Innate immunity: a key player in the mobilization of hematopoietic stem/progenitor cells.

The mobilization of hematopoietic stem/progenitor cells (HSPCs) from bone marrow into peripheral blood (PB) is still not fully understood. Different chemokines, cytokines, growth factors, and neurotransmitters have been described that facilitate this process. However, mounting evidence suggests that mobilization of HSPCs is a part of the immune response and is mediated by innate immunity. We discuss evidence showing that complement system cleavage fragments play a crucial role in both the retention and mobilization of HSPCs by modulating their responsiveness to stromal-derived growth factor-1 (SDF-1) gradient (by C3-derived anaphylatoxins) and by modulating the release of granulocytes into PB that subsequently facilitate the egress of HSPCs (by C5-derived anaphylatoxins).