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MZB1 is an endoplasmic reticulum (ER)-resident chaperone that facilitates immunoglobulin assembly and secretion, particularly polymeric IgM and dimeric IgA. Accumulating evidence links MZB1 to the regulation of intracellular Ca²⁺ signaling, integrin activation, and ER stress responses, which could influence innate-like B-cell functions and the maintenance of immune homeostasis. Aberrant expression of MZB1 has been implicated in the development of autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), as well as in tumor progression, including hepatocellular carcinoma and multiple myeloma. In certain cancer, MZB1 expression is associated alterations in the tumor microenvironment that may be permissive for tumor progression. Conversely, its ability to enhance antibody responses suggests potential applications in immunotherapy. This review integrates current insights into MZB1 biology, summarizes its role in disease pathogenesis, and discusses future directions for therapeutic targeting.