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Figure 3: Activation of the complement cascade by CRP complexes. CRP interacts with bacteria that express repeating ligands like PC on the Gram-positive pathogen S. pneumoniae. A single CRP pentamer interacts with one globular head group of a C1q molecule. Interaction of C1q with multiple CRP molecules leads to C1 activation, C4 and C2 cleavage, and the formation of a C3 convertase. The cleavage of C3 in turn forms a C5 convertase. This step is limited by CRP recruitment of the inhibitory protein fH. Thus the cleavage of C5 resulting in C5a generation and formation of the MAC is blocked.