Costimulatory receptor–ligand interactions including OX40–OX40L have been intensively studied in life‐threatening influenza immunopathology, because the detrimental outcome occurs mostly as a result of excessive inflammatory responses (Kopf et al, 1999; Humphreys et al, 2003, 2006; Lin et al, 2009; Snell et al, 2010; Braciale et al, 2012). The gene discussed is TNFSF4; the disease is influenza.