**Core Concept**
Type IV hypersensitivity reactions, also known as cell-mediated or delayed-type hypersensitivity reactions, involve the activation of T lymphocytes. These reactions are typically mediated by T helper 1 (Th1) cells and are responsible for the clinical manifestations observed in conditions such as contact dermatitis, tuberculin skin test, and transplant rejection.

**Why the Correct Answer is Right**
The skin test is used to diagnose type IV hypersensitivity reactions by assessing the immune response to specific antigens, such as tuberculin in the Mantoux test. When an individual with a sensitized immune system is exposed to the antigen, T lymphocytes are activated, leading to the release of cytokines and the recruitment of other immune cells to the site of antigen exposure. This results in the characteristic skin reaction observed in type IV hypersensitivity reactions. The skin test is a useful tool for assessing the individual's immune response to specific antigens and is commonly used in the diagnosis of conditions such as tuberculosis and contact dermatitis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because skin tests are not used for type I hypersensitivity reactions, which are mediated by IgE antibodies and involve immediate allergic reactions such as anaphylaxis and asthma.
**Option B:** This option is incorrect because skin tests are not used for type II hypersensitivity reactions, which are mediated by IgG and IgM antibodies and involve cytotoxic reactions such as hemolytic disease of the newborn.
**Option C:** This option is incorrect because skin tests are not used for type III hypersensitivity reactions, which are mediated by immune complexes and involve systemic reactions such as serum sickness.

**Clinical Pearl / High-Yield Fact**
It is essential to note that skin tests should only be performed in individuals who are not immunocompromised, as they may not be able to mount an appropriate immune response. Additionally, skin tests should be interpreted in the context of the individual's medical history and clinical presentation.

**Correct Answer: C. Type IV hypersensitivity reactions.