Introduction
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.
