Determination of serum C9 level by immunodiffusion. Elevation in patients with infectious or allergic skin diseases.

The measurement of the ninth component of complement (C9) in human serum can be easily carried out by the Mancini method using antiserum raised against purified C9. The average C9 level in 29 healthy adults was 44.5 plus or minus 10.6 mug/ml. The serum C9 level was often elevated in patients with infectious or allergic skin diseases. The C9 level remained normal in most patients with collagen diseases. The availability of the monospecific antiserum has permitted identifying C9 in human serum as alpha2-globulin.