CD20 antigen structure and organization into membrane bilayers. (A) The CD20 protein has 4 cell membrane-spanning domains with both the amino and carboxy termini of the protein located within the cytoplasm. CD20 is closely associated with other proteins, in particular the C-terminal src kinase-binding protein (Cbp), CD40, and major histocompatibility complex class II proteins (MHC II). (B) Antibody binding to CD20 induces rapid translocation of the molecule to lipid rafts. Cbp can concentrate the C-terminal Src kinases (Csk) such as Lyn, Flyn, or Lck, which phosphorylate Cbp, leading to induction of kinase activity.