Several recognition proteins of the defence collagen family associate with proteases to initiate the complement cascade. The associated proteases, which are the subject of this review, mediate the proteolytic activation trigger. MBL-associated serine proteases (MASPs) mainly activate the lectin complement pathway (LP), while C1r and C1s activate the classical complement pathway (CP). This chapter will briefly introduce the current structural knowledge on these effector proteases and question what we know on MASPs structures, their common properties and how they differ from the C1r/s proteases. We will primarily focus on the structural comparison of the N-terminal domains, where the collectin binding sites are located and highlight novel aspects on their interaction with collagens. We also aim to highlight further molecular details associated to functional specificities, and to mention questions remaining open and needing further investigations. This chapter complements other reviews that describe the main general lines of complement activation mechanisms (Merle et al. 2015), the proposed structure-based scheme of the lectin pathway activation (Kjaer et al. 2013) and previous comparisons of LP and CP proteases (Gál et al. 2007; Pike and Wijeyewickrema 2013).