Hypersensitivity Flashcards
What is hypersensitivity?
Excessive or inappropriate immune response that results in a disease
What are the possible causes of hypersensitivity reactions?
- Inappropriate immune response against host tissue (autoimmunity)
- Inappropriate immune response against commensal microbes or inert environmental antigens (e.g. food)
- Excessive or inappropriate response to pathogenic microbes resulting in extensive tissue destruction
How are hypersensitivity reactions classified?
- The type of immune response (antibody- v. cell-mediated)
- Mechanism of tissue injury
What types of hypersensitivity reactions are antibody mediated?
- Type I
- Type II
- Type III
- Type V
What is the pathogenic immune mediator in type I (immediate) hypersensitivity?
IgE
What is the mechanism of tissue injury in type I (immediate) hypersensitivity?
Mast cells and their mediators:
- Vasoactive amines (chiefly histamine): increased vascular permeability, contraction of intestinal, bronchial, and arterial smooth muscle
- Leukotrienes and prostaglandins: bronchoconstriction, increased vascular permeability, mucus production
- Cytokines (IL-4, IL-5, and IL-13): recruitment and activation of inflammatory cells
What are examples of type I (immediate) hypersensitivity diseases?
- Allergic reactions
- Anaphylaxis (systemic allergic reaction)
- Asthma
- Eczema
What are examples of allergens?
- Dust mites
- Peanuts
- Dyes
- Creams
- Fragrances in skin care products
- Eggs
- Pollen
- Spores
- Drugs
What is the etiology of allergy?
Occurs in genetically predisposed (atopic) individuals
What are diseases caused by allergy?
- Allergic rhino-conjunctivits (hay fever): due to respiratory exposure to the allergen
- Asthma: due to respiratory exposure
- Food allergy: due to GI exposure
- Atopic dermatitis (eczema): due to skin exposure
- Anaphylaxis: due to presence of the allergen in the blood
How are allergic reactions managed?
- Identification of the causative allergen and its avoidance
- Anti-inflammatory agents and antihistamines to treat symptoms
- Administration of epinephrine (EpiPen) in anaphylaxis
How are allergies diagnosed?
- Very high levels of serum IgE
- Skin testing to specific allergens
What is the pathogenic immune mediator in type II hypersensitivity?
IgM and IgG against antigens bound to cells or tissues
What is the mechanism of tissue injury in type II hypersensitivity?
- Phagocytosis
- Antibody-dependent cell mediated cytotoxicity (ADCC)
- Receptor blocking
- Complement-mediated lysis
What are examples of type II hypersensitivity diseases?
- ABO incompatibility
- Rh incompatibility
What is a complication of ABO incompatibility in blood transfusions or pregnancy?
Hemolysis and subsequent thrombosis
What is the pathogenic immune mediator in type V hypersensitivity?
Stimulating antibodies against antigens bound to cells or tissues
What is the mechanism of tissue injury in type V hypersensitivity?
Overactivity of the target organ
What are examples of type V hypersensitivity diseases?
Grave’s disease
How does type V hypersensitivity differ from type II?
- In type II, the reaction leads to direct tissue damage
- In type V, the reaction leads to overactivity of the target organ (e.g. the thyroid gland in Grave’s disease)
What is the pathogenic immune mediator in type III hypersensitivity?
Circulating immune complexes of antigens and IgM or IgG, due to failure to clear immune complexes
What is the mechanism of tissue injury in type III hypersensitivity?
Ag–Ab complexes activate the complement and Fc receptors, resulting in activation and recruitment of leukocytes
What are examples of type III hypersensitivity diseases?
- SLE
- Rheumatoid arthritis
What is the pathogenic immune mediator in type IV hypersensitivity?
CD4+ T cells or CD8+ CTLs—an exaggerated interaction between the antigen and normal T cell-mediated responses
What is the mechanism of tissue injury in type IV hypersensitivity?
- Macrophage activation (M1) resulting in cytokine-mediated inflammation
- Direct cell killing by CTLs
What are examples of type IV hypersensitivity diseases?
Tuberculosis
