IMMUNE LYMPH SYSTEM
Essentials of Histology
The immune system
is represented by immune cells and accessory cells associated to reticular
fibers , reticular cells and lymphatic capillares. The immune cells are
represented by a T and B lymphocytes, and plasma cells , which have
the specific role in immune response (cellular or humoral) .The accessory
cells are macrophages, monocytes and neutrophils and others. These cells
perform a key role in the phagocytosis and secretion of soluble factors
– citocins and quimiotaxins.
In the mucosa
of different regions of the digestive and respiratory tract the immune
system are represented of MALT (mucosa associated lymphatic tissue) in
the form lymphoid follicles. These lymphoid follicles are roughly spherical
aggregates and have no limiting capsule. In low magnification, the
follicle is called primary, when appears as a homogeneous darkly stain
mass. On the other hand, when a lighter staining central region, known
as a germinal center, is observed, the lymphoid follicle is called secondary.
The next level of complexity is seen as permanent aggregates of lymphoid
follicles in organs of digestive system – the tonsils , Peyer’s patches
and appendix.
The tonsils
, which are confluent aggregates of more or less unencapsulated lymphatic
tissue, are arranged in the form of an incomplete ring in the walls of
the pharynx and nasopharynx and at the base of the tongue.
The Peyer’s
patches, that are large masses of confluent lymphatic nodules, situated
in the walls of the small intestine, particularly the ileum and in the
appendix. In the latter one, it appears as an extremely large mass of confluent
lymphatic follicles in the organ wall.
The immune cells
– lymphocytes – development in bone marrow and then enter the bloodstream.
The pré-T lymphocytes entering the thymus to become immunocompetent
cells. These cells, in the spleen and lymph node, constituting a thymus-dependent
zone, which form a periarterial lymphatic sheath and paracortical region
of lymph node, respectively.
The central
lymphatic organs - bone marrow and thymus – are sites where
new lymphocytes are autonomously produced. The peripheral lymphatic
organs are the places, from where lymphocytes respond to antigens,
and forming lymph node, spleen, MALT and others.
The thymus is
organized in a cortical and a medullar regions. This organ produces continuously
T-lymphocytes and thymic hormones, which stimulated others lymphatic organs.
An unusual feature of
the postcapillary venules, structures
found in all lymphatic tissues, is the presence of a high endothelial lining,
and as consequence, commonly referred as high endothelial venules or HEVs.
Recirculating T and B – lymphocytes recognize and adhere to these
HEVs in order to return to the tissue. So, HEVs are the sites whereby
T and B-cells cross over from blood to lymph in their path of recirculation
through the body.
The lymph nodes
are distributed along the course of the major tributaries, structures that
drain the lymph into the thoracic duct and right lymphatic duct, known
as lymphatic vessels. These organ possesses a capsule, an outer cortex,
inner cortex and a medulla, and perform two major fuctions. First
one is to filter the lymph and the second one form B-lymphocyte.
The spleen is
designed to facilitate immune responses to antigens that have gained access
to the circulating blood. The spleen
carries out two major functions: first, produces B-lymphocytes that
become activated as plasm cells, which are capable of producing humoral
antibodies
directed against blood-borne antigens.
The second function is collect and destroy defective blood cells.
The large population
of resident macrophages are responsible for the continuous removal
of senescent or damaged blood cells
and platelets, circulating debris and any suspended particulate matter
that may be present in the blood.
