(Primary) Familial Hemophagocytic Lymphohistiocytosis– aka Familial Erythrophagocytic Lymphohistiocytosis
1° HLH/ FHL
Macrophage Activation Diseases
PRF1, STX11, STXBP2, MUNC13-4 , RAB27A, UNC13D; X linked: SH2D1A,
Autosomal recessive, but if X-linked: inheritance is dominant.
1° HLH affects 1:50,000 people worldwide. 
Timing of symptoms:
Fevers often > 39°C 1-2 times/day for >2 weeks, most often occurring in the evening with arthralgia, rash and other symptoms.
High fevers. Increased CSF protein. High intracranial pressure (ICP). Multifocal inflammation of the gray and white matter, intracranial bleeding, generalized atrophy or brain edema, seizures and/or coma. 
Blindness due to brain inflammation. 
High risk for respiratory infections triggering fevers, edema, systemic inflammation and macrophage activation syndrome (MAS). 
Hepatomegaly, splenomegaly, or hepatosplenomegaly. Liver disease (hepatitis) is common. High risk of death from multi-organ failure in 2+ months if untreated. 
Lymphoma. Hemophagocytosis– spleen and lymph nodes. Splenomegaly. 
Joints bones muscles cartilage:
Hemophagocytosis in the bone marrow. Delayed closure of the bones of the skull in infants, with a bulging fontanel often noted. Neck stiffness, abnormal muscle tone, impaired muscle coordination, paralysis.  
High: ESR, CRP, triglycerides, liver Function tests (LFTs), especially ALT, soluble CD25, ferritin, sIL2r. Low: platelets, fibrinogen. Low NK cell cytotoxic function, neutropenia, anemia. 
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Leading center in the US for HLH is Cincinnati Children's.