T-cell markers – identifying cytotoxic T lymphocytes Cytotoxic T lymphocytes (killer T cells) are classically identified by which surface marker co-receptor?

Introduction / Context:
CD markers distinguish immune cell subsets. Recognizing the marker for cytotoxic T lymphocytes (CTLs) is foundational for interpreting flow cytometry and understanding antigen recognition in cell-mediated immunity.

Given Data / Assumptions:

• CD4 associates with MHC class II; CD8 with MHC class I.
• CTLs recognize peptides presented by MHC class I on most nucleated cells.
• Markers like CD25 indicate activation, not lineage.

Concept / Approach:

CTLs are CD8+ T cells. Their TCRs survey peptides bound to MHC class I and, upon activation, kill infected or transformed cells via perforin/granzyme and Fas–FasL pathways. CD4 identifies helper T cells (MHC II restricted) rather than CTLs.

Step-by-Step Solution:

Verification / Alternative check:

Flow cytometry gating of CD3+CD8+ populations correlates with cytotoxic function; CD8 deficiency impairs viral clearance, validating the association.

Why Other Options Are Wrong:

• CD4: helper T cells.
• CD7, CD9: not lineage-defining for CTLs.
• CD25: IL-2 receptor alpha; indicates activation/regulatory T cell features.

Common Pitfalls:

• Equating activation markers with lineage markers.

Final Answer:

CD8