**Core Concept**
The Mitsuda reaction is a clinical test used to assess the immune response to leprosy. It is a delayed hypersensitivity reaction, also known as a Type IV hypersensitivity reaction, which involves the activation of T lymphocytes in response to the leprosy bacillus, *Mycobacterium leprae*. This reaction is crucial in the diagnosis and management of leprosy.

**Why the Correct Answer is Right**
The Mitsuda reaction is read after 21 to 28 days of the intradermal injection of inactivated leprosy bacilli, also known as Mitsuda's antigen. The reaction is characterized by the formation of a papule or nodule at the injection site, which is indicative of a positive immune response. The reaction is mediated by the activation of T lymphocytes, particularly CD4+ T cells, which recognize the antigens presented by antigen-presenting cells, such as dendritic cells and macrophages.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Mitsuda reaction is not read immediately after the injection of Mitsuda's antigen. The reaction is a delayed hypersensitivity response, which requires several days to develop.

**Option B:** This option is incorrect because the Mitsuda reaction is not typically read after 14 days. While some reactions may start to appear within this timeframe, the optimal time for reading the reaction is between 21 to 28 days.

**Option C:** This option is incorrect because the Mitsuda reaction is not read after 1 week. As mentioned earlier, the reaction is a delayed hypersensitivity response, which requires several weeks to develop.

**Clinical Pearl / High-Yield Fact**
It is essential to note that a positive Mitsuda reaction is not a definitive diagnostic criterion for leprosy, but rather a useful indicator of immune response. A negative reaction does not exclude the possibility of leprosy, and further diagnostic tests, such as bacteriological examination and histopathological examination, may be necessary to confirm the diagnosis.

**Correct Answer:** D.