Immunoglobulin Binding Protein (BiP) is a member of heat shock protein 70 famaily, and is also known as an autoantigen in rheumatoid arthritis (RA) patients. Serum anti-BiP antibody is detected up to 60% of RA patients, and recent reports demonstrated that serum anti-BiP antibody is also detected in systemic lupus erythematosus patients. Notably, anti-citrullinated BiP antibody is revealed as another member of anti-citrullinated protein/peptide antibodies (ACPAs). Since ACPAs are supposed to be closely associated with RA pathogenesis, immune responses to citrullinated BiP could play an important role in RA. Indeed, immunization of citrullinated BiP exacerbated inflammatory arthritis in mice. Moreover, T cell responses to BiP were reported in human RA and mice models. In mice models, native BiP administration induced IL-4 and IL-10 producing CD4(+) T cells and regulated inflammatory arthritis. In this way, immune responses to BiP are various, and dysregulation of the balances between pro-inflammatory and regulatory responses to BiP could lead to the autoimmune responses and diseases.