This article summarizes the tolerance mechanisms of B cells and B-cell tolerance checkpoint defects in T1D, as well as the potential pathogenic mechanisms of B cells in T1D, including the secretion of autoantibodies by autoantibody-secreting cells, serving as APCs presenting self-antigens to CD4+ T cells, participating in coexistence with CD8+ T cells around the islets, participating in early regulation of the pancreatic microenvironment in disease, and differentiating into Bregs to exert a protective effect against T1D. The gene discussed is CD4; the disease is type 1 diabetes mellitus.