Note a delayed type of hypersensitive reactions
**Core Concept**
Delayed type hypersensitivity reactions, also known as type IV hypersensitivity reactions, are a type of immune response that occurs over a period of hours to days. This reaction is primarily mediated by T lymphocytes, specifically T helper 1 (Th1) cells, which activate macrophages to induce inflammation and tissue damage.
**Why the Correct Answer is Right**
The delayed type hypersensitivity reaction is initiated when an antigen-presenting cell (APC) presents an antigen to a T cell receptor on a T helper 1 (Th1) cell. This interaction activates the Th1 cell to release cytokines, such as interferon-gamma (IFN-γ), which in turn activate macrophages to produce reactive oxygen species (ROS) and nitric oxide (NO). The activated macrophages then cause tissue damage and inflammation, leading to the clinical manifestations of the reaction.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it refers to an immediate type hypersensitivity reaction, which is mediated by IgE antibodies and mast cells, not T lymphocytes.
**Option B:** This option is incorrect because it refers to a type of autoimmune reaction, which is not a delayed type hypersensitivity reaction.
**Option C:** This option is incorrect because it refers to an allergic reaction, which can be either immediate or delayed, but is not a specific type of hypersensitivity reaction.
**Clinical Pearl / High-Yield Fact**
It's essential to note that delayed type hypersensitivity reactions are often seen in the context of tuberculosis, where the reaction is known as the Mantoux test, and is used to diagnose latent tuberculosis infection.
**Correct Answer:** D.