CHAPTER 10
Lymphoid System
179
Introduction and Key Concepts
for the Lymphoid System
The
lymphoid system
is composed of
lymphocytes
,
lymphoid
organs
, and
lymphatic vessels
. The structure of lymphatic ves-
sels is discussed in Chapter 9, “Circulatory System.” Lymphoid
organs include the
bone marrow
(see Chapter 8, “Blood and
Hemopoiesis”),
lymph nodes
,
thymus
,
spleen
, and
mucosa-
associated lymphatic tissue
(
MALT
), such as tonsils and Peyer
patches. Most lymphoid organs contain lymphatic nodules or
diffuse lymphatic tissues and play an important role in provid-
ing sites for lymphocytes to come into contact with antigens;
promote proliferation and maturation of lymphocytes; and
promote B lymphocytes to become plasma cells, which pro-
duce antibodies. Lymphoid organs can be divided into two
groups: (1)
Primary lymphoid organs
, also called
central lym-
phoid organs
, are the sites where lymphocytes differentiate and
develop the ability to recognize foreign antigens and distinguish
nonself from self. Primary lymphoid organs include the
bone
marrow
for B lymphocytes and the
thymus
for T lymphocytes.
(2)
Secondary lymphoid organs
, also called
peripheral lym-
phoid organs
, are where mature lymphocytes (both B and T
cells) encounter foreign antigens and the immune response
takes place. Secondary lymphoid organs include
MALT
,
lymph
nodes
, and the
spleen
.
Cells in the Lymphoid System
Lymphocytes
can be classiF
ed into three major types based
on their immunologic functions:
B lymphocytes
(
B cells
),
T lymphocytes
(
T cells
), and
null cells
. B cells and T cells are the
two main cell types found in lymphoid organs. Lymphocytes
originate in the bone marrow and develop and mature in pri-
mary lymphoid organs. Exposure to foreign antigens initiates the
immune response in secondary lymphoid organs. It is impossible
to distinguish between the T and B cells without using immuno-
histochemical stains. However, they have a tendency to reside
in certain regions of the lymphoid organs. ±or example, most
B cells reside in lymphatic nodules of the secondary lymphoid
organs, whereas T cells reside in the thymus, paracortex of the
lymph nodes, and periarterial lymphatic sheath (PALS) of the
spleen.
B cells
participate in the
humoral immune response
,
and
T cells
are involved with the
cell- mediated immune responses
.
Other cells in the lymphoid organs include
plasma cells
and
antigen-presenting cells
.
B Lymphocytes
B lymphocytes
originate from precursor cells in the
bone
marrow
and become
naive
(
virgin
)
B cells
in the bone mar-
row. These B cells develop their
surface antibody
(
Ig
), which
enables them to recognize
nonself antigens
. If B cells rec-
ognize self-antigens
during the maturation process, these
B cells will undergo
apoptosis
(negative selection [±ig. 10-3]).
Naive B cells migrate from the bone marrow to the
second-
ary lymphoid organs
through the blood circulation. If naive
B cells do not meet a speciF
c foreign antigen, they will die in a
short time. If they encounter such an antigen, recognizing and
binding to the antigen will allow them to survive and become
active B cells
. Activated B cells undergo cell division and dif-
ferentiate into
plasma cells
and
memory B cells
. Memory
B cells have a long life and can live for decades in circulating
blood in an inactive state. They can differentiate into plasma
cells, which produce antibodies to participate in the humoral
immune response.
T Lymphocytes
T lymphocytes
also originate from precursor cells in the bone
marrow, but they do not mature in the bone marrow.
Pro–T
lymphocytes
enter the blood circulation and travel to their
primary lymphoid organ
(
thymus
) to F
nish their maturation
(±ig. 10-4A). They develop into
thymocytes
in the cortex of the
thymus and undergo a differentiation process to become
naive
(
virgin
)
T cells
. Naive T cells have surface markers on their
cytoplasmic membrane and have a short life as do naive B cells
Figure 10-11A–D
Lymph Nodes
Figure 10-12A,B
High Endothelial Venules (HEVs), Paracortex of a Lymph Node
Figure 10-12C
Clinical Correlation: Hodgkin Lymphoma
Thymus
Figure 10-13A
Thymus
Figure 10-13B
Thymus, Cortex
Figure 10-13C
Thymus, Medulla
Spleen
Figure 10-14A
Spleen
Figure 10-14B
White Pulp, Spleen
Figure 10-14C
Red Pulp, Spleen
Figure 10-15A
Splenic Circulation
Figure 10-15B
Periarterial Lymphatic Sheath, Spleen
Figure 10-16
Red Pulp of the Spleen
Table 10-2
Lymphoid Organs
Synopsis 10-3
Pathological and Clinical Terms for the Lymphoid System
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