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Objectives
After working through this module, you should
be able to:
Differentiate the terms: hypersensitivity,allergy, anergy and atopy.
Explain the immunological mechanismsunderlying type I hypersensitivity reaction.
Describe the principal causes and symptomsassociated with each type of type I
hypersensitivity reaction. Discuss current approaches to the
prevention or relieve of type Ihypersensitivity response.
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Introduction
Definition:
Hypersensitivity
Term used when an immune
response results inexaggerated or inappropriate
reactions harmful to
the host
Allergy
Term which is often equated with
hypersensitivity
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Introduction
Definition:
Anergy
Term used for a state of
unresponsiveness which isspecific for a specific epitope
AtopyTerm used for a strong familialpredisposition for
immediate-hypersensitivity reactions
and is
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Introduction
The immune system is a destructiveorganization in the human body whichconstantly destroys invadingmicroorganisms from the environmentusing mechanisms such as complementfixation, ADCC, cytotoxic T cells, andreactions involving many othercytotoxic factors.
Some of the immune reactions maymayactually damage host tissuesactually damage host tissues due to anover response to certain antigensresulting in hypersensitivity.
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Introduction
Many reactions in hypersensitivityappear to have no apparent beneficialeffect on the human body.
There are 4 different types of hypersensitivity reactions, namelytypes I,II,III and IV; each type carries a
unique mechanism that can causetissue damage.
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HYPERSENSITIVITY REACTIONS
A hypersensitivity reaction may bedefined as:
an immune or inflammatory
response that occurs in anexaggerated or inappropriate form,or in an inappropriate situation
Occurs only following second orsubsequent contact with a particularantigen
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Characteristics of 4 types of hypersensitivity
Type Name Response
I Allergy Ab-mediated (IgE)
II Cytotoxic Ab-mediated (non-IgE, to cell
surfaces)
III Immune complex Ab-mediated (non-IgE, tosoluble molecules)
IV Delayed T cell mediated
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Type I Hypersensitivity
Type I hypersensitivity, also knownas the immediate typehypersensitivity
Is the most common form of theallergic reaction
The response occurs shortly after theexposure of an antigen to the bodywhich had previously been sensitizedto that specific antigen.
Type I hypersensitivity may result in
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TYPE I HYPERSENSITIVITY
An immediate response =anaphylaxis, allergy
Allergens – food, medicines, dust etc.
Basis of Type I response is productionof IgE instead of other antibodyisotypes, regulated by cytokines
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Mechanism of Type IHypersensitivity
antigen
IgE
Mast Cell
Degranulation
Mediators
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Mechanism of Type IHypersensitivity
IgE attaches via the Fc portion toFcε receptor on mast cell and theircirculating counterpart the basophil.
Cross-linking of IgE by the antigenstimulates the mast cell todegranulate and release the
mediators
2 types of mediators
- preformed
- newly synthesized
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Type I Hypersensitivity
PreformedMediators Function Antagonist
Histamine
HeparinProtease
Contraction of smooth muscle
AnticoagulantActivation of kinin andcomplement
Antihistamine
ProtaminenProteaseinhibitor
Eosinophil.Chemotactic.Factor of
anaphylaxis.
Chemotaxis Unknown
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Type I Hypersensitivity
Mediators Function Antagonist
Prostaglandin Inhibitdegranulation
IndometachinAspirin
Leukotrienes Contraction of smoothmuscle
Indometachin
Thromboxane Plateletaggregation
Indometachin
Platelets
activation
Platelet
aggregation
Phospholipas
e
• Newly synthesize mediators
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Type I Hypersensitivity
Clinical examples of type I hypersensitivity. Asthma
Urticaria
Anaphylaxis
How do you diagnose allergies?
Skin testingIgE levels
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i i i
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Type I Hypersensitivity
Principle of treatment with reference tothe stages of type I hypersensitivity
i. Avoidance of the allergen
ii. Medication to relieve symptomsantihistamines, topical nasal steroids,sodium cromoglycate
iii. Immunotherapyallergy immunization or desensitization
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Type I Hypersensitivity
Current approaches to the prevention ofhypersensitivity responses.
Immunotherapy
i. Desensitizationincrease IgG decrease
IgE
ii. recombinant cytokinesbalance Th1 /Th2 activity
f
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References
Roderick Nairn and Matthew Helbert. Immunology
for Medical Students. Elsevier Mosby. 2003.
Stites DP, Terr Al and Parslow TG. MedicalImmunology. 10th Edition, Appleton & Lange,Prentice-hall International Inc. 2001.
Roitt I, Brostoff J and Male D. Immunology. 6th Edition, Gower Medical Publishing, London. 2001
Cedric Mims, Hazel M. Dockrell, Richard V.Goering,Ivan Roitt, Derek Wakelin and Mark Zuckerman.Medical Microbiology. 3rd Edition, Elsevier Mosby.2004.
Marjorie Kelly Cowan and Kathleen Park Talaro.