Plays a role in mast cell activation. Monomer and homodimer. Expressed on mast cells (at protein level) (PubMed:10630292, PubMed:10982834). Also expressed at much lower levels on natural killer cells (at protein level) (PubMed:10982834). In contrast to the related Lilrb4a protein, does not contain any copies of a cytoplasmic motif that is referred to as the immunoreceptor tyrosine-based inhibitor motif (ITIM).