In clinical immunology, which cell type characteristically expresses the CD3 surface antigen on its membrane (a pan-T-cell marker used in flow cytometry and diagnostics)?

Introduction / Context:
CD markers (cluster of differentiation) are cell-surface molecules used to identify and classify immune cells. CD3 is a classic pan-T-cell marker tightly associated with the T-cell receptor (TCR) complex. This question assesses recognition of core immunophenotyping markers used in diagnostics and research.

Given Data / Assumptions:

• We are comparing common leukocyte subsets: T cells, B cells, granulocytes, monocytes, and NK cells.
• We focus specifically on the presence of CD3 at the cell surface.
• Standard human immunology conventions are used (flow cytometry panels).

Concept / Approach:
CD3 is a multi-chain signaling complex (gamma, delta, epsilon, zeta) noncovalently associated with the TCR. It is essential for signal transduction after antigen recognition. Therefore, all mature T cells (both CD4+ and CD8+) express CD3. Other leukocytes do not display CD3 under normal conditions.

Step-by-Step Solution:

Verification / Alternative check:
Standard clinical flow cytometry panels gate lymphocytes by scatter, then CD3 versus CD19/CD20. T cells are CD3+; B cells are CD3− and CD19+/CD20+. NK cells appear CD3− CD56+/CD16+.

Why Other Options Are Wrong:

• Granulocytes: No TCR/CD3 complex; typically CD15+/CD66b+.
• Monocytes: CD14 high; lack CD3.
• B cells: CD19/CD20, not CD3.
• NK cells: CD16/CD56; specifically CD3 negative.

Common Pitfalls:

• Confusing CD3 (pan-T) with CD4/CD8 (subsets).
• Assuming NK-T cells are NK cells; invariant NKT are CD3+ (still T-lineage), whereas classic NK cells are CD3−.

Final Answer:
T cells (all mature T-lymphocytes)