLA-DRB1*1501

D. Type I hypersensitivity reactions can be divided into immediate and late stages

 Acute (minutes) versus Chronic (5-12 hours) Reactions  Immediate allergic reactions is then followed by a late-phase response

 Acute – Immediate  Peaks within minutes after allergen injection or inhalation and then subsides  Wheal and flare   Bronchial constriction in asthma IgE crosslinking  rapid degranulation  Release of preformed inflammatory mediators    Histamine, serotonin Mast cell chymase, tryptase, carboxypeptidase and cathepsin G  breaks down tissue matrix proteins (remodeling of connective tissue matrix) TNF a

Mast cell stained for protease chymase demonstrating abundant granules residing in the cytoplasm.



Chronic – Late

 Caused by influx of inflammatory leukocytes (including eosinophils)  Chronic allergic inflammation  Tissue damage  Edema, long-lasting

 Chemokines  Heparin  Lipid mediators derived from membrane phospholipids  Form a precursor called arachidonic acid  Many anti-inflammatory agents inhibit arachidonic acid metabolism (e.g. aspirin)  Arachidonic acid forms:  Leukotrienes    Prostaglandins Thromboxanes Platelet activating factor

Fig. 10.5: Mast cell products involved in allergic reactions.

Fig. 10.7

Mast cell production of prostaglandins and leukotrienes by different enzyme pathways starting with arachidonic acid.

Fig. 10.8: Eosinophils display a unique staining pattern with bilobed nuclei and stain pink with eosin. Eosinophils are specialized granulocytes that release toxic mediators in IgE-mediated responses.

Fig. 10.9: Products of activated eosinophils.

Fig 10.16: Immediate and late-phase reactions to house dust mite allergen (HDM) injected intradermally. Saline injection = control.

Wheal = raised area of skin around injection site; flare = redness (erythema) spreading out from the wheal.

E. Two types of anaphylaxis

 1. Systemic anaphylaxis  Generalized response to systemically administered Ag (e.g. IV) or rapidly absorbed from gut  Immediate: a lot of mast cell products released quickly  Smooth muscle constriction of bronchioles  breathing difficulties  Epiglottal swelling  Asphyxiation  Can be fatal

 Arterioles dilate  Arterial blood pressure decreases  Capillary permeability increases (increases vascular permeability  Fluid loss into tissue spaces  Edema  Late phase reaction = sustained edema  Circulatory shock  Can be fatal

 Examples of allergens:  Penicillin (or cephalosporins)  Penicillin = hapten  beta lactam ring reacts with amino groups on host proteins  conjugates form  Bee, wasp or hornet venom  Peanuts or brazil nuts  Anti-sera

 2. Localized anaphylaxis  Atopic (out of place) allergy  Examples:  Allergic rhinitis (hay fever) – URT   Airborne allergens: pollen, spores, animal dander, house dust mite feces Allergens diffuse across the mucus membranes of nasal passages   Mast cells sensitized in mucus membrane upon primary exposure Upon secondary exposure – itchy, runny eyes and nose, sneezing coughing

 Bronchial asthma = allergic asthma – LRT    Air sacs (alveoli) fill with fluid and mucus Wall of bronchi constricted Bronchodilators relax muscles, making breathing easier (inhalers)     Anticollinergic Sympathetic activators Metaproterenol Albuterol  Hives (food allergy)   Vomiting and diarrhea = local response Urticaria = systemic response

Fig. 10.23: Inflammation of the airways in chronic asthma restrict breathing A = section through bronchus of individual who died from asthma. MP = mucus plug – restricts airway. White plug depicts remaining passageway in bronchial lumen.

B = Bronchial wall at higher magnification demonstrating presence of inflammatory infiltrate consisting of eosinophils, neutrophils, and lymphocytes. L = lumen of bronchus.

 In vivo skin testing can help to identify responsible allergens  rapid inflammation  Diameter of swelling measured  Wheal-and-flare reactions  Cutaneous allergic response  Develops within 1-2 minutes  minutes lasts ~30

F. Desensitization

 Subcutaneous injections of Ag  IgG Abs  to produce can compete with IgE Ab, and neutralize allergens before they reach mast cells  Tiny amounts injected initially, then dose is increased  Diverts IR from TH 2 to TH 1  Decreases IgE production  65-75% effective treatment of inhaled allergens

G. Treatment

 Inhibit allergic reactions – Examples  Desensitization (described above)  Experimental:  Inhibit IL-4, IL-5 and/or IL-13 or CD40L to reduce IgE responses   Use cytokines that enhance TH1 responses  g IFN, a IFN, IL-10, IL-12, and TGF b Block Fc e R (e.g. with modified Fc components that lack variable domains)

 Block allergic response effector pathways    Epinephrine    Endothelial tight junctions reform Relaxation of smooth muscle Stimulation of heart (increase BP) Anti-histamines   Block histamine receptors Decrease urticaria (hives) Corticosteroids  Reduce inflammation

Figure 10.20: Effect of epinephrine on blood pressure Time 0 = point at which anaphylactic response began. Arrows = times when epinephrine was administered.

III. Type II (Cytotoxic) Hypersensitivity

 A. Host cells are killed or lysed  Cell surface antigens  B. IgG (mainly) or IgM Abs react with cell surface receptors, matrix associated Ag or modified cell membranes  Complement is activated    C’ binds Ig (C1q) C’ cascade results in formation of membrane attack complex (MAC) Holes are punched in target cells  Death

 FcR bind Ig:Ag complexes  FCR-bearing accessory cells are activated (e.g. macrophages, neutrophils and NK cells)  Especially important mechanism used by splenic macrophages  clearance of cells  Opsonization induced via FcR + CR1

 Antibody-dependent cell-mediated cytotoxicity (ADCC) is induced in NK cells  NK cells secrete preformed perforin and granzyme from cytoplasmic granules  Perforin forms a pore in target cell – transmembrane polymerization  Granzyyme (aka fragmentin) = 3 serine proteases – digest host proteins and activate endonucleases  DNA is degraded into ~200 by multimers (subunits) = APOPTOSIS

 Examples  Hemolytic disease of the newborn (Erythroblastosis fetalis) (Abs to Rh Ags)

Hemolytic Disease of the Newborn (Erythroblastosis fetalis) Type II hypersensitivity Alloantibodies resulting from Rh incompatibilities between mother and father Spacing of Rh antigen is too far to activate C’ or cause agglutination.

Fetal RBC destroyed by macro phages causing edema. This may in turn lead to heart failure, edema and fetal death (hydrops fetalis).

More examples:

 Mismatched blood transfusion (Abs to A/B Ags)  Autoimmune hemolytic anemia (Abs to self Ag on RBC)  Autoimmune thrombocytopenia purpura (Abs to platelet integrin  abnormal bleeding/hemorrhaging)  Goodpastuer’s Syndrome (renal failure due to anti-basement membrane collagen Abs)

 Pemphigus vulgaris (skin blisters – anti epidermal cadherin Abs)  Acute rheumatic fever (cross-reactive Abs to cardiac muscle generated following Streptococcus group A infection  myocarditis, arthritis, heart valve scarring)  Drug allergies (e.g. penicillin) (drug combines with cell proteins)

Penicillin interferes with the bacterial enzyme transpeptidase after binding to the active site in the enzyme.

Penicillin may also bind to surface proteins on human cells (RBC = most common). This creates a new epitope that can act like a foreign Ag.

Fig. 10.27: Penicillin-protein conjugates stimulate the production of anti-penicillin antibodies. Penicillin-modified RBC get coated with C3b as a bystander effect of C’ activation by bacterial activating surfaces for which the penicillin was administered. This initiates the process by inducing opsonization by macrophages.

 RBC and platelets are especially susceptible to lytic effects of Type II hypersensitivity, owing to reduced levels of C’ regulatory proteins than other cells have.

 Ab can alter signaling properties of cells in autoimmunity  Grave’s Disease   Agonist Ab  Hyperthyroidism Ab = anti TSH receptor specific  thyroid hormone overproduction of  Myasthenia Gravis (MG)   Antagonist Ab  Blocks neuromuscular transmission Anti-acetylcholine receptor specific  weakness progressive  MORE LATER - AUTOIMMUNITY

IV. Type III (Immune complex) Hypersensitivity

A. Description of immune complexes

 Form through association of Ab with multivalent soluble Ag  Complexes become deposited on blood vessel walls or tissue sites and activate C’  Inflammation induced (C5a)  Pathogenicity depends on size of complex  Large = cleared by C’ fixation (Ab excess)  Small = deposited (Ag excess)

B. Damage to host tissue

 Blood vessels  Vasculitis  Kidney glomerular basement membrane  Glomerulonephritis  Synovial tissue of joints  Arthritis or Arthralgia  Skin  Butterfly rash in SLE

The pathology of type III hypersensitivity reactions is determined by the sites of immune-complex deposition.

 Mechanism:  C’ is activated      Basophils and platelets degranulate Histamine and other inflammatory mediators are released Vascular permeability increases Platelets aggregate and form microthrombi (blood clots) on vessel walls  Burst, hemorrhaging of skin Recruitment of PMNL by chemotaxis  Further degranulation, enzyme release and host damage  vasculitis

C. Five types of disease

 Arthus reaction  Serum sickness  Persistent viral, bacterial or protozoan infection in face of weak Ig response  Continuous autoantibody production  Immune complexes formed at body surfaces

D. Examples

 Arthus Reaction   A skin reaction occuring in sensitized (already immune) individuals where Ag is injected into the dermis and reacts with IgG in extracellular spaces This in turn leads to C’ fixation/activation (mast cell degranulation) and recruitment of phagocytic cells leading to inflammation  Increased fluid and protein release  Increased phagocytosis  Blood vessel occlusion by platelets  Experimental model for I.C. disease

Localized deposition of immune complexes within a tissue causes a type III hypersensitivity reaction.

 Serum Sickness  Systemic reaction to a large dose of Ag (7 10 days after injection)  Ag is poorly catabolized and remains in circulation long enough to be available following primary immune response  Chills, fever, urticaria, arthritis and glomerulonephritis

 Examples:  Horse serum used to treat pneumococcal pneumonia prior to antibiotics usage  Anti-venin – horse anti snake venom  Mouse anti-lymphocyte globulin used for immunosuppression of transplantation (mouse MoAb)  Streptokinase (bacterial enzyme) to treat heart attack victims  Antibiotics (penicillin or cephalosporin)

 Serum sickness is usually a self-limited disease  Symptoms improve as host Abs increase to zone of Ab excess  Can be fatal if kidneys shut down or hemorrhaging occurs in brain   Treatment  Prednisone (anti-inflammatory – corticosteroid) and Benadryl (anti-histamine) Prior sensitization is NOT prerequisite  Reaction can occur on first encounter if Ag isn’t readily cleared from circulation and is present at high concentration

Serum sickness is a classic example of a transient immune-complex mediated syndrome.

 Persistent viral, bacterial or protozoan infections  Results in chronic immune complex formation (IC)  Examples:  Subacute bacterial endocarditis  Acute glomerulonephritis  Chronic viral hepatitis

 Autoantibody produced continuously  Prolonged IC formation  Systemic lupus erythematosus (SLE)  Glomerulonephritis, arthritis, vasculitis  AutoAbs to DNA, RNA and proteins associated with nucleic acids

 Immune complex formed at body surfaces (lungs) (IgG not IgE)  Exposure to very large doses of inhaled allergens  Inflammation of alveolar wall of lung  Farmer’s lung  mold spores  Inhalation of hay dust or Gas exchange compromised

V. Type IV Hypersensitivity

A. Features

 T-cell mediated immune responses  Includes:  Delayed-type hypersensitivity  Contact hypersensitivity  Gluten-sensitive enteropathy (Celiac disease)

B. Mechanism

 Delayed-type hypersensitivity = DTH    T DTH recruited Soluble Ag Cell-associated Ag  cytotoxicity  macrophages, TH 1 TH 1 activation activation  T cyt  Cytokines and chemokines produced  IL-2, g IFN, IL-3, TNF a , TNF b and GM-CSF  Other cells recruited  Macrophages, basophils, other lymphocytes  Tissue can be severely damaged

 Cytokines, chemokines and cytotoxins made by TH 1 during Type IV Hypersensitivity Reactions  Chemokines  Recruitment of macrophages to the site of Ag deposition  Cytokine  g IFN  Macrophage activation, release of inflammatory mediators  IL-3/GM-CSF  Increased monocyte synthesis in bone marrow

 Cytotoxins – TNF a  TNF a  TNF a and TNF b activates macrophage and TNF b  blood vessel adhesion molecules expressed (activation of endothelial cells)  cells infiltrate, edema  TNF b cells  cytotoxic to macrophages and other

 T cyt may also be involved in Type IV hypersensitivity reactions  Cell-mediated cytotoxicity and g IFN production  Modified peptides associate with class I (e.g. pentadecacatechol of poison ivy = lipid soluble)

 The time course of a delayed type hypersensitivity reaction  Acquired IR  1 st phase:  Uptake, processing and presentation of Ag  2 nd phase:   Previously primed TH 1 cells migrate to site of infection and become activated T cells secrete mediators that result in recruitment of macrophages  Inflammation ensues fluid and protein accumulate  Lesion  Induration

C. Examples

 Tuberculin hypersensitivity  Tuberculosis skin test (Mantoux test, Heath test – multipronged skin prick)  Purified protein derivative (PPD) from

Mycobacterium tuberculosis

 Injected intradermally  After 48 hours, induration (swelling/lesion) indicates positive reaction  Related to degree of sensitivity  Indicates prior exposure to

M. tuberculosis

 Other microbial products used in Type IV skin testing include  Histoplasmic (for histoplasmosis –

capsulatum

– fungus)

Histoplasma

  Coccidiodin (for coccidiodomycosis – fungus) Lepromin (for Hansen’s disease –

Mycobacterium leprae

)  Brucellergen (for brucellosis – bacteria –

Brucella

spp.)

 Allergic contact dermatitis  Haptens combine with skin proteins  Pentadecacatechol (poison ivy)  Cosmetics  Metals (jewelry)  Nickel  Gold  Transplantation (Graft) Rejection

 Autoimmune diseases  Rheumatoid arthritis (joint inflammation)  Multiple sclerosis and Experimental allergic encephalomyelitis (EAE) (demyelination)  Diabetes mellitus (IDDM) (pancreatic beta cell destruction)  Gluten-sensitive enteropathy – Celiac disease  Ag = Gliadin  Malabsorption results from villous atrophy in small intestine

Fig. 10.33: Summary