Leukopoiesis

Leukopoiesis is the production and development of white blood cells (leukocytes). As a component of hematopoiesis, it ensures a supply of neutrophils, monocytes, lymphocytes, and other subtypes essential for immune defense. In adults, leukopoiesis occurs primarily in the bone marrow; during fetal life the liver and spleen contribute before bone marrow becomes the main site after birth.

Leukopoiesis begins with hematopoietic stem cells that differentiate into two main lineages: common myeloid progenitors and common lymphoid progenitors. The myeloid lineage gives rise to granulocytes (neutrophils, eosinophils, basophils), monocytes/macrophages, and dendritic cells. The lymphoid lineage produces B cells, T cells, and natural killer (NK) cells. B cells mature in the bone marrow; T cells mature in the thymus.

Granulopoiesis and monocytopoiesis describe the maturation of granulocytes and monocytes within the marrow. Progenitors progress through stages to mature neutrophils, eosinophils, basophils, and monocytes, which enter the bloodstream and migrate to tissues as needed. Lymphopoiesis yields lymphocytes, with B and T cells populating peripheral lymphoid organs.

Leukopoiesis is regulated by cytokines and growth factors, including G-CSF, GM-CSF, M-CSF, and interleukins such as IL-3 and IL-7. Signals adapt to homeostatic needs and infection, enabling emergency production and release of leukocytes.

Clinically, disturbances of leukopoiesis produce leukopenia or leukocytosis and can contribute to leukemias or bone marrow failure. Chemotherapy or radiation suppress leukopoiesis, increasing infection risk, while growth factors like G-CSF are used to stimulate production after cytotoxic therapy.