CHAPTER 10
Lymphoid System
191
SYNOPSIS 10-2
Lymphoid Organs
In
primary lymphatic organs
, lymphocytes differentiate and mature; B cells’ primary lymphoid organ is bone marrow;
T cells’ primary lymphoid organ is the thymus.
In
secondary lymphatic organs
, lymphocytes encounter and respond to foreign antigens; secondary lymphoid organs
include MALT, lymph nodes, and spleen.
Lymphatic nodules
are spherical structures that contain accumulated lymphocytes. They include primary nodules and
secondary nodules.
Primary nodules
contain mostly small (inactivated) B cells and do not have a germinal center.
Secondary nodules
contain mostly large (activated) B cells and have a germinal center (light area in the center).
Lymphatic nodules
contain mostly B cells.
The
thymus
,
paracortex
of the lymph node, and
PALS
in the spleen contain mostly T cells.
Afferent
lymphatic vessels
Artery
Medullary
sinuses
Efferent
lymphatic vessel
Postcapillary venules
(HEV in paracortex)
Venules
Small vein (medulla)
Vein
Subcapsular
sinuses
Small artery
(b
in medulla)
ranches of artery
Peritrabecular
sinuses
Arterioles
(paracortex and cortex)
capillaries
(nodules of cortex)
Vascular channels (blood):
Lymphatic channels (lymph):
Secondary
nodule
Afferent
lymphatic vessel
Subcapsular sinus
Primary
nodule
Medullary
cords
Medullary
sinuses
Efferent
lymphatic vessel
Trabecula
Trabecula
Germinal center
Germinal center
of secondary
of secondary
nodule in cortex
nodule in cortex
HEV
in
paracortex
Trabecula
Peritrabecular
sinus
Germinal center
of secondary
nodule in cortex
Medulla
Paracortex
Artery
Vein
Cortex
Cortex
Cortex
Capsule
P
a
ra
co
rte
te
x
Medula
M
edula
Paracortex
P
a
r
a
c
o
r
t
e
x
Paracorte
x
Medula
Paracortex
Figure 10-10.
Overview of the lymph node.
This is a representation of a
lymph node
. It is covered by a capsule consisting of a layer of connective tissue, which extends into the
substance of the node to form trabeculae. The lymph node is divided into three regions:
cortex
,
paracortex
, and
medulla
. (1) The
cortex
is composed of a row of lymphatic nodules; the majority are
secondary nodules
with
germinal centers
. Occasionally, primary
nodules (without germinal centers) may be found in the cortex region. (2) The
paracortex
lies between the cortex and medulla; most
T cells reside in this region.
HEVs
are located in paracortex and are the sites where circulating lymphocytes enter the node. (3) The
medulla
is composed of
medullary cords
and
medullary sinuses
. Lymph enters the lymph node through
afferent lymphatic vessels
;
courses through the
subcapsular
,
peritrabecular
, and
medullary sinuses
; and exits the lymph node through the
efferent lymphatic
vessel
(follow the
dotted magenta line
). The artery and vein enter and exit by passing through the hilum of the lymph node.
Lymph Nodes
Comparison of Lymph and Blood Flow
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