A fundamental difference between the immune and other body systems, such as the nervous, endocrine, and digestive systems, is that many of the cells involved in the immune response are highly motile. They use the blood vessels and lymphatic vessels in order to move into and out of organized lymphoid tissue and to reach the site of an infection. For an effective acquired immune response, an intricate series of cellular events must occur. Antigen must be detected and then processed by antigen‐presenting cells (APCs), which subsequently make contact with and activate helper T‐cells to stimulate B‐cells and cytotoxic T‐cell precursors. Additionally, various factors, such as cytokines, are required to support lymphocyte proliferation and bring about cellular differentiation. Memory cells for secondary responses must be formed and the whole response coordinated so that it is adequate but not excessive and is appropriate to the type of infection being dealt with. The integration of the complex cellular interactions that form the basis of the immune response takes place within the secondary lymphoid tissue, which consists of the lymph nodes, spleen, and the mucosa-associated lymphoid tissue (MALT) lining the respiratory, gastrointestinal, and genitourinary tracts.
The Location Of The Immune System
The skin and mucosal outer surfaces of the body provide a first line of defense. If these are breached then the cells of what is conventionally referred to as the immune system will be encountered. Virtually all (the exception being the follicular dendritic cell) cells of the immune system are generated from multipotent hematopoietic stem cells in the bone marrow, and the majority of them mature within the bone marrow prior to being released into the blood circulation and subsequently entering the tissues. Cells of the innate response are found throughout the body. For example, resident tissue macrophages, mast cells in connective and mucosal tissues, NK cells in many locations, neutrophils recruited to the site of an infection, and so on. With respect to the lymphocytes of the adaptive response, although the B‐cells become fully mature within the bone marrow, T‐cell precursors must travel from the bone marrow to the thymus where they reach full maturity (Figure 6.1). The bone marrow and thymus are therefore referred to as the primary lymphoid tissue – the location where mature lymphocytes are produced. Any location in the body outside of the primary lymphoid tissues is referred to by immunologists as the “periphery.” The initiation of adaptive immune responses by lymphocytes takes place in specialized areas of the periphery – the secondary lymphoid tissues (MALT, lymph nodes, and spleen).
The term “leukocyte” is used to describe the white blood cells but one should remain cognisant of the fact that the blood circulation acts largely as a distribution network for these cells and that they carry out their functions mostly within the lymphoid and other body tissues. This may be a good point at which to pose the question, how does one categorize a cell as belonging to the immune system? Like many very simple questions there is no easy answer to this one. Thus, erythrocytes are perhaps not usually considered a part of the immune system despite the fact that their possession of complement receptors provides them with an important role on the clearance of immune complexes from the circulation. Likewise endothelial cells are also not normally classed as cells of the immune system despite their fundamental role in alerting leukocytes to an infection. The message here is clear: Mother Nature does not compartmentalize the different body systems in the rigid way that we sometimes try to.