Complement factor I deficiency is a disorder that affects the immune system. People with this condition are prone to recurrent infections, including infections of the upper respiratory tract, ears, skin, and urinary tract. They may also contract more serious infections such as pneumonia, meningitis, and sepsis, which may be life-threatening.

Some people with complement factor I deficiency have a kidney disorder called glomerulonephritis with isolated C3 deposits. Complement factor I deficiency can also be associated with autoimmune disorders such as rheumatoid arthritis or systemic lupus erythematosus (SLE). Autoimmune disorders occur when the immune system malfunctions and attacks the body's tissues and organs.

Complement factor I deficiency is caused by mutations in the CFI gene. This gene provides instructions for making a protein called complement factor I. This protein helps regulate a part of the body's immune response known as the complement system. The complement system is a group of proteins that work together to destroy foreign invaders (such as bacteria and viruses), trigger inflammation, and remove debris from cells and tissues. This system must be carefully regulated so it targets only unwanted materials and does not attack the body's healthy cells. Complement factor I and several related proteins protect healthy cells by preventing activation of the complement system when it is not needed.

Mutations in the CFI gene that cause complement factor I deficiency result in abnormal, nonfunctional, or absent complement factor I. The lack (deficiency) of functional complement factor I protein allows uncontrolled activation of the complement system. The unregulated activity of the complement system decreases blood levels of another complement protein called C3, reducing the immune system's ability to fight infections. In addition, the immune system may malfunction and attack its own tissues, resulting in autoimmune disorders.

Normal Function

The CFI gene provides instructions for making a protein called complement factor I. This protein helps regulate a part of the body's immune response known as the complement system. The complement system is a group of proteins that work together to destroy foreign invaders (such as bacteria and viruses), trigger inflammation, and remove debris from cells and tissues. This system must be carefully regulated so it targets only unwanted materials and does not attack the body's healthy cells. Complement factor I and several related proteins protect healthy cells by preventing activation of the complement system when it is not needed.

Health Conditions Related to Genetic Changes

Complement factor I deficiency

At least 10 mutations in the CFI gene have been identified in people with complement factor I deficiency, a disorder characterized by immune system dysfunction. The mutations result in abnormal, nonfunctional, or absent complement factor I.

The lack (deficiency) of functional complement factor I protein allows uncontrolled activation of the complement system. The unregulated activity of the complement system decreases blood levels of another complement protein called C3, reducing the immune system's ability to fight infections. In addition, the immune system may malfunction and attack its own tissues, resulting in autoimmune disorders.

Age-related macular degeneration

MedlinePlus Genetics provides information about Age-related macular degeneration

Atypical hemolytic-uremic syndrome

MedlinePlus Genetics provides information about Atypical hemolytic-uremic syndrome

C3 glomerulopathy

MedlinePlus Genetics provides information about C3 glomerulopathy

Other disorders

Mutations in the CFI gene have also been found in people with glomerulonephritis with isolated C3 deposits. This condition, which may also occur in people with complement factor I deficiency, is characterized by kidney malfunction that can be serious or life-threatening. The CFI gene mutations identified in this disorder result in an abnormal or nonfunctional version of complement factor I. The defective protein allows uncontrolled activation of the complement system. The overactive complement system attacks certain kidney cells, which damages the kidneys and leads to a loss of protein in the urine (proteinuria).

A common variation (polymorphism) in the CFI gene has also been associated with age-related macular degeneration (AMD). AMD is a leading cause of vision loss among older adults. It is characterized by damage to the retina and a loss of sharp vision (visual acuity). Researchers suggest that the CFI gene variation that has been associated with AMD changes the way the gene is activated (expressed). It is unclear how this change is related to the development of AMD. A combination of genetic and environmental factors likely determines the risk of developing this complex eye disorder.

Other Names for This Gene

• AHUS3
• C3b-INA
• C3b-inactivator
• C3B/C4B inactivator
• C3BINA
• CFAI_HUMAN
• complement component I
• complement control protein factor I
• complement factor I heavy chain
• complement factor I preproprotein
• FI
• IF
• KAF
• Konglutinogen-activating factor
• light chain of factor I

Genomic Location

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Complement factor I deficiency is a rare disorder; its exact prevalence is unknown. At least 38 cases have been reported in the medical literature.