Part of the innate immune responses against microbial infection (PubMed:12529506, PubMed:27572839). Specifically recognizes a set of N-terminally truncated immunoglobulins that are produced via cleavage by proteases from a range of pathogenic bacteria and fungi, including L.pneumophila, M.hyorhinis, S.pneumoniae, S.aureus and C.albicans (PubMed:27572839). Recognizes epitopes that are in part in the variable region of the immunoglobulin light chains, but requires also the constant region for signaling (PubMed:27572839). Binds to a subset of cleaved IgM, IgG3 and IgG4 molecules, but does not bind cleaved IgA1 (PubMed:27572839). Binding of N-terminally truncated immunoglobulins mediates activation of neutrophils (PubMed:27572839). In monocytes, activation leads to the release of CSF2, CF3, IL6, CXCL8 and CCL3 and down-regulates responses to bacterial lipopolysaccharide (LPS), possibly via down-regulation of TLR4 expression and reduced signaling via TLR4 (PubMed:22479404). In eosinophils, activation by ligand binding leads to the release of RNASE2, IL4 and leukotriene C4 (PubMed:12529506). Does not bind class I MHC antigens (PubMed:19230061). Homodimer. Detected on the surface of all peripheral blood monocytes, neutrophils, basophils and eosinophils (at protein level) (PubMed:12529506, PubMed:22479404). Expression levels are very low or not detectable on monocytes, T-cells, B-cells, dendritic cells and natural killer (NK) cells (PubMed:9548455).