SH2D1A and Epstein-Barr virus infection: Similarly, patients with a mutation in the SLAM-associated protein (SAP) encoding gene Sh2d1a, a signalling protein expressed by TFH cells critically required for the formation of stable T∶B conjugates [221], [222] and GC-TFH cell expression of IL-4 [223], develop X-linked lymphoproliferative (XLP) disease characterised by an increased susceptibility to a number of pathogens (particularly Epstein-Barr virus infection, which can be fatal in children) due to abortive B cell responses [224].