Hypersensitivity Reactions and Allergies

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Questions and Answers

What is the primary mechanism of Type I hypersensitivity reactions?

IgG-mediated complement activation

T-cell mediated cytotoxicity

Antibody-dependent cellular cytotoxicity

IgE-mediated mast cell degranulation (correct)

Which of the following is a pre-formed granule mediator released by mast cells during Type I hypersensitivity reactions?

Prostaglandins

Interleukins

Histamine (correct)

Cytokines

What is the purpose of the skin prick test in diagnosing Type I allergies?

To detect allergen-specific IgE on mast cells (correct)

To induce an allergic reaction in the patient

To test for T-cell mediated immune responses

To measure IgE levels in the blood

Which of the following allergens is commonly associated with cross-reactive allergy reactions?

Pollen Signup and view all the answers

What is the primary function of heparin, a mediator released by mast cells during Type I hypersensitivity reactions?

Anticoagulation Signup and view all the answers

Which of the following is a characteristic of Type I hypersensitivity reactions?

Immediate reaction, occurring within minutes of exposure Signup and view all the answers

What is the primary location of mast cells in the body?

Connective tissue throughout the body Signup and view all the answers

How many allergens are typically tested at a time using the skin prick test?

10-15 Signup and view all the answers

What is the main goal of allergen-specific immunotherapy in IgE-mediated allergy?

To induce regulatory T cells and produce IgG antibodies Signup and view all the answers

What is the proposed mechanism behind the hygiene hypothesis?

Lack of exposure to infections leads to an underactive immune system Signup and view all the answers

What is the purpose of gradual dose increments in desensitization therapy for allergy-causing drugs?

To eliminate all available cell-bound IgE Signup and view all the answers

What is the role of regulatory T cells in allergen-specific immunotherapy?

They promote IgG production and suppress IgE production Signup and view all the answers

What is the primary mechanism of cellular destruction in hypersensitivity type II reactions?

All of the above Signup and view all the answers

What is the characteristic of genes associated with asthma susceptibility?

They are expressed in airway epithelial cells and regulate Th2 cells and ILC2 Signup and view all the answers

What is the primary role of IgG antibodies in allergen-specific immunotherapy?

They neutralize allergens before they can react with IgE Signup and view all the answers

What is the primary goal of seasonal allergen-specific immunotherapy?

To achieve permanent desensitization to specific allergens Signup and view all the answers

Study Notes

Hypersensitivity Reactions or Allergies

Pathological immune system-induced reactions to harmless antigens (allergens)
Antigen-specific reactions

Gell and Coombs Classification of Hypersensitivity Reactions

Type I Hypersensitivity: IgE-mediated
- Immediate hypersensitivity or anaphylactic reaction
- Allergic symptoms appear within 2-30 minutes after re-exposure to allergen
- Allergen-bound IgE crosslinks FcεRI on mast cells or basophils leading to degranulation and cell activation
- Pre-formed and activation-induced mediators cause immediate and late symptoms
- Reaction can be local or systemic

Pre-formed Granule Mediators

Histamine: causes vasodilation, increased vascular permeability, contraction of airway smooth muscle, mucus overproduction, and pain and itching
Heparin: anticoagulant
Proteases (e.g. tryptase, chymase, carboxypeptidase): tissue matrix remodeling
Mast cells: distributed in connective tissue throughout the body, near blood and lymph vessels, skin, peritoneal cavity, gut and airway mucosa, and blood basophils (2%)

Ag Dose and Route

Determines type and severity of symptoms

Cross-reactive Allergy Reactions

Between pollen and foods (Oral Allergy Syndrome / Pollen-Food Allergy Syndrome)
People allergic to birch pollen proteins develop an allergy after eating certain foods containing structurally homologous proteins

Skin Prick Test

For detection of type I allergies
Positive if previously sensitized mast cells loaded with allergen-specific IgE are present
Allergen re-exposure via the skin induces degranulation
Wheal area appears within 15-20 minutes
Test for: food and drug allergies, pollen, mold, house dust mite, animal hair/dander, milk, wheat, fish, tree nuts, eggs, legumes, insect venoms, penicillin

Therapy of IgE-mediated Allergy

Desensitization: Allergen-specific immunotherapy (AIT)
Permanent desensitization to allergens (e.g. airborne)
Intradermal administration of increasing doses of Ag for 2-3 years
Production of IgG that binds to Ag and neutralizes it before reacting with IgE
Based on induction of regulatory T cells (iTreg cells) that promote IgG production and suppress IgE production

Hygiene Hypothesis

Less hygienic environment: exposure to certain infections and environmental micro-organisms during infancy and childhood leads to a general state of non-responsiveness
Hygienic environment: children with genetic susceptibility to atopy and low exposure to infections and environmental microorganisms are more prone to developing atopic/allergic disease

Asthma Susceptibility Loci

Genes expressed in airway epithelial cells
Regulate differentiation and function of Th2 cells and type 2 innate lymphoid cells (ILC2)
Genes with other or unknown functions

Hypersensitivity Type II

IgG-mediated cytotoxic reactions
IgG or IgM binds Ag located on the cell surface
Cell destruction via complement activation, ADCC, and/or opsonization and phagocytosis via FcR/CR

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Description

This quiz covers the pathogenesis of hypersensitivity reactions, including the different types of reactions, and the Gell and Coombs classification. It focuses on the immune system's response to harmless antigens and the resulting allergic symptoms.