Disentangling inborn and acquired immunity in human twins.

The human geneticist Archibald Garrod noted in 1931 that, "It is, of necessity, no easy matter to distinguish between immunity which is inborn and that which has been acquired" (The Inborn Factors in Disease). In this issue of Cell, Brodin et al. show that the heritability of blood counts rapidly decreases with age for the lymphoid subsets responsible for adaptive immunity, unlike cells from other hematopoietic lineages.