Caries cannot develop without the presence of a host, which comprises tooth structure, the saliva that surrounds a tooth, and the immune responses of saliva and plasma in the oral cavity. In the caries process, particular attention is paid to the enamel - the hard, outermost layer - because it is the primary contact with cariogenic bacteria, and where demineralization first begins. It is also the only tissue of the tooth that does not have the ability to grow or repair itself after maturation. Saliva is also important to consider in the caries process because it has protective properties that can reduce caries risk: it neutralizes pH in the vicinity of the tooth, assists in remineralization, acts as an antibacterial agent, and plays a role in the immune responses to cariogenic oral bacteria.