Arthus Reaction: Local Type III, Timing vs Serum Sickness

The Arthus reaction is a local type III (immune complex) hypersensitivity reaction: antigen injected into the skin meets high levels of preformed IgG, and the antigen–antibody complexes that form in and around small vessels activate complement and recruit neutrophils, producing a local vasculitis. The classic scenario is a tetanus- or diphtheria-toxoid booster given to someone who is already highly immune, followed within hours (usually 4–12 hours, according to ACIP) by severe pain, swelling, induration, edema, hemorrhage and occasionally necrosis at the injection site. Timing separates it from its look-alikes: type I reactions start within minutes, a type IV tuberculin reaction is read at 48–72 hours, and serum sickness, the systemic type III reaction, appears about 1–3 weeks after exposure.

The Q-bank vignette

A 32-year-old carpenter returns to the emergency department 10 hours after receiving a tetanus-diphtheria (Td) booster for a nail puncture wound. He could not remember his vaccine history, but records now show tetanus-containing boosters given 1 and 3 years ago at other hospitals. The injection site on his left deltoid is exquisitely painful, swollen, red and firm, with a dusky, hemorrhagic center. He has no hives, itching, wheezing, lip swelling or lightheadedness. Temperature is 37.4°C (99.3°F); other vital signs are normal.

Which mechanism best explains this reaction?

• A. Cross-linking of IgE on mast cells with release of histamine
• B. IgG binding to an antigen fixed on the surface of host cells
• C. Local formation of antigen–antibody complexes in vessel walls with complement activation
• D. Circulating antigen–antibody complexes depositing in vessels throughout the body
• E. Cytokine release from previously sensitized T lymphocytes

Answer: C, an Arthus (local type III) reaction. Repeated boosters have left him with high circulating antibody against the toxoids, and the new dose places a high local concentration of antigen in the tissue, so complexes form on the spot. ACIP's description matches: severe pain, swelling, induration, edema, hemorrhage and occasionally necrosis, usually 4–12 hours after vaccination, and the reaction usually resolves without sequelae. Other references give slightly different windows (OpenStax: 3–10 hours after injection; one Elsevier text: 2–8 hours), but all place the Arthus reaction in hours, not minutes or days.

Why C is right: the Arthus cascade

1. Prior sensitization: earlier exposures produced high titers of IgG, not IgE.
2. Local antigen load: the injection deposits a large amount of antigen in the dermis and subcutis.
3. Complexes form in place: antigen and antibody meet in and around small-vessel walls, so the complexes form locally instead of circulating first.
4. Complement: the complexes activate complement; C3a and C5a increase vascular permeability and recruit neutrophils, and Fc receptors on inflammatory cells amplify the response.
5. Neutrophil injury: neutrophils degranulate, releasing lysosomal enzymes that damage the vessel wall.
6. Result: a local vasculitis with edema, hemorrhage, thrombosis and sometimes necrosis.

If the site were biopsied, you would expect an immune complex small-vessel vasculitis: neutrophils in and around vessel walls, fibrinoid necrosis, hemorrhage and thrombi. The eponym comes from Maurice Arthus, who described the reaction in 1903 after repeatedly injecting horse serum into the skin of rabbits.

Why every other answer choice is wrong

A. Type I (IgE and mast cells). Antigen cross-links IgE on mast cells and basophils, releasing histamine and leukotrienes. Anaphylaxis typically begins within 15 minutes, with urticaria, angioedema, wheezing, stridor or hypotension, and the treatment is IM epinephrine right away. This lesion appeared hours later and is painful and indurated rather than itchy and urticarial.

B. Type II (antibody against a fixed antigen). IgG or IgM binds antigens that are part of cells or basement membranes, causing complement-mediated lysis, opsonization or receptor dysfunction; examples are Coombs-positive hemolytic anemia, anti-GBM disease and hyperacute graft rejection. A vaccine toxoid is a soluble antigen, not a structure on host cells, so it forms immune complexes instead. (Cross-reactive antibodies against fixed tissue antigens are also how molecular mimicry injures the heart in rheumatic fever.)

D. Serum sickness (systemic type III). Same family, different scale and clock. Antibodies made after exposure to a foreign protein or drug form circulating complexes that lodge in vessels throughout the body, typically 7–10 days after a drug and 1–3 weeks after antivenom. Expect fever, rash (often urticarial), arthralgias and lymphadenopathy, with low C3 and C4; the kidneys are commonly affected. Our patient has one local lesion after a few hours. More in the serum sickness guide.

E. Type IV (delayed-type, T cells). Sensitized T cells release cytokines and recruit macrophages; antibody plays no part. The model is the tuberculin skin test, a T-cell-mediated delayed-type reaction read at 48–72 hours by measuring induration; contact dermatitis is another example. Because both Arthus and type IV reactions are indurated, timing decides: hours = Arthus, days = type IV (see the type IV Q-bank breakdown).

The hypersensitivity clock

Lupus nephritis is the chronic, organ-level version of immune complex injury; see the lupus nephritis breakdown.

Arthus reaction vs serum sickness

The next tetanus dose

• A past Arthus-type reaction after a diphtheria- or tetanus-toxoid vaccine is a precaution, not a contraindication, for DTaP, Tdap, DT and Td.
• Defer the next tetanus-toxoid-containing dose until at least 10 years have passed since the last one (CDC Pink Book). ACIP adds that these patients should not receive Td more often than every 10 years even for wound prophylaxis.
• Routine boosters are every 10 years; for someone with 3 or more prior doses, a wound that is not clean and minor needs a booster only if more than 5 years have passed. Checking records before boosting is how the over-immunization behind this case is avoided.

Worked example: the systemic version

Vignette. A 48-year-old man receives antivenom after a rattlesnake bite. Twelve days later he develops fever, an urticarial rash, painful knees and wrists and enlarged lymph nodes. Urinalysis shows mild proteinuria; C3 and C4 are low. What is the mechanism?

Answer: Serum sickness, a systemic type III reaction. Antibodies against the foreign antivenom protein form circulating immune complexes that deposit in small vessels of the skin, joints and kidney. Merck recommends H1 blockers and a tapering course of oral corticosteroids.

Why not the others?

• Arthus reaction: confined to the injection site and starts within hours, not a systemic illness nearly 2 weeks later.
• Type I anaphylaxis: starts within minutes of the infusion, with urticaria, wheeze or hypotension.
• DRESS (type IV): a T-cell drug reaction that can start as late as 12 weeks after a medication is begun; think new drug, fever, rash, eosinophilia and organ involvement rather than a foreign protein 12 days earlier.

Exam traps

• Hours, not minutes or days. Hours (usually 4–12) is Arthus; minutes is type I; 48–72 hours is type IV; 1–3 weeks is serum sickness.
• Induration does not mean type IV. An Arthus lesion is firm too; the timing and the hemorrhage or necrosis give it away.
• Arthus needs antibody already present. It follows repeated exposure, typically too many boosters, not a first-ever dose.
• Both Arthus and serum sickness are type III. When both are offered, the question is local vs systemic.
• Anaphylaxis gets epinephrine; an Arthus reaction gets supportive care and a 10-year pause before the next tetanus-toxoid dose.

Key takeaways

• Arthus = local type III: injected antigen + high preformed IgG → complexes in vessel walls → complement (C3a, C5a) → neutrophils → local vasculitis.
• Onset within hours (ACIP: usually 4–12 h): pain, swelling, induration, edema, hemorrhage, sometimes necrosis; usually resolves without sequelae.
• Classic trigger: too-frequent tetanus or diphtheria toxoid boosters; defer the next dose until 10 years after the last.
• Serum sickness = systemic type III: 1–3 weeks, fever, rash, arthralgias, lymphadenopathy, low C3 and C4.
• Type I = minutes, IgE; type IV = 48–72 hours, T cells.

Sources

• Preventing Tetanus, Diphtheria, and Pertussis Among Adults: Use of Tdap (Arthus reactions) — ACIP, MMWR (2006)
• Chapter 21: Tetanus — CDC Pink Book
• Overview of Allergic and Atopic Disorders (Gell and Coombs classification) — Merck Manual Professional Edition
• Hypersensitivities — OpenStax Microbiology
• Drug Hypersensitivity (serum sickness, DRESS) — Merck Manual Professional Edition
• Serum sickness — DermNet
• Snakebites (antivenom serum sickness) — Merck Manual Professional Edition
• Anaphylaxis — Merck Manual Professional Edition
• Clinical Testing Guidance for Tuberculosis: Tuberculin Skin Test — CDC
• Overview of Vasculitis (leukocytoclastic vasculitis, fibrinoid necrosis) — Merck Manual Professional Edition
• Arthus Reaction (excerpts from Elsevier books and journals: history, mechanism, timing) — ScienceDirect Topics