Lymph Drainage - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 179

and nasal-associated and bronchial-associated lymphoid tissues. These 2 groups of
secondary lymphoid organs trap and concentrate antigens at many loci throughout
the body to initiate an adaptive immune response. The spleen detects blood-borne
pathogens. T-cell maturation occurs in the thymus.
During embryonic development, circulating hematopoietic cells gather at
predestined sites throughout the body and form B- and T-cell-specific regions
of secondary lymphoid organs. Tertiary lymphoid structures appear with local,
prolonged inflammation. Paracrine signaling regulates the organization and
induction of immunocyte populations that lead to the formation of secondary
and tertiary lymphoid structures.

4.2.1 Thymus

The thymus is located in the upper anterior region of the chest cavity behind the
sternum and partly in the neck. In the thymus, lymphocyte precursors from the
bone marrow become thymocytes to subsequently mature into T lymphocytes.
The thymus becomes atrophic during puberty.
The thymus possesses 2 lobes. Each lobe is enclosed in a capsule. It is composed
of numerous lobules that contain multiple small follicles. Each follicle is made of
a medullary and a cortical region. Each follicle is surrounded by a vascular plexus.
The cortex is mainly composed of lymphoid lineage cells supported by a network
of epithelial reticular cells.
Early and late events in thymocyte development occur in the cortex and medulla,
respectively. The medulla has a coarser reticulum and concentric corpuscles of Has-
sall filled with lymphoid lineage and granular cells and enclosed by epithelioid cells.

4.2.2 Spleen

The spleen and tonsils are large lymphoid organs that have functions similar to
lymph nodes, although the spleen filters blood cells rather than lymph. The spleen
is located in the left upper abdomen below the rib cage. It destroys old erythrocytes
and serves as a center of the reticuloendothelial system.
Like the thymus, the spleen possesses only efferent lymphatic vessels. It is
subdivided into 2 regions — the red and white pulp. The red pulp that operates as
a blood filter contains blood sinuses (or sinusoids), splenic cords of reticular fibers,
and a marginal zone along the white pulp. The white pulp that participates in fighting
infections is composed of Malpighian corpuscles made of lymphoid follicles rich in
B lymphocytes and peri-arteriolar lymphoid sheaths rich in T lymphocytes.
In the spleen, follicular dendritic and fibroblastic reticular cells represent B- and
T-cell zone stromal cells, respectively. Fibroblastic reticular cells form continuous
sheaths around a mesh of collagen fibers and act as a backbone for the T-cell zone.