MHC class I is present on-
**Core Concept**
The Major Histocompatibility Complex (MHC) class I is a group of genes that play a critical role in the immune system, particularly in the presentation of peptide fragments to T-cells. MHC class I molecules are expressed on the surface of almost all nucleated cells. This expression is crucial for the recognition of self versus non-self by the immune system.
**Why the Correct Answer is Right**
MHC class I molecules are present on all nucleated cells, as they present endogenously synthesized peptides, such as those from viruses, to CD8+ T-cells. This is a key component of cell-mediated immunity, allowing for the recognition and elimination of infected cells or tumor cells. The expression of MHC class I on nearly all nucleated cells is a fundamental aspect of their function in immune surveillance.
**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because while some options may be partially correct, they do not fully encompass the breadth of MHC class I expression.
**Option B:** Similarly, this option is incorrect as it does not accurately represent the widespread expression of MHC class I molecules.
**Option C:** This choice is also incorrect for the same reason, lacking the comprehensive nature of MHC class I expression.
**Option D:** Except for the correct answer, all other options are incorrect as they do not fully describe the presence of MHC class I on nearly all nucleated cells.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that MHC class I molecules are expressed on virtually all nucleated cells, which is crucial for their role in immune surveillance and the presentation of antigens to cytotoxic T-cells.
**Correct Answer:** Correct Answer: D. All nucleated cells.