Structure of the complement component 1 (C1) complex

A model of the C1 complex showing the intertwining of C1s-C1r-C1r-C1s in the C1q subcomponent. This structure permits the catalytic domains of C1s to contact the catalytic domains of C1r. The Fc portion of immunoglobulin G (IgG) and immunoglobulin M (IgM) engages the globular head of C1.