Table 1.

HLH and MAS terminology^*

Cytokine storm: Encompasses primary and acquired HLH syndromes
Primary HLH: HLH caused by a genetic disorder	Acquired HLH: HLH that develops as a complication of another condition
FHL: HLH caused by bi-allelic mutations in one of the known FHL genetic loci (PRF1, UNC13D, STX11, STXBP2)	Infection-associated HLH: HLH that results from an infectious trigger such as EBV
HLH associated with genetic disorders: HLH that occurs in the setting of a known genetic disorder, most commonly PIDs (XP1, XLP2, ITK, XMEN, CD27 deficiency, CHS, GS type 2, HPS type 2, CGD)	Rheumatologic HLH/MAS: HLH in the setting of a rheumatologic condition (sJIA, AOSD, SLE, KD, autoinflammatory diseases)
	Malignancy-associated HLH: HLH that develops in a patient with malignancy
	Chemotherapy-associated HLH: HLH that develops as a result of chemotherapy for malignancy

^*There is no agreed-upon nomenclature for HLH- and MAS-related diseases.

This table reviews how the given terms are used in this manuscript AOSD adult-onset Still disease, CGD chronic granulomatous disease, CHS Chediak-Higashi syndrome, EBV Epstein-Barr virus, FHL familial hemophagocytic lymphohistiocytosis, GS Griscelli syndrome, HLH hemophagocytic lymphohistiocytosis, HPS Hermansky-Pudlak syndrome, ITK IL-2-inducible T-cell kinase deficiency, KD Kawasaki’s disease, MAS macrophage activation syndrome, PID primary immunodeficiency, sJIA systemic juvenile idiopathic arthritis, SLE systemic lupus erythematosus, XLP1 X-linked lymphoproliferative disease type 1, XLP2 X-linked lymphoproliferative disease type 2, XMEN X-linked immunodeficiency with magnesium defect, EBV infection, and neoplasia