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The first component of complement is a calcium-dependent complex of the 3 subcomponents C1q, C1r, and C1s. The serine protease subunits C1r and C1s form a Ca(2+)-dependent C1s-C1r-C1r-C1s tetramer. C1r activates C1s so that it can, in turn, activate C2 and C4. C1s is a glycoprotein of approximately 88 kDa, which is activated by cleavage into an A chain and a B chain.