C1INHAntigen

C1INH antigen refers to the concentration of C1 esterase inhibitor (C1-INH) protein in blood plasma as measured by an immunoassay. C1-INH is a serine protease inhibitor that controls the complement system’s classical pathway by inhibiting C1r and C1s, and it also regulates the kallikrein-kinin and contact systems, limiting bradykinin production and plasma kallikrein activity. Deficiency or dysfunction of C1-INH leads to excessive activation of these pathways and is a key feature of hereditary and acquired angioedema.

The C1-INH antigen test is used diagnostically to quantify the amount of C1-INH protein. In hereditary angioedema, low C1-INH antigen levels indicate Type I disease, while normal antigen with reduced functional activity indicates Type II disease. Acquired C1-INH deficiency presents with low levels as well. C1-INH functional assays assess how well the protein inhibits target proteases and are often interpreted alongside C4 levels, which are typically low in active disease.

Genetics: C1-INH is encoded by the SERPING1 gene and is primarily produced by the liver, with additional expression in other tissues. Clinically, measurements of C1-INH antigen are part of a diagnostic workup for suspected angioedema and help guide management decisions, including therapy selection.

See also: hereditary angioedema, SERPING1, bradykinin, C4.