Okay, let's tackle this question. The question is about the Leucocyte Common Antigen, also known as CD45. First, I need to recall what CD45 is. It's a protein tyrosine phosphatase expressed on all nucleated cells except red blood cells. So, the question is asking which cells express this antigen.

The options aren't provided, but the correct answer is likely related to all white blood cells. Let me think. CD45 is a marker for leukocytes, which includes T cells, B cells, natural killer cells, monocytes, and granulocytes. So the correct answer would be an option that includes all these cell types. If the options include something like "All leukocytes" or "All white blood cells," that's the right choice.

Now, the distractors. Let's say an option says "Only T cells" – that's incorrect because CD45 is on all leukocytes. Another might be "Only B cells" – same issue. Another option could be "All nucleated cells," which is wrong because RBCs lack CD45. Also, if an option mentions "Hematopoietic stem cells," maybe that's incorrect if the question is about mature cells.

The core concept here is understanding CD45's expression. The clinical pearl is that CD45 is used in flow cytometry to identify leukocytes and detect blasts in leukemias. So, the key takeaway is that CD45 is present on all leukocytes except RBCs. The correct answer should be the option that includes all white blood cells.

**Core Concept** Leucocyte Common Antigen (LCA/CD45) is a transmembrane protein tyrosine phosphatase expressed on all **nucleated hematopoietic cells** (leukocytes, monocytes, lymphocytes, granulocytes, and dendritic cells) but absent on mature erythrocytes. It is a critical marker in immunophenotyping for leukemia/lymphoma classification.

**Why the Correct Answer is Right** The correct option identifies that CD45 is expressed on **all leukocytes** (white blood cells) including T-cells, B-cells, NK cells, monocytes, and granulocytes. It is absent on red blood cells, platelets, and non-hematopoietic cells. CD45 regulates T-cell receptor and B-cell receptor signaling by dephosphorylating key signaling molecules like ZAP-70 and Syk.

**Why Each Wrong Option is Incorrect**
**Option A:** "Only T-cells" – Incorrect. CD45 is pan-leukocyte, not restricted to T-cells.
**Option B:** "Only B-cells" – Incorrect. B-cells express CD45 but so do all other leukocytes.
**Option C:** "All nucleated cells" – Incorrect. CD45 is absent on erythrocytes and non-hematopoietic nucleated cells (e.g., fibroblasts).

**Clinical Pearl** CD45 is a **flow cytometry gate marker** to distinguish leukemic blasts from non-hematopoietic cells. In acute leukemias, CD45 dim expression on blasts is a hallmark of immature cell origin.

**Correct Answer: B. All leukocytes**

✓ Correct Answer: A. Lymphoma