Which of the following examples represents a Type IV hypersensitivity reaction?

A Type IV hypersensitivity reaction, also known as delayed-type hypersensitivity, is primarily mediated by T-cells and does not involve antibodies. The primary mechanism involves the activation of sensitized T lymphocytes upon re-exposure to a specific antigen. This process can lead to a range of inflammatory responses that typically manifest 24 to 48 hours after exposure to the antigen.

The example of contact dermatitis, which is presented in this question, illustrates this type of hypersensitivity well. In contact dermatitis, exposure to certain substances, such as poison ivy or nickel, leads to a delayed immune response. Sensitized T-cells recognize the hapten-modified skin proteins as foreign, resulting in an inflammatory reaction that causes red, itchy rashes.

In contrast, the other options represent different types of hypersensitivity reactions. Hemolytic transfusion reactions are examples of Type II hypersensitivity, where antibodies target specific blood cells, leading to their destruction. Anaphylaxis is a severe and rapid type of reaction known as Type I hypersensitivity, characterized by IgE-mediated release of histamines and other chemicals. Angioedema often occurs in conjunction with anaphylaxis or as an allergic response, relating to fluid accumulation in deep layers of the skin and mucous membranes