Activation of emergency hematopoiesis. Emergency hematopoiesis is initiated by PAMP, damage-associated molecular patterns (DAMP), or acute blood loss that trigger release of cytokines from hematopoietic stem and progenitor cells (HSPCs), mature blood cells, and nonhematopoietic bystander cells. Emergency cytokines activate hematopoiesis in a manner commensurate to the strength of the signal, with the main burden of proliferation/differentiation borne by ST-HSCs and MPPs, whereas dormant HSCs are shielded.