Human C4b-binding protein (C4BP) protects host tissue, and those pathogens able to hijack this plasma glycoprotein, from complement-mediated destruction. For example, C4BP localises to apoptotic and necrotic cells, via its affinity for DNA, whereupon this regulator helps to prevent complement activation and subsequent inflammation and tissue damage. The first two CCP modules of the C4BP alpha-chain, plus the four residues connecting them, are necessary and sufficient for binding a bacterial virulence factor, the Streptococcus pyogenes M4 (Arp4) protein. Structure determination of this region of C4BP by NMR reveals two tightly coupled CCP modules in an elongated arrangement. Chemical shift perturbation studies demonstrate that the N-terminal hyper-variable, region of M4 binds to a site including strand 1 of CCP module 2. This interaction is accompanied by an intermodular reorientation within C4BP. The DNA-binding site of C4BP also involves CCP1 and CCP2 of the alpha=chain.  Chemical shift changes locate the binding site for DNA to a groove at the CCP1/2 interface enabling the use of data-driven docking to produce a model of the C4BP12-DNA complex. The work described in this thesis thus provides detailed pictures of interactions whereby a pathogen evades complement and that help protect host tissue during programmed cell death.