The O323 antibody reacts with human CD27 (TNFRSF7), a cell surface homodimer of ≥ 55 kDa subunits, which provides co-stimulatory signaling in support of the T cell (TCR) and B cell (BCR) receptors. By comparison with CD28, whose TCR co-stimulatory signal can trigger cell proliferation, CD27 signaling appears to promote cell survival and differentiation to effector / memory stages. Also in contrast with CD28, the CD27 receptor may be shed following interaction with its ligand CD70, which is typically expressed on activated dendritic cells, T cells and B cells. With respect to B cells, CD27 is considered to be a phenotypic marker for memory B cells. CD27 has been included within a group of phenotypic markers for identifying human B regulatory cells (Bregs), a cell type proposed to regulate CD4+ T cell proliferation and Foxp3 / CTLA-4 expression in Treg cells.