The type of resistance thus developed in the host is called acquired immunity . The term immune usually means the ability to resist infectious disease. Immunity refers to the relative state of resistance of the host to a specific pathogen. Acquired immunity, itself, is sometimes divided into two types based on how it is acquired by the host. Active immunity can persist a long time in the host, up to many years in humans.
Passive immunity is typically short-lived and usually persists only a few weeks or months.
Bacteria are composed of various macromolecular components that are antigenic to their host and bacterial antigens interact with the host immunological system in a variety of ways. Furthermore, the interaction between these immune tissues and intestinal bacteria results in the production of serum and secretory antibodies that are directed against bacterial antigens. These antibodies could potentially protect the host from invasion by the normal flora, or they could cross react with antgenically-related pathogens. For example, antibodies against normal E. coli could react with closely-related pathogenic Shigella dysenteriae . These type of antibodies are sometimes called natural or cross-reactive antibodies .