Associates with immunoglobulin M (IgM) heavy and light chains and promotes IgM assembly and secretion. May exert its effect by acting as a molecular chaperone or as an oxidoreductase as it displays a low level of oxidoreductase activity (By similarity). Isoform 2 may be involved in regulation of apoptosis. Helps to diversify peripheral B- cell functions by regulating Ca(2+) stores, antibody secretion and integrin activation. {ECO:0000250, ECO:0000269|PubMed:11350957, ECO:0000269|PubMed:21688198}. Acts as a hormone-regulated adipokine/pro-inflammatory cytokine that is implicated in causing chronic inflammation, affecting cellular expansion and blunting insulin response in adipocytes. May have a role in the onset of insulin resistance. This is the function of MZB1 (marginal zone B and B1 cell specific protein, Ensembl gene identifier ENSG00000170476).