Which of the following is type III hypersensitivity reaction: September 2006
**Core Concept**
Type III hypersensitivity reactions involve the formation of antigen-antibody complexes (immune complexes) that deposit in tissues and trigger an inflammatory response. This type of reaction is mediated by antibodies of the IgG and IgM classes, which can bind to antigens and form insoluble complexes.
**Why the Correct Answer is Right**
The correct answer is a type III hypersensitivity reaction because it involves the formation of immune complexes that deposit in tissues and trigger an inflammatory response. This type of reaction is often seen in conditions such as systemic lupus erythematosus (SLE) and serum sickness. The immune complexes activate the complement system, leading to the release of pro-inflammatory mediators and the recruitment of leukocytes to the site of immune complex deposition.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option does not describe a type III hypersensitivity reaction. Type I hypersensitivity reactions involve IgE antibodies and the release of histamine from mast cells.
* **Option B:** This option describes a type IV hypersensitivity reaction, which involves T-cell mediated immunity and is often seen in contact dermatitis and tuberculosis.
* **Option C:** This option does not describe a type III hypersensitivity reaction. Type II hypersensitivity reactions involve IgG and IgM antibodies that bind to cell surface antigens, leading to cell destruction.
**Clinical Pearl / High-Yield Fact**
Type III hypersensitivity reactions are often seen in conditions where there is an imbalance between the formation and clearance of immune complexes. This can occur in conditions such as SLE and rheumatoid arthritis, where the formation of immune complexes is not balanced by adequate clearance mechanisms.
**Correct Answer:** C.