 However, the idea that the immune system contributes to organismal identity maintenance has to be weighed against evidence suggesting that immunity contributes to the accommodation of change. At the end of the nineteenth century, Metchnikoff proposed that phagocytes (primitive immune cells) were agents of organismal transformation, driving metamorphic and developmental alterations (Metchnikoff 1901 [1905]; Tauber and Chernyak 1991). This early view has been supported by recent evidence that the immune system actively participates in mediating transformations in organismal identity by constantly redefining and modulating immune reactivity towards endogenous and exogenous molecules (Tauber 2017). This is most apparent during development as illustrated by the activity of brain-resident macrophages (microglia), which, by impacting angiogenesis/vascularization, as well as the migration, proliferation and apoptosis of neurons, influence social behavioral and sexual identity (Wynn, Chawla, and Pollard.2013; Lenz and Nelson 2018).