1. Originate from hematopoietic stem cell in bone marrow.
2. Gives rise to two major lineages:
a) myeloid progenitor cell that gives rise to: monocytes, macrophages, dendritic cells, meagakaryocytes, granulocytes and mast cells
b) lymphoid progenitor cell that gives rise to: T cells (matures in thymus), B cells, natural killer (NK) and dendritic cells