Layered immunity of the developing thymus.

In this issue of Blood, Elsaid et al reinforce the temporal and spatial complexity of immune layering during embryogenesis by intricately dissecting 2 distinct waves of early thymic progenitors (ETPs) that differentially contribute to normal thymic organogenesis and homeostasis. The concept of “layered immunity,” in the form of waves of different immune cells derived from developmentally distinct progenitor populations, was initially described for innate-like lymphocytes, including B1-B cells and γ-δ T cells., Cellular compartments generated by layered immunity now extend beyond subsets of innate-like B and T cells to include tissue-resident macrophages, innate lymphoid cells (ILC), and mast cells.