myeloid

Myeloid refers to the myeloid lineage of blood cells, which originate from hematopoietic stem cells in the bone marrow and differentiate into several mature cell types. The term contrasts with the lymphoid lineage, which gives rise to lymphocytes. The word myeloid derives from Greek myelos, meaning bone marrow.

During hematopoiesis, a common myeloid progenitor gives rise to two main branches: megakaryocyte-erythroid progenitor (MEP) producing erythrocytes (red blood cells) and platelets, and granulocyte-monocyte progenitor (GMP) producing granulocytes (neutrophils, eosinophils, basophils) and monocytes. Monocytes can differentiate into macrophages and dendritic cells in tissues. Other cells such as mast cells also originate from the myeloid lineage through related progenitors.

Myeloid cells play essential roles in the innate immune system and in tissue homeostasis. Neutrophils and macrophages phagocytose pathogens and debris, dendritic cells present antigens to activate adaptive immunity, and mast cells release mediators that influence inflammation and allergic responses. Erythrocytes transport oxygen, and platelets participate in blood clotting and wound repair.

In medicine, the term myeloid is used to classify disorders of the hematopoietic system. Myeloid neoplasms include acute myeloid leukemia (AML), myelodysplastic syndromes, and myeloproliferative neoplasms such as polycythemia vera or chronic myeloid leukemia, reflecting abnormal differentiation or proliferation of myeloid cells.

See also: hematopoiesis, lymphoid lineage, myeloid-derived cells.