The Arthus reaction, characterized by localized edema, hemorrhage, and necrosis after repeated subcutaneous injections of an antigen, is an example of which type of hypersensitivity?

1. A

   Type I Hypersensitivity

2. B

   Type II Hypersensitivity

3. C

   Type III Hypersensitivity

4. D

   Type IV Hypersensitivity

Appeared in: NORCET 8 Prelims-2025

Explanation

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• The Arthus reaction is a localized form of Type III hypersensitivity, characterized by tissue inflammation and necrosis.
• It is caused by the deposition of antigen-antibody (immune) complexes in the walls of small blood vessels at the site of antigen injection.
• These complexes activate the complement system, a part of the immune system, which then attracts inflammatory cells called neutrophils.
• The neutrophils release enzymes that cause inflammation and damage to the blood vessels (vasculitis), leading to the clinical signs of localized edema, hemorrhage, and tissue death.

Why Other Options Were Wrong

• Option A: Type I hypersensitivity is an immediate reaction, occurring within minutes. It is mediated by IgE antibodies and mast cells, not immune complexes.
• Option B: Type II hypersensitivity is a cytotoxic reaction where antibodies (IgG or IgM) bind directly to antigens on the surface of cells, leading to cell destruction. The Arthus reaction involves complexes deposited in tissues, not antibodies attacking cells directly.
• Option D: Type IV hypersensitivity is a delayed-type reaction, typically taking 24-72 hours to develop. It is mediated by T-cells and macrophages, not antibodies and immune complexes.

Related Visual

• Visual 1: Diagram - An illustration of the Type III hypersensitivity mechanism. The visual should depict an antigen and antibody forming an immune complex, its deposition in a blood vessel wall, the subsequent activation of the complement system, the recruitment of neutrophils, and the resulting tissue damage and inflammation.

Clinical Relevance

• Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Classification of hypersensitivity reactions as background academic context rather than a clinical decision trigger.
• Recognizing the type of hypersensitivity reaction is crucial for correct diagnosis and management. The 4-12 hour onset of the Arthus reaction helps distinguish it from immediate (Type I) and delayed (Type IV) reactions.
• This type of localized reaction can be observed in clinical practice after the administration of certain vaccines (like tetanus or diphtheria toxoids) or drugs to individuals who already have high levels of circulating IgG antibodies.
• What if? If the patient developed widespread hives, wheezing, and a drop in blood pressure within 15 minutes of an injection, it would indicate a Type I (anaphylactic) reaction. This is a medical emergency requiring immediate administration of epinephrine, completely different from the management of a localized Type III reaction.

How to Approach the Question

• First, analyze the question stem to identify the key features of the reaction described: it is localized ('localized edema, hemorrhage, and necrosis') and occurs after 'repeated subcutaneous injections' of an antigen.
• Recall the fundamental mechanisms of the four types of hypersensitivity.
• Match the features from the stem to the correct mechanism. The description points to damage caused by antigen-antibody complexes, which is the hallmark of Type III hypersensitivity. The 'repeated injections' suggest the presence of pre-formed antibodies.
• Eliminate the other options based on their mechanisms: Type I is an immediate, IgE-mediated allergic reaction. Type II involves antibodies attacking cell surfaces directly. Type IV is a delayed, T-cell-mediated reaction.
• Conclude that the Arthus reaction, with its characteristic localized immune complex deposition and subsequent inflammation, is a classic example of Type III hypersensitivity.

Concept Tested & Keywords

• Concept Tested: Classification of hypersensitivity reactions
• Stem keywords: Arthus reaction, localized edema, hemorrhage, necrosis, hypersensitivity
• Lead-in keywords: is an example of
• Negative lead-in flag: false

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QLSy6QjDIZny5yzApDBo19

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