C1I

C1I is most commonly encountered as an abbreviation for the C1 esterase inhibitor, also called C1-inhibitor (C1-INH), a serine protease inhibitor in the serpin family that regulates the classical complement pathway. The protein is encoded by the SERPING1 gene in humans and is produced primarily by the liver and vascular endothelium, circulating in blood plasma.

Function: It inhibits the proteases C1r and C1s of the C1 complex, as well as kallikrein and factor XIIa, thereby controlling activation of the complement and kallikrein-kinin systems. This limits inflammation and vascular permeability during immune responses.

Clinical significance: Deficiency or dysfunction of C1INH leads to hereditary angioedema (HAE), with episodic swelling of skin and mucous membranes. Types I and II reflect quantitative and functional deficiency; acquired C1INH deficiency is observed in certain lymphoproliferative or autoimmune conditions. Treatments include C1-INH replacement therapy (plasma-derived or recombinant) and agents that inhibit kallikrein or block bradykinin receptors.

Other uses and disambiguation: In other contexts, C1I may refer to field-specific codes, identifiers, or products. If a different meaning is intended, additional context is required.