The CD70 antigen (CD70) is a cytokine that is found on the surface of activated but not resting T and B lymphocytes (Bowman et al. 1994). CD70 can bind to the CD27 antigen receptor (CD27), a dimeric membrane glycoprotein belonging to the tumor necrosis factor receptor family and found on the surface of most T lymphocytes (Camerini et al. 1991). This complex is thought to influence T-, B- and NK-cell functions (Salzer et al. 2013). Defects in CD27 can cause lymphoproliferative syndrome 2 (LPFS2), an autosomal recessive immunodeficiency disorder associated with persistent symptomatic EBV viremia, hypogammaglobulinemia, and impaired T-cell-dependent B-cell responses and T-cell dysfunction (van Montfrans et al. 2012, Salzer et al. 2013).