Mediates B-cell B-cell interactions. May be involved in the localization of B-cells in lymphoid tissues. Binds sialylated glycoproteins; one of which is CD45. Preferentially binds to alpha-2,6-linked sialic acid. The sialic acid recognition site can be masked by cis interactions with sialic acids on the same cell surface. Upon ligand induced tyrosine phosphorylation in the immune response seems to be involved in regulation of B-cell antigen receptor signaling. Plays a role in positive regulation through interaction with Src family tyrosine kinases and may also act as an inhibitory receptor by recruiting cytoplasmic phosphatases via their SH2 domains that block signal transduction through dephosphorylation of signaling molecules. Interacts with LYN, SYK, PIK3R1/PIK3R2, PLCG1, SHC1, INPP5D and GRB2 upon phosphorylation. May form a complex with INPP5D/SHIP, GRB2 and SHC1. Interacts with PTPN6/SHP-1 upon phosphorylation (By similarity). B-lymphocytes. Contains 3 copies of a cytoplasmic motif that is referred to as the immunoreceptor tyrosine-based inhibitor motif (ITIM). This motif is involved in modulation of cellular responses. The phosphorylated ITIM motif can bind the SH2 domain of several SH2-containing phosphatases. Phosphorylated on tyrosine residues by LYN. Phosphorylation of Tyr-783 and Tyr-843 are involved in binding to SYK. Phosphorylation of Tyr-828 is involved in binding to GRB2. Phosphorylation of Tyr-863 is involved in binding to SYK, PLCG2 and PIK3R1/PIK3R2. Belongs to the immunoglobulin superfamily. SIGLEC (sialic acid binding Ig-like lectin) family. Siglec-2