The O323 monoclonal antibody specifically recognizes CD27 which is also known as Tumor necrosis factor receptor superfamily member 7 (TNFRSF7), T14, Tp55, or S152. CD27 exists as a ~110-120 kDa disulfide-linked homodimer comprised of two single-pass type I transmembrane glycoproteins that are encoded by CD27 (CD27 molecule). CD27 is expressed on medullary thymocytes and T cells, with higher expression on activated T cells, and subsets of mature B cells and natural killer (NK) cells. A soluble 28-32 kDa form of CD27 is produced by lymphocytes upon cellular activation. Binding of the CD27 antigen, expressed on T cells, to its ligand, CD70 (CD27L), provides a costimulatory signal, leading to T cell proliferation, production of cytotoxic T cells, and enhanced production of cytokines. Binding of CD70 to CD27 expressed on B cells leads to B cell proliferation and the generation of plasma cells and immunoglobulin production. The CD27 antigen becomes hyperphosphorylated on serine residues upon activation of T cells. Signaling through the CD27 antigen activates NFκB and stress activated protein kinase (SAPK)/c Jun N terminal kinase (JNK).