C3 convertase C3bBb, which is stabilized by the presence of plasma properdin (Kemper et al. 2010) . Properdin is a protein released by activated neutrophils (properdin is also found in macrophages and T cells) and which stabilizes the convertase by binding to C3b and preventing its cleavage by Factors H and I. Recent studies suggest that properdin can directly bind to apoptotic and necrotic cells and initiate complement activation (Kemper et al. 2010 ).