Questions: Type III and Type IV Hypersensitivity Reactions

Question 1

A patient develops progressive kidney disease and joint inflammation. Biopsy of kidney tissue shows granular deposits of IgG and complement components along the glomerular basement membrane. Which hypersensitivity type is responsible, and what is the cellular mechanism causing tissue damage?

• A. Type IV hypersensitivity — T cells recognize antigen presented in kidney tissue and recruit macrophages that damage the glomerular membrane
• B. Type III hypersensitivity — immune complexes deposited in the kidney activate complement, generating C3a/C5a that recruit neutrophils, which release destructive enzymes into glomerular tissue
• C. Type I hypersensitivity — IgE-coated mast cells in kidney tissue degranulate in response to the antigen, causing local inflammation
• D. Type II hypersensitivity — antibodies target kidney-specific antigens directly on the glomerular basement membrane and fix complement

Answer: (B)

Granular IgG + complement deposits (not linear deposits) are the hallmark of Type III — they reflect immune complexes depositing at filtration surfaces, not antibodies targeting tissue antigens directly (which would give linear deposits in Type II). The complement activation generates anaphylatoxins C3a and C5a that recruit neutrophils. Neutrophils attempt to phagocytose the complexes but instead degranulate their lytic enzymes into the surrounding tissue, causing the vasculitis and glomerulonephritis. Type IV would show mononuclear infiltrates with no immunoglobulin deposits.

Question 2

Why do immune complexes preferentially deposit in the glomeruli of the kidney, the synovial membranes of joints, and the walls of small blood vessels rather than in other tissues?

• A. These tissues express Fc receptors that actively bind the antibody portion of immune complexes
• B. These are filtration sites with high blood pressure and slow blood flow where intermediate-sized complexes become mechanically trapped
• C. These tissues express the antigens that are bound by the antibodies in the complexes, making deposition antigen-specific
• D. The complement components generated during complex formation are produced specifically in kidney and joint tissues

Answer: (B)

The kidney glomerulus and joint synovium are filtration beds with high hydrostatic pressure that force plasma through tight spaces — ideal traps for circulating intermediate-sized complexes. The pathology correlates with sites of mechanical filtration, not with the specific antigen targeted by the antibodies. This is why Type III diseases affect multiple organs simultaneously (systemic lupus affects kidneys, joints, and vasculature at once) regardless of what the specific autoantigen is. Large complexes are cleared by phagocytes in the spleen and liver; small complexes pass through filters; it is the intermediate-sized complexes that escape clearance and become trapped.

Question 3

Type IV (delayed-type) hypersensitivity involves antibodies, just like Types I–III, but the antibodies are produced more slowly — which is why the reaction takes 24–72 hours rather than minutes.

• T. True
• F. False

Answer: False

Type IV hypersensitivity is the only hypersensitivity that involves NO antibodies. It is mediated entirely by T cells — specifically sensitized CD4+ T cells that release IFN-γ and TNF-α, which activate macrophages, and CD8+ cytotoxic T cells in some reactions. The 24–72 hour delay occurs because T cells must travel to the site, secrete cytokines, and recruit macrophages — a slower cellular mobilization compared to pre-formed IgE on mast cells. The tuberculin skin test and contact dermatitis are purely T-cell phenomena with no antibody involvement.

Question 4

The clinical distinction between Type III and Type IV hypersensitivity can be confirmed by biopsy: Type III shows granular immunoglobulin deposits visible by immunofluorescence, while Type IV shows mononuclear cell infiltrates with no antibody deposits.

• T. True
• F. False

Answer: True

This histological distinction directly reflects the mechanistic difference. Type III is antibody-driven: immune complexes deposit in tissue, so immunofluorescence with anti-IgG or anti-complement antibodies reveals granular deposits at the sites of pathology (glomeruli, vessel walls). Type IV is T-cell-driven: no antibodies are deposited, so immunofluorescence is negative. The cellular infiltrate in Type IV consists of mononuclear cells — T cells and macrophages — not the neutrophilic infiltrate of acute Type III reactions. These histological signatures directly guide treatment choices.

Question 5

Both Type III and Type IV hypersensitivity arise from adaptive immune responses to antigens, yet they cause tissue damage through completely different effector mechanisms. Explain why the same 'antigen exposure in a sensitized individual' leads to such different pathologies.

Model answer: The difference lies in which arm of adaptive immunity was sensitized. Type III depends on antibody production (B cell responses): when antigen persists or is re-encountered, it forms soluble complexes with IgG antibodies, and it is the complement activation by these complexes that drives pathology — neutrophils are recruited by C3a/C5a and damage tissue trying to clear the complexes. Type IV depends on T cell sensitization: memory CD4+ T cells recognize antigen presented on MHC class II and release Th1 cytokines (IFN-γ, TNF-α) that activate macrophages, causing delayed inflammatory tissue damage with no antibody involvement. The same triggering event (antigen re-exposure) activates different weapons depending on which was primed during the original sensitization.

This question targets the mechanistic understanding that makes the hypersensitivity classification clinically useful. Students who understand why Type III leads to neutrophil-mediated vasculitis while Type IV leads to macrophage-driven granuloma formation can predict which treatments will work: corticosteroids suppress T cell activation in Type IV, while treating Type III might also require targeting complement or reducing antigen/immune complex load.