Proposed model for the pathway regulating IFN-γ secretion in immature B cells. Ly49D stimulation by its ligand, MHC class I, activates a cascade leading to the secretion of IL-12 and IL-18. Both IL-12 and IL-18 bind to their receptor, expressed on immature B cells, resulting in the main (direct pathway, via IL-15) or less effective (indirect) activation of IFN-γ secretion. IFN-γ secretion results in the inhibition of the migration of immature B cells to the lymph node and sites of inflammation.