Immunodeficiency with factor I anomaly

Immunodeficiency with factor I anomaly, also known as complement factor I deficiency or factor I deficiency, is a rare primary immunodeficiency disorder caused by deficiency or dysfunction of complement factor I (CFI), a serine protease that plays a critical role in regulating the complement system. Complement factor I is responsible for cleaving and inactivating complement components C3b and C4b, thereby preventing excessive complement activation. When factor I is absent or dysfunctional, uncontrolled complement activation leads to secondary consumption and depletion of complement component C3, resulting in impaired opsonization and immune defense. The condition primarily affects the immune system, leaving patients highly susceptible to recurrent, severe infections caused by encapsulated bacteria such as Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae. Common clinical manifestations include recurrent sinopulmonary infections, meningitis, sepsis, and otitis media, typically presenting in early childhood. Some patients may also develop autoimmune manifestations, including glomerulonephritis and systemic lupus erythematosus-like symptoms, due to impaired immune complex clearance. Additionally, factor I deficiency has been identified as a risk factor for atypical hemolytic uremic syndrome (aHUS). Treatment is primarily supportive and includes prompt and aggressive antibiotic therapy for infections, prophylactic antibiotics in some cases, and vaccination against encapsulated organisms. Fresh frozen plasma infusions may be used to temporarily restore complement factor I levels during acute infections. Long-term management focuses on infection prevention and monitoring for autoimmune complications. Genetic counseling is recommended for affected families.

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