The study investigates the ontogenetic lineages of conventional dendritic cells (cDCs), specifically focusing on the heterogeneity of cDC2 subsets. The authors identify distinct pre-cDC2 subsets in the bone marrow that bias towards generating cDC2As or cDC2Bs. They show that a Siglec-H+ pre-cDC2A population in the bone marrow preferentially gives rise to cDC2As, while a Siglec-H+ fraction of pre-cDCs generates cDC2Bs. The results suggest that the specification of cDC2A and cDC2B lineages begins in the bone marrow and that these subsets are ontogenetically determined lineages rather than cell states imposed by the peripheral tissue environment. Single-cell RNA sequencing further reveals that pre-cDC2s can be divided into two groups, with one group resembling cDC2As and the other cDC2Bs. The study also demonstrates that the bone marrow contains two populations of pre-cDC2s that can be segregated according to SiglecH expression, corresponding to cDC2As and cDC2Bs. Additionally, the authors show that lymphotoxin and Notch ligands sustain the specification of pre-cDC2As, while the specification of pre-cDC2Bs is influenced by other environmental cues. Lineage tracing experiments confirm the distinct ontogeny of cDC2A and cDC2B lineages, and the findings are conserved across species. Overall, the study provides insights into the ontogenetic determination of cDC2 subsets and their potential plasticity in response to environmental cues.The study investigates the ontogenetic lineages of conventional dendritic cells (cDCs), specifically focusing on the heterogeneity of cDC2 subsets. The authors identify distinct pre-cDC2 subsets in the bone marrow that bias towards generating cDC2As or cDC2Bs. They show that a Siglec-H+ pre-cDC2A population in the bone marrow preferentially gives rise to cDC2As, while a Siglec-H+ fraction of pre-cDCs generates cDC2Bs. The results suggest that the specification of cDC2A and cDC2B lineages begins in the bone marrow and that these subsets are ontogenetically determined lineages rather than cell states imposed by the peripheral tissue environment. Single-cell RNA sequencing further reveals that pre-cDC2s can be divided into two groups, with one group resembling cDC2As and the other cDC2Bs. The study also demonstrates that the bone marrow contains two populations of pre-cDC2s that can be segregated according to SiglecH expression, corresponding to cDC2As and cDC2Bs. Additionally, the authors show that lymphotoxin and Notch ligands sustain the specification of pre-cDC2As, while the specification of pre-cDC2Bs is influenced by other environmental cues. Lineage tracing experiments confirm the distinct ontogeny of cDC2A and cDC2B lineages, and the findings are conserved across species. Overall, the study provides insights into the ontogenetic determination of cDC2 subsets and their potential plasticity in response to environmental cues.