**Core Concept**
HMB-45 is a monoclonal antibody that recognizes an antigen present in melanocytic tumors, specifically melanoma cells. It is used as a diagnostic marker in immunohistochemistry to identify these cells.

**Why the Correct Answer is Right**
HMB-45 reacts with an antigen called gp100, which is a melanocyte-specific protein. This protein is expressed in melanoma cells and is involved in the regulation of melanogenesis. The gp100 antigen is a target for cytotoxic T cells in the immune system, and its recognition by HMB-45 is crucial for the diagnosis of melanoma. The gp100 antigen is also involved in the development of melanoma, and its expression is associated with a poor prognosis.

**Why Each Wrong Option is Incorrect**
**Option A:** HMB-45 is not a marker for S100 protein, which is another protein expressed in melanocytes. While S100 protein is also used as a diagnostic marker for melanoma, it is not the target of HMB-45.
**Option B:** HMB-45 is not a marker for cytokeratins, which are proteins expressed in epithelial cells. Cytokeratins are used as diagnostic markers for various types of carcinomas, but not for melanoma.
**Option C:** HMB-45 is not a marker for desmin, which is a protein expressed in muscle cells. Desmin is used as a diagnostic marker for rhabdomyosarcoma and other muscle tumors, but not for melanoma.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that HMB-45 is a sensitive marker for melanoma, but it can be false-negative in cases of desmoplastic melanoma. Therefore, a combination of HMB-45 and other markers, such as S100 protein, may be necessary for a definitive diagnosis.

**Correct Answer:** C. Melanoma cells.