Man and microbes are committed to a perennial evolutionary conflict in which the human immune system represents a powerful tool of protection against invasion. The liver plays an important role in the immune defense because of its central position adjacent to the gastrointestinal tract, representing the first line of defense against ingested pathogens and various antigens from food.

There are two types of immune response innate and adaptive. Innate immunity represents the first line of immune defense in which cells such as phagocytes, natural killer (NK) cells, and NK T cells recognize highly conserved antigens from the invading microorganisms and provide a prompt nonspecific inflammatory response. The principal intrahepatic defenders are Kupffer cells – resident macrophages, strongly supported by the action of NK cells, previously also known as Pit cells, which represent approximately 50% of the lymphocyte pool in a healthy liver. In contrast, the adaptive immune system is more phylogenetically advanced and includes more highly specialized cells such as T and B lymphocytes, produced and differentiated in the lymphoid organs. On stimulation, these cells undergo sophisticated processes of immune diversification enabling them to mount specific immune responses to different invading antigens from the environment. These events are much slower and involve mechanisms such as cell-to-cell interaction, proliferation of B cells, production of antibodies and cytokines, and activation of effector cytotoxic cells.