All are type II hypersensitivity reaction except: September 2007
**Core Concept**
Type II hypersensitivity reactions involve the binding of antibodies to the surface of cells or tissues, leading to cell destruction through mechanisms such as complement activation, antibody-dependent cellular cytotoxicity (ADCC), or phagocytosis. This type of reaction is typically mediated by IgG and IgM antibodies, which are directed against antigens on the surface of cells or in connective tissue.
**Why the Correct Answer is Right**
The correct answer is a type of hypersensitivity reaction that does not involve the binding of antibodies to the surface of cells or tissues. Instead, it is mediated by the release of pre-formed chemical mediators from mast cells, basophils, or other cells, leading to the symptoms of an allergic reaction. This type of reaction is an example of a type I hypersensitivity reaction, which is mediated by IgE antibodies.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because type II hypersensitivity reactions involve the binding of antibodies to the surface of cells or tissues, which leads to cell destruction. Type II reactions are characterized by the presence of IgG and IgM antibodies.
**Option B:** This option is incorrect because type II hypersensitivity reactions are mediated by the binding of antibodies to the surface of cells or tissues, leading to cell destruction. Type II reactions are not mediated by the release of chemical mediators.
**Option C:** This option is incorrect because type II hypersensitivity reactions involve the binding of antibodies to the surface of cells or tissues, leading to cell destruction. Type II reactions are not characterized by the release of histamine from mast cells.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that type I hypersensitivity reactions are mediated by IgE antibodies and are characterized by the release of pre-formed chemical mediators, such as histamine, from mast cells and basophils.
**Correct Answer:** A. Type I hypersensitivity reaction.