Allergy - Hypersensitivity Reactions

What is hypersensitivity, and when does it typically occur?

An exaggerated immune response causing tissue damage, typically on second exposure to an allergen"

How are hypersensitivity reactions classified?

Based on immune mechanism and time course into four types:

• Type I (Immediate)

• Type II (Antibody-Mediated Cytotoxic),

• Type III (Immune Complex-Mediated)

• Type IV (Delayed-Type) [NO ANTIBODY INVOLVEMENT]

—

ACID:

• A = Type I: Anaphylaxis (Allergy – IgE)

• C = Type II: Cytotoxic (IgG/IgM)

• I = Type III: Immune Complexes

• D = Type IV: Delayed (T-cell)

What mediates Type I hypersensitivity, and what cells are involved?

IgE antibodies; involves mast cells, basophils, and eosinophils"

What happens during the first exposure in Type I hypersensitivity?

Allergen stimulates IgE production by B cells, which binds to Fcε receptors on mast cells, basophils, and eosinophils, sensitizing them"

What triggers degranulation in Type I hypersensitivity during second exposure?

Allergen cross-links bound IgE on sensitized cells"

What are the main mediators released during Type I hypersensitivity degranulation?

Histamine, leukotrienes, and heparin"

What physiological effects are caused by Type I hypersensitivity?

Smooth muscle contraction, mucus secretion, vasodilation, and increased vascular permeability"

What are examples of Type I hypersensitivity reactions?

Anaphylaxis, allergic rhinitis, asthma, eczema, hay fever"

What antibodies mediate Type II hypersensitivity, and what do they bind to?

IgG or IgM antibodies; bind to surface antigens on host cells"

What are the mechanisms of tissue damage in Type II hypersensitivity?

Complement activation, opsonization and phagocytosis, antibody-dependent cell-mediated cytotoxicity (ADCC)"

What are examples of Type II hypersensitivity reactions?

Mismatched blood transfusion, autoimmune hemolytic anemia, Rhesus incompatibility, Myasthenia Gravis"

What characterizes Type III hypersensitivity?

IgG or IgM antibodies form immune complexes with soluble antigens, which deposit in tissues"

What are the consequences of immune complex deposition in Type III hypersensitivity?

Complement activation and inflammation, causing vasculitis, glomerular nephritis, or arthritis"

What are examples of Type III hypersensitivity reactions?

Systemic lupus erythematosus (SLE), Farmer's lung disease"

What mediates Type IV hypersensitivity, and what cells are primarily involved?

T lymphocytes, especially CD4+ Th1 cells and CD8+ cytotoxic T cells"

What is the time course of Type IV hypersensitivity reactions?

Occurs 48-72 hours after exposure"

What role do cytokines play in Type IV hypersensitivity?

Released by T cells to recruit macrophages, neutrophils, and mast cells, causing inflammation"

Are antibodies involved in Type IV hypersensitivity?

No antibodies are involved"

What are examples of Type IV hypersensitivity reactions?

Contact dermatitis, tuberculin skin test (Mantoux test), type 1 diabetes mellitus, multiple sclerosis"