factor I
a plasma enzyme that regulates both classical and alternative pathways of complement activation (see complement). In the classical pathway, it blocks the formation of C3 convertase by catabolizing C4b; in the alternative pathway, it inactivates C3b by cleaving it to form iC3b, requiring also a cofactor such as factor H, MCP (membrane cofactor protein), or CR1 (complement receptor 1).
Medical dictionary.
2011.