Anaphylactic shock is caused by which type of hypersensitivity reaction?

**Core Concept**
Anaphylactic shock is an acute, life-threatening allergic reaction characterized by the release of histamine and other mediators from mast cells, leading to vasodilation, increased vascular permeability, and bronchoconstriction. This phenomenon is mediated by IgE antibodies, which play a crucial role in triggering the allergic response.

**Why the Correct Answer is Right**
The correct answer is type I hypersensitivity reaction, also known as immediate hypersensitivity. This type of reaction occurs when IgE antibodies bind to the surface of mast cells and basophils, leading to cross-linking of the IgE molecules upon exposure to the allergen. This cross-linking causes the release of histamine and other chemical mediators, resulting in the characteristic symptoms of anaphylaxis, including vasodilation, increased vascular permeability, bronchoconstriction, and smooth muscle contraction.

**Why Each Wrong Option is Incorrect**
**Option A:** Type II hypersensitivity reaction is incorrect because it involves IgM or IgG antibodies directed against cell surface antigens, leading to cell destruction or activation, but not the release of histamine and mediators seen in anaphylaxis.

**Option B:** Type III hypersensitivity reaction is incorrect because it involves the formation of antigen-antibody complexes that deposit in tissues, leading to an inflammatory response, but not the rapid release of histamine and mediators seen in anaphylaxis.

**Option C:** Type IV hypersensitivity reaction is incorrect because it involves T cell-mediated responses, which are typically delayed and do not involve the immediate release of histamine and mediators characteristic of anaphylaxis.

**Clinical Pearl / High-Yield Fact**
The most common causes of anaphylaxis include food allergies (e.g., peanuts, tree nuts, fish, shellfish, milk, eggs, wheat), insect stings, medications (e.g., penicillin, latex), and certain medical procedures (e.g., blood transfusions). Prompt recognition and treatment of anaphylaxis with epinephrine and other medications can be life-saving.

**Correct Answer:** C. Type I hypersensitivity reaction.