CD61 (integrin β3) is a surface marker primarily expressed on platelets and megakaryocytes, used to identify megakaryocytic lineage in blood and bone marrow. It participates in platelet aggregation and thrombus formation. CD61 is also detectable in megakaryoblastic leukemias and platelet-rich thrombi, making it useful diagnostically for platelet lineage identification and assessing platelet-related disorders.

The principal CD markers for megakaryocytes are CD41 (integrin αIIb) and CD61 (integrin β3), which together form the platelet GPIIb/IIIa complex. Mature megakaryocytes and platelets also express CD42b (GPIbα). CD34 marks progenitors but usually declines with maturation. These markers are routinely used in flow cytometry and immunohistochemistry to identify megakaryocytic lineage.

Abnormal megakaryocytes are seen in myeloproliferative neoplasms—especially essential thrombocythemia and primary myelofibrosis—where they show hyperplasia and atypia. Megakaryocytic dysplasia also occurs in myelodysplastic syndromes, and malignant proliferation defines acute megakaryoblastic leukemia. These conditions cause thrombocytosis, marrow fibrosis or cytopenias and need hematology evaluation with blood counts, bone marrow biopsy, and molecular testing.

Megakaryocytes generally express the leukocyte common antigen CD45, especially at progenitor stages, but expression often diminishes with maturation; very mature megakaryocytes may be CD45-low or negative. Platelets (anucleate fragments) lack CD45. Thus CD45 is variably present on the megakaryocytic lineage and is not a reliable sole marker of mature megakaryocytes.

CD markers used to identify leukemias include: B‑cell ALL—CD19, CD10, CD22 (±CD20); T‑cell ALL—CD3, CD2, CD7; AML—myeloid markers CD13, CD33, CD117 and MPO, often with CD34 in blasts; CLL—co‑expression of CD19 and CD5 with CD23; hairy cell—CD11c, CD25, CD103. Definitive diagnosis requires specialist interpretation with flow cytometry and molecular tests.

CD61 refers to the gene ITGB3, which encodes the integrin beta-3 subunit. Also known as platelet glycoprotein IIIa (GPIIIa), integrin beta-3 pairs with alpha subunits (commonly alphaIIb) to form receptors involved in platelet aggregation, adhesion, and cell–matrix interactions. Mutations in ITGB3 can cause bleeding disorders such as Glanzmann thrombasthenia.