The immunological system is comprised of the lymphoid tissues and organs of the body. Lymphoid tissues are widely distributed: they are concentrated in bone marrow, lymph nodes, spleen, liver, thymus, and Peyer’s patches scattered in linings of the GI tract. The lymphoid system is encompassed by the system of mononuclear phagocytes. Lymphocytes are the predominant cells, but macrophages, dendritic cells, and plasma cells are also present. Lymphocytes are cells which circulate, alternating between the circulatory blood stream and the lymphatic channels.
