Which of these is CD marker of cytotoxic cells?
**Core Concept**
The question is testing the student's knowledge of cell surface markers, specifically those associated with cytotoxic cells. CD markers are proteins on the surface of immune cells that help identify their function and subtype. Cytotoxic cells, such as cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells, play a crucial role in eliminating infected cells and tumor cells.
**Why the Correct Answer is Right**
Cytotoxic cells express a specific CD marker that helps distinguish them from other immune cells. The correct CD marker for cytotoxic cells is CD8. This is because CD8 molecules are present on the surface of CTLs, which recognize and bind to antigen-presenting major histocompatibility complex (MHC) class I molecules on the surface of target cells. This interaction triggers the cytotoxic response. CD8 is a key marker for identifying CTLs and other cytotoxic cells in the immune system.
**Why Each Wrong Option is Incorrect**
**Option A:** CD4 is a marker for helper T cells, not cytotoxic cells. Helper T cells play a crucial role in activating other immune cells, but they do not directly kill infected cells or tumor cells.
**Option B:** CD19 is a marker for B cells, which are a type of lymphocyte involved in producing antibodies. While B cells can play a role in the immune response, they are not cytotoxic cells.
**Option C:** CD16 is a marker for natural killer (NK) cells, which are a type of cytotoxic cell. However, CD16 is not the most specific marker for cytotoxic cells, as it can also be expressed on other immune cells.
**Clinical Pearl / High-Yield Fact**
CD8 is a key marker for identifying cytotoxic T lymphocytes (CTLs) and other cytotoxic cells in the immune system. In the context of immunology, understanding the CD markers associated with different immune cells is crucial for diagnosing and treating immune-related disorders.
**Correct Answer:** C. CD8.