Arthus phenomenon is an example of which hypersensitivity-
**Core Concept**
The Arthus phenomenon is an antibody-mediated hypersensitivity reaction that occurs when pre-formed antibodies react with antigens in the presence of complement, leading to localized vasculitis and tissue damage. This type of hypersensitivity is characterized by the formation of immune complexes, which deposit in the small blood vessels, triggering an inflammatory response.
**Why the Correct Answer is Right**
The Arthus phenomenon is a classic example of type III hypersensitivity, which involves the formation of antigen-antibody complexes that activate the complement system, leading to the release of various inflammatory mediators. This reaction typically occurs in response to the administration of antigens to individuals with pre-existing antibodies against those antigens. The activation of the complement system results in the formation of C3a and C5a, which are potent chemotactic agents that attract neutrophils to the site of immune complex deposition, leading to inflammation and tissue damage.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Arthus phenomenon is not an example of type I hypersensitivity, which involves the release of histamine from mast cells and basophils, leading to allergic reactions such as anaphylaxis.
**Option B:** This option is incorrect because the Arthus phenomenon is not an example of type IV hypersensitivity, which involves the activation of T lymphocytes and the release of cytokines, leading to cell-mediated immunity.
**Option C:** This option is incorrect because the Arthus phenomenon is not an example of a cytotoxic hypersensitivity reaction, which involves the direct killing of cells by antibodies or T cells.
**Clinical Pearl / High-Yield Fact**
The Arthus phenomenon is a classic example of a type III hypersensitivity reaction, which can occur in individuals with pre-existing antibodies against certain antigens. It is an important consideration in the diagnosis and treatment of conditions such as systemic lupus erythematosus (SLE), where immune complex deposition can lead to tissue damage and inflammation.
**Correct Answer: D. Type III hypersensitivity.**