Type III hypersensitivity is one of those Step 1 topics that feels “conceptual” until you realize it’s basically a predictable sequence: circulating immune complexes form → they deposit in tissues → complement turns on → neutrophils arrive → collateral damage. If you can visualize that timeline, you can answer most question stems—even when they disguise it behind lupus nephritis, serum sickness, or post-strep glomerulonephritis.
Big Picture Definition (What makes Type III “Type III”?)
Type III (immune complex–mediated) hypersensitivity occurs when antigen–antibody (IgG or IgM) immune complexes form in the circulation and deposit in tissues, triggering:
- Complement activation (especially via classical pathway)
- Recruitment of neutrophils
- Inflammation and tissue damage (vasculitis, nephritis, arthritis, etc.)
Key distinguishing features
- Antibodies are directed against soluble antigens, not fixed cell surface or ECM antigens.
- Injury is caused by deposited immune complexes, not direct antibody binding to a cell (Type II) or T-cell mediated injury (Type IV).
Step 1 Pathophysiology: The 6-Step Story You Should Memorize
1) Soluble antigen exposure
Sources include:
- Exogenous (drugs, animal proteins, microbes)
- Endogenous (self antigens in autoimmune disease—e.g., SLE)
2) IgG/IgM bind antigen → immune complexes form
Complex size matters:
- Large complexes tend to be cleared more efficiently.
- Small-to-intermediate complexes are more likely to persist and deposit (classic testable concept).
3) Immune complexes circulate and deposit in “filtration/high-pressure” sites
High-yield deposition sites:
- Blood vessels (small vessels) → vasculitis, palpable purpura
- Glomeruli → nephritic syndrome
- Joints → inflammatory arthritis
- Skin → urticarial or purpuric lesions
4) Complement activation (classical pathway)
Immune complexes activate complement → generates:
- C3a, C5a (anaphylatoxins) → increase vascular permeability, mast cell degranulation
- C5a → neutrophil chemotaxis (very HY)
5) Neutrophils attempt to clear deposits → “frustrated phagocytosis”
Neutrophils bind immune complexes via Fc receptors but can’t “eat” complexes embedded in tissue → release:
- Proteases
- Reactive oxygen species
→ tissue injury
6) Clinical inflammation and organ-specific injury
The organ involved determines presentation.
What It Looks Like Clinically (Classic Presentations)
Time course clue
- Often 1–2 weeks after exposure (e.g., serum sickness after a drug, or post-infectious GN after strep)
Common clinical patterns
- Vasculitis: palpable purpura, skin lesions, systemic symptoms
- Nephritis: hematuria, RBC casts, hypertension, periorbital edema
- Arthritis/arthralgias
- Fever
Serum sickness (systemic Type III)
Triad (super HY):
- Fever
- Urticaria
- Arthralgia
Often with proteinuria, lymphadenopathy. Historically from foreign serum (antitoxins), but Step exams love drugs as triggers (e.g., penicillin) and monoclonal antibodies can show up too.
Arthus reaction (localized Type III)
Localized immune complex deposition after intradermal injection (think booster vaccines in sensitized individuals):
- Local edema
- Hemorrhage
- Necrosis
- Occurs hours after exposure (localized, not systemic)
High-Yield Diseases/Associations (Must-Know List)
Autoimmune and post-infectious
- SLE: immune complex deposition across multiple organs
- Classically causes glomerulonephritis, arthritis, rash
- Poststreptococcal glomerulonephritis (PSGN)
- Occurs after strep throat/impetigo; nephritic syndrome
- Polyarteritis nodosa (PAN) (can be immune complex–mediated, esp. with HBV)
- Medium-vessel vasculitis; renal and GI ischemia; spares lungs (classic board clue)
- IgA vasculitis (Henoch-Schönlein purpura)
- Often presented as immune complex disease with IgA deposition (classically after URI)
- Palpable purpura, abdominal pain, arthralgia, renal involvement
Infection-related immune complex disease
- Reactive arthritis is not Type III (it’s more of a post-infectious inflammatory arthritis); don’t mix it up with immune complex arthritis.
- Subacute bacterial endocarditis can cause immune complex phenomena (e.g., glomerulonephritis).
Quick “Type III vs Type II” sorting tip
- Type II: antibody binds fixed target on cells/tissues → cytotoxicity or receptor effects (e.g., AIHA, Goodpasture, Graves)
- Type III: antibody binds soluble antigen → immune complexes deposit (e.g., SLE, PSGN, serum sickness)
Diagnosis: What NBME/Step Questions Test
Labs
- Low complement (consumption) → ↓C3, ↓C4 is a classic systemic clue (especially SLE, serum sickness)
- Elevated inflammatory markers (nonspecific)
Biopsy and microscopy (very testable)
Immune complex deposition produces granular staining patterns on immunofluorescence.
| Disease | Site | IF pattern | Buzzword |
|---|---|---|---|
| PSGN | Glomerulus | Granular (“lumpy-bumpy”) | “Starry sky” |
| SLE nephritis | Glomerulus | Granular (“full house” often) | Anti-dsDNA, anti-Sm |
| Goodpasture (Type II, contrast) | GBM | Linear | Anti-GBM |
Board-style clue: If you see granular IF, think immune complex (Type III). If you see linear IF, think direct antibody binding (Type II).
Clinical vignette triggers
- “1–3 weeks after infection” + hematuria/RBC casts → PSGN
- “Drug exposure” + fever/urticaria/arthralgia + low complement → serum sickness
- “Multisystem autoimmune” + nephritis + low complement → SLE
Treatment Principles (How management maps onto the mechanism)
Type III treatment is basically:
- Remove/eradicate the antigen source if possible
- Control inflammation and prevent end-organ damage
General approach
- Stop offending drug (serum sickness–like reactions)
- Treat underlying infection when relevant
- NSAIDs/antihistamines for mild symptoms (arthralgia, rash)
- Corticosteroids for significant inflammation or organ involvement
- Immunosuppressants (disease-specific; e.g., lupus nephritis regimens)
- Plasmapheresis in select severe immune complex states (context-dependent; more common in some rapidly progressive immune-mediated diseases)
Organ-specific examples
- PSGN: supportive care (BP control, diuretics); antibiotics to eradicate strep (prevents spread, doesn’t always reverse renal injury)
- Lupus nephritis: immunosuppression tailored to class/severity (often steroids + steroid-sparing agents)
First Aid Cross-References (What to flip to)
Use these as “anchor pages” in First Aid for the USMLE Step 1 (edition page numbers vary):
- Hypersensitivity reactions: the summary table differentiating Types I–IV (Type III = immune complex deposition + complement)
- Autoimmune diseases:
- SLE: immune complex disease, nephritis patterns, low complement
- PAN: association with hepatitis B (immune complex contribution)
- Renal pathology:
- PSGN: subepithelial humps (EM), granular IF
- Compare with Goodpasture: linear IF (Type II)
Pro tip for Step: When FA shows granular IF, mentally write “immune complexes = Type III” in the margin.
High-Yield “Don’t Miss” Facts (Rapid Review)
- Type III = immune complexes (IgG/IgM) deposit in tissues
- Complement activation → low C3/C4 (consumption)
- C5a recruits neutrophils → inflammation and tissue injury
- Granular IF = immune complex deposition (contrast: linear = Type II)
- Classic diseases:
- SLE
- PSGN
- Serum sickness
- Arthus reaction
- Classic clinical triad of serum sickness: fever + urticaria + arthralgia
- Deposition likes glomeruli, joints, small vessels (filtration/high-pressure zones)
Practice-Style Mini Stems (Self-Check)
-
A patient develops fever, urticaria, and arthralgias 10 days after starting a new antibiotic. Labs show low C3. Mechanism?
→ Immune complex deposition with complement activation (Type III) -
Child develops cola-colored urine and periorbital edema 2 weeks after impetigo. IF shows granular deposits.
→ PSGN, Type III -
Biopsy shows linear IgG along the basement membrane.
→ Not Type III (this is Type II; think Goodpasture)