Helicobacter pylori produces a 550 kDa, multimeric, nickel-containing urease that catalyses the hydrolysis of urea to yield ammonia and carbonic acid. The ure gene cluster, comprised of seven genes, encodes the two structural subunits UreA (26.5 kDa) and UreB (60.3 kDa), and five accessory proteins: UreI, UreE, UreF, UreG and UreH. Accessory proteins are required for nickel ion insertion into the apoenzyme. The native protein consists of six copies each of UreA and UreB; two nickel ions are coordinated into each UreB active site. Urease is found in the cytosol, but may also localize on the surface (although this may be an artefact) and elicits a strong serum immunoglobulin response. Urease aids in colonization of the host by neutralizing gastric acid and providing ammonia for bacterial protein synthesis. Host defences are avoided by urease by continuing to neutralize acid locally and by shedding urease, which may be bound by immunoglobulin, from the surface of the bacterium. Host tissues can be damaged directly by the urease-mediated generation of ammonia and indirectly by urease-induced stimulation of the inflammatory response, including recruitment of leukocytes and triggering of the oxidative burst in neutrophils.