Wheal & flare reaction is what type of hypersensitivity reaction?
## **Core Concept**
The wheal and flare reaction is a skin response that occurs within minutes of exposure to an allergen, leading to localized edema (wheal) and redness (flare). This reaction is a classic example of an immediate hypersensitivity reaction, primarily mediated by IgE antibodies. It involves the release of histamine and other chemical mediators from mast cells.
## **Why the Correct Answer is Right**
The wheal and flare reaction, also known as a type I hypersensitivity reaction or immediate hypersensitivity reaction, occurs when IgE antibodies bound to mast cells encounter their specific allergen. This binding causes the mast cells to degranulate and release histamine, bradykinin, and other mediators. Histamine increases vascular permeability, leading to edema (wheal), and also causes vasodilation, resulting in erythema (flare). This reaction is the basis for skin prick tests used to diagnose allergies.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because Type II hypersensitivity reactions involve IgG or IgM antibodies directed against antigens on cell surfaces or in connective tissues, leading to cell destruction or tissue damage through mechanisms like complement activation or antibody-dependent cellular cytotoxicity.
- **Option B:** This option is incorrect because Type III hypersensitivity reactions involve the formation of antigen-antibody complexes that deposit in tissues, leading to complement activation and inflammation. This does not match the immediate nature of the wheal and flare reaction.
- **Option D:** This option is incorrect because Type IV hypersensitivity reactions are delayed-type hypersensitivity reactions, mediated by T cells rather than antibodies. They occur hours to days after exposure to an allergen and involve inflammation and tissue damage.
## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the wheal and flare reaction is a hallmark of allergic reactions and is used diagnostically in skin prick tests. It typically peaks within 15-20 minutes after allergen exposure. A notable point for exams is that this reaction is IgE-mediated, and substances like antihistamines can suppress the reaction, while corticosteroids have a minimal effect on the acute phase but can affect the late phase.
## **Correct Answer:** . Immediate hypersensitivity reaction