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The complement system is an essential component of the innate and acquired immune system (1), and consists of a series of proteolytic cascades that are initiated by the presence of microorganisms. In health, activation of complement is precisely controlled through membrane-bound and soluble plasma-regulatory proteins including complement factor H (fH; ref. 2), a 155 kDa protein composed of 20 domains (termed complement control protein repeats). Many pathogens have evolved the ability to avoid immune-killing by recruiting host complement regulators (3) and several pathogens have adapted to avoid complement-mediated killing by sequestering fH to their surface (4). Here we present the structure of a complement regulator in complex with its pathogen surface-protein ligand. This reveals how the important human pathogen Neisseria meningitidis subverts immune responses by mimicking the host, using protein instead of charged-carbohydrate chemistry to recruit the host complement regulator, fH. The structure also indicates the molecular basis of the host-specificity of the interaction between fH and the meningococcus, and informs attempts to develop novel therapeutics and vaccines., Neisseria meningitidis is a human adapted pathogen of global importance as a leading cause of bacterial meningitis and septic shock (5). Owing to Neisserial strain variation, the vaccines currently available [...]