Part of the innate immune responses against microbial infection. 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. Recognizes epitopes that are in part in the variable region of the immunoglobulin light chains, but requires also the constant region for signaling. Binds to a subset of cleaved IgM, IgG3 and IgG4 molecules, but does not bind cleaved IgA1. Binding of N-terminally truncated immunoglobulins mediates activation of neutrophils. 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. In eosinophils, activation by ligand binding leads to the release of RNASE2, IL4 and leukotriene C4. Does not bind class I MHC antigens.