300
UNIT IV
Infection, Inflammation, and Immunity
Thymus
The thymus is an elongated, bilobed structure located in
the mediastinum above the heart and serves as a specialized
immune system organ. Each lobe is surrounded by a connec-
tive tissue capsule layer and is divided into lobules. The lob-
ules can be divided into an outer cortex and a central medulla,
which play different roles in the process of T-lymphocyte
maturation. The outer cortex contains densely packed imma-
ture T lymphocytes (thymocytes). The inner medulla is a less
dense area of tissue that contains fewer but more histologically
mature lymphocytes. The medulla is comprised of Hassall
corpuscles but also stores DCs and macrophages (Fig. 13.13).
The thymus is essential to the development of the
immune system because it is responsible for the production
of mature, immunocompetent T lymphocytes. The thymus
is a fully developed organ at birth, weighing approximately
15 to 20 g. It is most active in the neonatal and preadoles-
cent periods. At puberty, when the immune cells are well
established in peripheral lymphoid tissues, the thymus begins
to atrophy and is replaced by adipose tissue. Nevertheless,
residual T-lymphocyte production continues throughout adult
life. Precursor T (pre-T) cells enter the thymus as function-
ally and phenotypically immature T cells. They then mature
during different cycles and then move from the cortex to the
medulla until they are released into the peripheral lymphoid
tissues. Rapid cell division, maturation, and selection occur
Lymphoid Organs
The central and peripheral lymphoid organs are respon-
sible for the production, maturation, and storage of large
numbers of immune system cells including the B and
T lymphocytes. These organs and tissues are widely distrib-
uted throughout the body and provide different, but often
overlapping, functions (Fig. 13.12). The central lymphoid
organs are comprised of the bone marrow and the thymus
and are responsible for immune cell production and mat-
uration. The tissues and cells of the peripheral lymphoid
system store the cells of the immune system where they
function to concentrate and process antigen as well as
support cellular processes necessary for development of
fully functioning, adaptive immune responses. The periph-
eral lymphoid tissues are comprised of the lymph nodes,
spleen, tonsils, appendix, Peyer patches in the intestine,
and mucosa-associated lymphoid tissues in the respiratory,
gastrointestinal, and reproductive systems. Networks of
lymph channels, blood vessels, and capillaries connect the
lymphoid organs and transport immune cells, antigens, and
cellular debris throughout the body.
Spleen
Adenoid
Tonsil
Thymus
Bronchus-
associated
lymphoid
tissue
Intestine
Axillary
lymph nodes
Peyer
patches
Bone
marrow
Appendix
Inguinal
lymph nodes
FIGURE 13.12
•
Central and peripheral lymphoid organs and tissues.
Capsule
Dead cell
Macrophage
Epithelial cell
Dendritic cell
Dividing
thymocyte
Hassall
corpuscles
FIGURE 13.13
•
Structural features of the thymus gland. The thymus
gland is divided into lobules containing an outer cortex densely packed
with dividing thymocytes or premature T cells and an inner medulla that
contains mature T lymphocytes, macrophages, dendritic cells, and
Hassall corpuscles.
