Understanding the structural organisation and mode of action of the initiating complex of the
classical pathway of complement activation (C1) has been a central goal in complement
biology since its isolation almost 50 years ago. Nevertheless, knowledge is still incomplete,
especially with regard to the interactions between its subcomponents C1q, C1r and C1s that
trigger activation upon binding to a microbial target. Recent studies have provided new
insights into these interactions, and have revealed unexpected parallels with initiating …