Lymphocyte impairment in COVID-19 resembles the cytotoxic dysfunction of CD8+ cytotoxic T lymphocytes and natural killer cells observed in familial haemophagocytic lymphohistiocytosis, in which T cell dysfunction is the result of heterozygous mutations in genes affecting the expression of perforin or other proteins involved in the trafficking and docking of cytolytic granules,1 and in patients who are predisposed to MAS, in whom IL-6 overexpression can reduce perforin and granzyme B concentration inside granules.31