CD20 is a transmembrane protein of the MS4A family, primarily expressed on the surface of B cells, accompanying B cell development from early stages to maturity. It plays a crucial role in B cell activation, proliferation, and differentiation, particularly in regulating calcium channel and signal transduction. CD20 is equally important in regulating B cell receptor-mediated signal transduction and immune response. Variations in its expression levels are closely associated with various diseases such as B cell lymphomas and autoimmune disorders. Although CD20 mutations are rare, changes in its expression levels significantly impact B cell function and the efficacy of disease treatment. Monoclonal antibody therapy targeting CD20 has emerged as an effective approach for these diseases, underscoring the pivotal role of CD20 in the treatment of B cell-related disorders.