Arthus reaction is one of those “sounds obscure, shows up on exams” immunology nuggets: a localized type III hypersensitivity reaction that happens when pre-existing IgG antibodies meet a high local concentration of antigen, forming immune complexes that light up complement and neutrophils. If you can picture “immune complex vasculitis—just confined to the skin at an injection site,” you’re already most of the way there.
Where Arthus Reaction Fits in Hypersensitivity (Step-Friendly Map)
The 10-second classification
| Type | Main mechanism | Key players | Timing | Classic examples |
|---|---|---|---|---|
| I | Immediate (IgE) | Th2, IgE, mast cells | Minutes | Anaphylaxis, atopy |
| II | Antibody-mediated | IgG/IgM vs cell surface/ECM | Hours | Autoimmune hemolytic anemia, Goodpasture |
| III | Immune complexes | IgG + soluble antigen → complement | Hours to days | Serum sickness, Arthus reaction, SLE, PSGN |
| IV | Delayed (T-cell) | Th1/Th17, CTLs, macrophages | 48–72 hours | Contact dermatitis, TB skin test |
Arthus = localized type III
Serum sickness = systemic type III
Definition (What It Is)
Arthus reaction is a localized immune complex–mediated (type III) hypersensitivity that occurs when an individual with high titers of IgG is exposed to antigen (often via intradermal or subcutaneous injection), leading to:
- Immune complex deposition in small vessels
- Complement activation
- Neutrophil recruitment
- Local vasculitis and tissue necrosis
Pathophysiology (What’s Actually Happening)
Think of it as a predictable sequence:
1) Sensitization (you already have antibodies)
The patient has pre-existing IgG antibodies from prior exposure (e.g., prior vaccination or boosters).
2) Antigen re-exposure at a local site
A new injection creates a high local antigen concentration.
3) Immune complex formation and deposition
Antigen + IgG forms immune complexes that deposit in vessel walls (postcapillary venules).
4) Complement activation (big Step 1 theme)
Immune complexes trigger the classical complement pathway:
- Generation of C3a and C5a
- C3a: anaphylatoxin (↑ vascular permeability)
- C5a: potent neutrophil chemotaxis + activation
5) Neutrophils cause “frustrated phagocytosis”
Neutrophils try to clear complexes stuck in tissue → release:
- Proteases
- Reactive oxygen species
Result: localized vasculitis, edema, hemorrhage, sometimes necrosis.
Timing (Very High Yield)
Arthus reaction occurs within hours of antigen exposure—classically 4–12 hours, sometimes up to ~24 hours.
Contrast:
- Type I: minutes
- Type IV: 48–72 hours (delayed)
Clinical Presentation (How It Shows Up)
Most commonly at an injection site.
Typical findings
- Pain, swelling, induration
- Erythema and warmth
- Edema
- Possible pruritus
- In more severe cases: hemorrhage, ulceration, necrosis
What it can mimic (exam trick)
- Cellulitis (but Arthus often occurs faster after injection and is immune-mediated)
- “Bad vaccine reaction” (yes—but mechanistically it’s type III)
Common Triggers / Settings
Historically and on exams, you’ll see:
- Tetanus-containing vaccines (especially after multiple boosters in a short interval)
- Diphtheria-containing vaccines
- Other protein antigens injected into someone with high IgG titers
Conceptual trigger: “High antibody + high local antigen”.
Diagnosis (What You’d See and How You Recognize It)
Arthus reaction is often a clinical diagnosis based on classic timing and localization.
Pathology clues (Step 1 histology-style)
- Immune complex deposition in vessel walls
- Fibrinoid necrosis of vessels (in severe vasculitis)
- Neutrophilic infiltrate
- Sometimes described as leukocytoclastic vasculitis (neutrophil debris)
Immunofluorescence (the buzzwords)
- Granular deposition of immunoglobulins and complement (vs linear in type II diseases like Goodpasture)
Treatment (What You Do)
Most cases are self-limited.
Supportive care
- Cold compresses
- NSAIDs for pain/inflammation
- Elevation if on an extremity
If more severe
- Topical corticosteroids for localized inflammation
- Systemic corticosteroids rarely, for extensive symptoms or significant necrosis (clinical judgment)
Prevention (board-relevant practical point)
- Avoid unnecessary frequent boosters in patients with high pre-existing titers when not indicated.
- Follow CDC/ACIP schedules—this is why spacing matters.
High-Yield Associations and Comparisons (Step 1/2 Gold)
Arthus vs Serum Sickness (both Type III!)
| Feature | Arthus reaction | Serum sickness |
|---|---|---|
| Distribution | Localized (injection site) | Systemic |
| Trigger | Antigen injection in sensitized patient | Heterologous proteins, some drugs |
| Timing | Hours (4–12h) | 1–2 weeks after exposure |
| Symptoms | Local pain, swelling, induration; possible necrosis | Fever, urticaria, arthralgias, lymphadenopathy, proteinuria |
| Mechanism | Immune complexes → complement → neutrophils | Same mechanism, widespread deposition |
Arthus vs Type I “vaccine allergy”
- Type I: immediate (minutes), IgE, hives/wheezing/anaphylaxis
- Arthus: hours later, IgG immune complexes, localized vasculitis-type reaction
Key complement/neutrophil facts to remember
- C5a = neutrophil chemotaxis (HY)
- Immune complex diseases often show low complement (consumption)—more relevant in systemic type III, but mechanism is the same.
First Aid Cross-References (Where to Anchor It)
In First Aid for the USMLE Step 1, Arthus reaction is typically referenced under:
- Immunology → Hypersensitivity reactions
- Type III: immune complex-mediated
- Examples include serum sickness and Arthus reaction
- Look for the classic mechanism phrasing:
- “Immune complex deposition → complement activation”
- “Neutrophil recruitment → tissue damage”
If you’re building a mental “FA-style” line, it’s:
Arthus reaction = localized type III hypersensitivity after antigen injection in a sensitized (high IgG) patient → immune complexes, complement, neutrophils → local vasculitis/necrosis.
USMLE-Style Pattern Recognition (How It’s Tested)
Vignette skeleton
- Patient recently got a booster (often tetanus)
- Several hours later develops marked pain, swelling, induration at injection site
- No airway symptoms, no immediate anaphylaxis
- Mechanism question: immune complexes / complement / neutrophils
What the question stem really wants
Pick:
- Type III hypersensitivity
- IgG immune complex deposition
- Complement activation (classical)
- Neutrophil-mediated tissue injury
Rapid Review (Last-Minute Checklist)
- Type: III
- Antibody: IgG (preformed)
- Trigger: local antigen injection in a sensitized person
- Timing: hours (4–12h)
- Mechanism: immune complexes → C3a/C5a → neutrophils → vasculitis/necrosis
- Presentation: painful swelling/induration at injection site ± ulceration
- Dx: clinical; granular IF if tested
- Tx: supportive ± steroids for severe cases