Factor I deficiency

Factor I consists of two disulfide-linked polypetides which regulate the alternative pathway-cleaving enzyme. The alternative pathway in uncontrolled in the patients resulting in continous activation and cleavage of native C3 throgh the alternative pathway and the production of C3b. Patients with factor I deficiency have a secondary deficiency of C3 with markedly reduced levels of C3 in their serum. Total hemolytic activity of the serum is very low or undetectable. Level of factor H, factor B, and properdin is reduced.

Alternative names

Complement component I deficiency


  • Defects of complement regulatory proteins


Autosomal recessive


+217030 I factor; IF

Cross references

Phenotypically related immunodeficiencies

IDR factfile for C4 binding protein α deficiency


Incidence is not known.