• Allergy and Immunology
• Paediatrics

A healthy male teenager presented to his primary care paediatrician for a routine check-up including receipt of vaccines. On clinical examination, isolated splenomegaly was noted, a few days later confirmed by MRI (figure 1). Extensive investigations excluded an underlying oncological, infectious or metabolic disease. Due to bicytopenia (haemoglobin 69 g/L, white blood count 1.52×10⁹/L, platelet count 151×10⁹/L) with suspected autoimmune pathogenesis, extended immunological diagnostics were performed. This revealed an increased serum IgG concentration (19.3 g/L (5.9–12.5)) and an increase in double-negative T cells (DNT) (14.7%, <6%) by flow cytometry of peripheral blood lymphocytes. In addition, serum vitamin B₁₂ (1530 pg/mL (182–1090)) and soluble Fas ligand levels (2428 pg/mL, <250) were elevated. In view of these highly suggestive findings of autoimmune lymphoproliferative syndrome (ALPS), functional and genetic diagnostics were performed to confirm the suspected diagnosis. Genetic investigation revealed a previously unknown heterozygous variant in FAS (NM_000043.6:c.806A>T, p.Asp269Val, not present in gnomAD), which was functionally verified …