Tositumomab is a murine monoclonal antibody which targets the CD20 antigen produced in mammalian cell. It is classified as a IgG2a lambda antibody.This protein may be involved in the regulation of B-cell activation and proliferation. Defects in MS4A1 are the cause of immunodeficiency common variable type 5 (CVID5); also called antibody deficiency due to CD20 defect. CVID5 is a primary immunodeficiency characterized by antibody deficiency, hypogammaglobulinemia, recurrent bacterial infections and an inability to mount an antibody response to antigen. The defect results from a failure of B-cell differentiation and impaired secretion of immunoglobulins; the numbers of circulating B-cells is usually in the normal range, but can be low. Belongs to the MS4A family.