Immune system consists of various components, namely organs, tissues and cells which belong to it according to functional criteria (performance of organism defense) as well as anatomic-physiological principle of their organization (organ-circulatory principle). According to this point of view immune system is composed of the central/primary organs (bone marrow and thymus), peripheral/secondary organs (lymph nodes, spleen, liver and lymphoid aggregates in different organs) and, finally, circulatory pathways of immunocompetent cells.
• Lymphoid system is anatomic equivalent and synonym of immune system. Interactions with other systems of organism are important for it (with blood cells and vessels of cardio-vascular system as well as with integumentary tissues such as mucous membranes and skin). These systems are the close partners of lymphocytic immunity system.
• Organ-circulatory principle of organization. A healthy adult organism contains 1013 lymphocytes or approximately each tenth cell is a lymphocyte. Anatomical-physiological principle of immune system arrangement is organ-circulatory. It means that lymphocytes do not permanently "dwell" in lymphoid organs, but intensively re-circulate between lymphoid organs and non-lymphoid tissue through lymphatic vessels and blood. Hence each lymph node admits ~109 lymphocytes per hour. The mechanism of lymphocyte migration is determined by specific interaction between certain surface molecules on lymphocytes and endothelial cells of vessel walls (such molecules are termed adhesins, selectins, integrins, homing RC). In fact each organ has a specific spectrum of both lymphocytes and their partners in immune response.
• Composition of the immune system. The following organs and tissues compose the immune system (Fig. 2.1).