## Core Concept
Type 4 hypersensitivity, also known as delayed-type hypersensitivity (DTH), is a cell-mediated immune response that involves T lymphocytes, particularly T helper 1 (Th1) cells, and macrophages. This type of hypersensitivity reaction typically occurs 48 to 72 hours after exposure to an antigen. It is characterized by the activation of T cells, which then orchestrate an inflammatory response.

## Why the Correct Answer is Right
The correct answer, Contact dermatitis, is a classic example of a type 4 hypersensitivity reaction. This condition occurs when the skin comes into contact with an allergen, such as nickel, latex, or certain chemicals in cosmetics or plants (e.g., poison ivy). The allergen is processed by antigen-presenting cells (APCs) and presented to T cells, which then become activated. Activated T cells proliferate and differentiate into effector cells, including Th1 cells and cytotoxic T cells, which mediate the inflammatory response. This response leads to the characteristic skin manifestations of contact dermatitis, including erythema, edema, and vesicles.

## Why Each Wrong Option is Incorrect
**Option A:** Immediate hypersensitivity reaction - This is incorrect because immediate hypersensitivity reactions (type 1 hypersensitivity) involve IgE antibodies and occur within minutes of exposure to an allergen, not 48 to 72 hours later.

**Option B:** Hemolytic disease of the newborn - This is incorrect because hemolytic disease of the newborn is an example of type 2 hypersensitivity, which involves IgG or IgM antibodies directed against antigens on the surface of red blood cells.

**Option C:** Serum sickness - This is incorrect because serum sickness is an example of a type 3 hypersensitivity reaction, which involves the formation of antigen-antibody complexes that deposit in tissues, leading to inflammation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that type 4 hypersensitivity reactions are cell-mediated and delayed, requiring 48 to 72 hours to develop. A classic clinical correlation is the tuberculin skin test (Mantoux test), which is used to assess for exposure to Mycobacterium tuberculosis and is a prime example of a type 4 hypersensitivity reaction.

## Correct Answer: . Contact dermatitis