Delayed hypersensitivity is due to –
**Core Concept**
Delayed hypersensitivity, also known as type IV hypersensitivity, is a cell-mediated immune response involving T lymphocytes, which takes time to develop (usually 48-72 hours) after exposure to an antigen. This type of hypersensitivity reaction is characterized by the activation of T helper 1 (Th1) cells and the release of cytokines, leading to inflammation and tissue damage.
**Why the Correct Answer is Right**
The correct answer is related to the role of T lymphocytes in cell-mediated immunity. T cells recognize and bind to antigens presented by antigen-presenting cells (APCs), such as dendritic cells or macrophages, through their T cell receptor (TCR). This interaction activates the T cells, which then proliferate and differentiate into effector T cells. The effector T cells release cytokines, such as interferon-gamma (IFN-Ξ³), which activate macrophages and other immune cells, leading to the inflammatory response characteristic of delayed hypersensitivity.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because delayed hypersensitivity is not primarily due to the activation of B lymphocytes, which are involved in humoral immunity and the production of antibodies.
**Option B:** This option is incorrect because type II hypersensitivity, also known as cytotoxic hypersensitivity, is mediated by antibodies that bind to antigens on the surface of cells, leading to cell destruction.
**Option C:** This option is incorrect because type III hypersensitivity, also known as immune complex-mediated hypersensitivity, is caused by the formation of antigen-antibody complexes that deposit in tissues and activate the complement system.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that delayed hypersensitivity reactions can be clinically significant, particularly in the context of organ transplantation, where they can lead to graft rejection. The tuberculin skin test, which measures the response to tuberculin, is a classic example of a delayed hypersensitivity reaction.
**Correct Answer: D. Cell-mediated immunity.**