Hypersensitivity Reactions Practice Questions

Correct answer: Histamine release

Histamine is a pre-formed vasoactive amine that acts rapidly on H1 receptors to cause smooth muscle contraction and increased capillary permeability. Leukotrienes and prostaglandins are synthesized de novo and contribute more significantly to the late-phase response.

Correct answer: Type II (Antibody-mediated)

Hemolytic disease of the newborn is a Type II hypersensitivity reaction where maternal IgG antibodies cross the placenta and target antigens on fetal red blood cells. This leads to opsonization and phagocytosis or lysis of the RBCs.

Correct answer: Type III hypersensitivity

The Arthus reaction occurs when antigens are injected into the skin of an individual with pre-existing circulating antibodies. Immune complexes form locally in vessel walls, triggering complement activation and neutrophilic infiltration, leading to fibrinoid necrosis.

Correct answer: CD4+ Th1 cells

Type IV hypersensitivity is mediated by T-cells rather than antibodies. In a tuberculin test, sensitized Th1 cells recognize the antigen and release cytokines like IFN-gamma to recruit and activate macrophages over 24 to 72 hours.

Correct answer: Type II

Graves disease is a form of Type II hypersensitivity where antibodies are directed against cell surface receptors. Instead of causing cell destruction, these antibodies mimic the natural ligand (TSH) and overstimulate the target cell.

Correct answer: Anti-GBM antibodies against alpha-3 chain of Type IV collagen

Goodpasture syndrome is a Type II hypersensitivity where autoantibodies target fixed antigens in the glomerular and alveolar basement membranes. Immunofluorescence shows a characteristic smooth, linear pattern of IgG deposition.

Correct answer: Formation of circulating immune complexes

Serum sickness is a systemic Type III hypersensitivity reaction. It occurs when large amounts of foreign antigen lead to the formation of immune complexes that exceed the body's clearance capacity, depositing in various tissues and activating complement.

Correct answer: Type 1 Diabetes Mellitus

In Type 1 Diabetes, CD8+ CTLs directly target and destroy insulin-producing beta cells in the pancreatic islets. While many Type IV reactions involve CD4+ cells (like contact dermatitis), islet destruction is primarily a CTL-mediated process.

Correct answer: Mast cell degranulation releasing histamine and heparin

The early phase occurs within minutes of allergen exposure and is dominated by the release of pre-formed mediators from mast cells. This causes the classic wheal-and-flare reaction, bronchospasm, and increased mucus secretion.

Correct answer: Type III

Fibrinoid necrosis occurs in Type III hypersensitivity (vasculitis) when immune complexes and complement activation cause severe damage to the vessel wall. The leakage of plasma proteins, including fibrin, creates a bright pink, smudgy appearance on H&E stains.

Correct answer: Interleukin-4 (IL-4)

IL-4 produced by Th2 cells is the primary signal for B-cells to undergo class-switch recombination to IgE. IL-5 is also involved but primarily functions in the recruitment and activation of eosinophils.

Correct answer: Immune complex deposition (Type III)

A granular pattern on immunofluorescence is characteristic of Type III hypersensitivity. It represents the random deposition of antigen-antibody complexes along the glomerular basement membrane or in the mesangium.

Correct answer: Nickel allergy from jewelry

Nickel is a hapten—a small molecule that is not antigenic on its own but becomes so when bound to a host protein. In contact dermatitis, these hapten-protein complexes are processed by Langerhans cells and presented to T-cells.

Correct answer: Acetylcholine receptor

In Myasthenia gravis, autoantibodies bind to acetylcholine receptors at the neuromuscular junction. This leads to receptor internalization or blockage, resulting in muscle weakness, a classic Type II hypersensitivity mechanism.

Correct answer: Eosinophil

Eosinophils are recruited during the late-phase response of Type I hypersensitivity (2–24 hours later). They release major basic protein and eosinophil cationic protein, which are toxic to epithelial cells and contribute to tissue damage.

Correct answer: C5a generated by complement activation

Complement activation by immune complexes generates C5a, which is a potent chemoattractant for neutrophils. Neutrophils then attempt to phagocytose the complexes but instead release lysosomal enzymes that damage the surrounding tissue.

Correct answer: Type III and Type IV mechanisms

Hypersensitivity pneumonitis involves Type III (immune complex) reactions in the acute phase and Type IV (granulomatous inflammation) reactions in the chronic phase after repeated inhalation of organic dusts.

Correct answer: Fas ligand (FasL)

Cytotoxic T-cells can induce apoptosis in target cells via the Fas-FasL pathway. FasL on the T-cell binds to Fas on the target cell, activating caspases and leading to programmed cell death.

Correct answer: Widespread vasodilation and plasma extravasation

Anaphylaxis involves a systemic release of mast cell mediators, leading to massive vasodilation and a sudden drop in blood pressure (shock). Loss of plasma into the interstitial space (edema) further reduces effective circulating volume.

Correct answer: Type II hypersensitivity

Hyperacute rejection is a Type II reaction where recipient antibodies (e.g., anti-ABO or anti-HLA) immediately bind to the vascular endothelium of the graft, triggering complement activation, thrombosis, and ischemic necrosis.