Development of human and zebrafish immune systems. The development of the immune system starts with hematopoiesis at ∼17 dpf in humans and 12 hpf in the zebrafish, with myeloid and erythroid cells arising in the ALPM and ICM, respectively (Jagannathan-Bogdan and Zon, 2013). In humans, myeloerythroid progenitor cells seed in the yolk sac before HSCs appear in the AGM at 27 dpf (Julien et al., 2016), a stage mirrored in zebrafish with the start of definitive hematopoiesis at 30 hpf in the AGM and transition into the caudal hematopoietic tissue (Jagannathan-Bogdan and Zon, 2013). At 72 hpf, vital markers for early lymphoid progenitors are present in developing immune organs, such as the early thymus and kidney in zebrafish (Willett et al., 1999; Langenau et al., 2004; Trede et al., 2004). This corresponds roughly to the colonization of immune cells in the bone marrow and thymus in the human fetus at 10.5 wpf (Kissa et al., 2008). Notably, at 72 hpf the zebrafish emerges from the chorion and into contact with the outside environment without fully developed CD4 + /CD8 + lymphocytes, which appear later at 3 wpf (Lam et al., 2004). This is contrasted to humans, in which lymphocytes are detectable at 12–13 wpf, well before birth at 40 wpf (Tavian et al., 2010). ALPM: anterior lateral plate mesoderm; ICM: intermediate cell mass; HSC: Hematopoietic stem cells; AGM: aorta-gonad-mesonephros.