Which of the following is an example of type IV hypersensitivity:
**Core Concept**
Type IV hypersensitivity, also known as delayed-type hypersensitivity, is a cell-mediated immune response that involves T lymphocytes and macrophages. It occurs when T cells recognize antigens presented by antigen-presenting cells (APCs) and release cytokines, leading to the activation of macrophages and the subsequent inflammatory response.
**Why the Correct Answer is Right**
The correct answer is an example of type IV hypersensitivity because it involves the activation of T cells and macrophages in response to an antigen. This type of reaction typically occurs 48-72 hours after exposure to the antigen and can lead to tissue damage and inflammation. The pathophysiology involves the recognition of antigens by T cells, which then activate macrophages and other immune cells to cause inflammation.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it describes a type I hypersensitivity reaction, which involves IgE antibodies and mast cell degranulation. Type I reactions are immediate and occur within minutes of exposure to the antigen.
**Option B:** This option is incorrect because it describes a type II hypersensitivity reaction, which involves IgG or IgM antibodies and the activation of complement. Type II reactions can cause cellular damage and inflammation, but they are not typically characterized by a delayed response.
**Option C:** This option is incorrect because it describes a type III hypersensitivity reaction, which involves immune complexes and the activation of complement. Type III reactions can cause inflammation and tissue damage, but they are not typically characterized by a delayed response.
**Clinical Pearl / High-Yield Fact**
A classic example of type IV hypersensitivity is the tuberculin skin test, which involves the injection of tuberculin into the skin and the subsequent development of a delayed-type hypersensitivity reaction. This reaction is mediated by T cells and macrophages and can occur 48-72 hours after exposure to the antigen.
**Correct Answer:** C.