Hematopoiesis is the process by which different types of blood cells are produced from a single type of cells called hematopoietic stem cells or HSCs. In adults, hematopoiesis occurs in the bone marrow. Bone marrow is surrounded by reticular cells, stromal cells, and an intricate web of blood vessels. Hematopoiesis occurs in the spaces between these blood vessels and cell components. HSCs continuously multiply and self-renew by undergoing asymmetric division. They produce two daughter cells, one of which can self-renew like the parent cell and maintain the HSC pool in the bone marrow. In contrast, specific molecular factors released in response to tissue injury, growth spurt, or infection help differentiate the second daughter cell into more specialized cells, including the myeloid progenitor cells and the lymphoid progenitor cells. HSC progenitors can produce all types of blood cells, including red blood cells, lymphocytes, macrophages, and natural killer cells. Once committed to their specific roles, these blood cells leave the bone marrow and enter the blood vessels to migrate to their site of action.