FLT3L, also known as flt3 ligand, is a small molecule that acts as a growth factor that increases the number of immune cells by activating the hematopoietic progenitors. In vivo, FLT3L also induces the mobilization of the hematopoietic progenitors and stem cells. This may help the system to kill cancer cells. Dendritic cells (DCs) provide the key link between innate and adaptive immunity by recognizing pathogens and priming pathogen-specific immune responses. FLT3L controls the development of DCs and is particularly important for plasmacytoid DCs and CD8 -positive classical DCs and their CD103 -positive tissue counterparts.