The first component of complement, C1, can be directly demonstrated in normal human serum by double immunodiffusion analyses since native macromolecular C1 gives a continuous line of precipitation with antisera to C1q, C1r, and C1s. All of the C1q, C1r, and C1s in normal serum were found to be in the C1 complex. These observations allow the quantitation of C1 in human serum by single radial immunodiffusion techniques.

Activation of C1 in human serum can be readily detected by double immunodiffusion studies with anti-C1q, anti-C1r, and anti-C1s because C1̄ exhibits a pattern quite different from that of native C1. In order to investigate this phenomenon, increasing doses of aggregated IgG were added to serum. Three major changes were apparent in the Ouchterlony patterns. First, spurring of the C1s precipitin line over that of macromolecular C1, indicating release of C1s from C1̄, was observed with the lowest doses of activator examined.

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