We have recently described a panel of monoclonal antibodies (mAb), that recognize two novel leukocyte surface antigens, BDCA-2 and BDCA-4. BDCA-2 is a novel type II C-type lectin specifically expressed by plasmacytoid dendritic cells (PDCs) that can internalize antigen for presentation to T cells. Furthermore, signaling via BDCA-2 may play a role in switching from interferon (IFN)-alpha/beta-controlled to interleukin (IL)-12-controlled immune response pathways, as triggering of BDCA-2 potently inhibits secretion of IFN-alpha/beta by PDCs and thereby promotes IL-12 p70 production in PDCs and other cells. Viruses may exploit this switch to escape innate antiviral immunity, but it may be beneficial for patients with systemic lupus erythematosus (SLE) if induced, for instance by anti BDCA-2 mAb treatment. BDCA-4 is shown here to be identical to neuropilin-1 (NP-1), a neuronal receptor for the axon guidance factors belonging to the class-3 semaphorin subfamily, and a receptor on endothelial and tumor cells for vascular endothelial growth factor (VEGF-A). In blood and bone marrow, BDCA-4/NP-1 is exclusively expressed on PDCs, but in tonsils also on a few other cells, primarily follicular B helper memory T cells (T(FH)).