180
UNIT 3
Organ Systems
(Fig. 10-4A,B). They will die if they do not meet an antigen.
Naive T cells migrate from the thymus to secondary lymphoid
organs where they encounter foreign antigens and become
active
T cells
. Once T cells are activated, they can boost the action of
cytotoxic T cells and macrophages and help to expedite pro-
liferation of B lymphocytes, which increase the production of
antibodies (see Fig. 10-5A). Activated T cells undergo cell divi-
sion to become
memory T cells
or
effector T cells
.
MEMORY T CELLS
have a much longer life than
naive
(
virgin
)
T cells
. They can survive for a long period in an inactive state
and can differentiate into effector T cells to participate in a stron-
ger and faster secondary immune response when they encounter
the same antigen for the second time. Memory T cells include
central memory T cells
and
effector memory T cells
.
Central
memory T cells
express
CCR7
(
chemokine receptor
) surface
molecules and secrete
interleukin-2
(
IL-2
) that stimulates B cells
to proliferate. They reside in secondary lymphoid organs, such
as the paracortex of the lymph nodes, and are capable of dif-
ferentiating into effector memory T cells.
Effector memory T
cells
do not express CCR7 surface molecules but secrete
IL-4
(to stimulate B cells and increase
immunoglobulin G
[
IgG
] and
IgM
). They often migrate to an infl ammatory site and develop
into effector T cells.
EFFECTOR T CELLS
include
helper T cells
,
cytotoxic T
cells
, and
regulatory (suppressor)
T cells
. (1)
Helper T cells
have the surface marker
CD4
, which restricts activation
to antigens only if it is presented by another cell in associa-
tion with major
histocompatibility complex
(
MHC
)
class II
.
Helper T cells include
Th0
,
Th1
, and
Th2 cells
.
Th0 cells
can
differentiate into Th1 and Th2 cells;
Th1 cells
secrete
IL-2
,
interferon-
g
, and
tumor necrosis factor
and down regulate Th2
cells’ response;
Th2
cells secrete
IL-4
,
IL-5
,
IL-6
, and
IL-10
,
which help promote antibody production, stimulate prolifera-
tion of eosinophil and mast cells, and down regulate Th1 cells’
response. Helper T cells do not directly kill infected cells or
pathogens but function indirectly to promote and activate other
immune cells. (2)
Cytotoxic
(
CD8
)
T cells
have the
CD8
surface
marker, which restricts activation to antigen only if it is pre-
sented by another cell in association with
MHC class I
. They
kill target cells, such as virus-infected cells, tumor cells, and
transplanted cells (grafts). (3)
Regulatory T cells
are also called
suppressor T cells
. They
suppress
the humoral and
cellular
immune responses and are involved with immunological
tolerance.
Null Cells
Null cells
resemble lymphocytes but do not have surface
markers,
which B and T cells have. They include
pluripotential hemopoi-
etic stem cells
(
PHSCs
) and
natural killer
(
NK
)
cells
.
PHSCs
function as stem cells and can give rise to various types of blood
cells.
NK cells
do not require exposure to antigens to become
activated. They function similarly to cytotoxic T cells but do
not have the surface markers CD8 or CD4. They kill invading
target cells, such as virus-infected cells and tumor cells.
Plasma Cells
Plasma cells
differentiate from B cells. These activated large cells
have clock-face nuclei, abundant rough endoplasmic reticulum,
and a Golgi apparatus in the cytoplasm (see Figs. 4-2 and 4-3).
They actively produce antibodies known as
immunoglobulins
(
Igs
), which are speci±
c for each type of antigen (Fig. 10-3).
Antigen-Presenting Cells
These cells
present antigens
to lymphocytes. Most of them are
MHC-II class, which have surface membrane molecules MHC-II
(histocompatibility complex). These cells present antigen to
T cells (Fig.10-4B). Antigen-presenting cells include
mac-
rophages
,
dendritic cells
,
Langerhans cells
, and
B cells
. In gen-
eral, B cells are both antigen-presenting and antigen-receiving
cells. They present antigens to T cells and also receive antigens
by either binding antigen to their receptors or through antigen-
presenting cells (
follicle dendritic cells
). Lymphocytes are acti-
vated after receiving an antigen.
Lymphatic Tissues and
Lymphoid Organs
Mucosa-Associated Lymphatic Tissues
Diffuse lymphatic tissues or nodules are often located in the con-
nective tissue, which support the wet epithelial membranes of
the body mucosae. The lymphatic tissues found in the mucosa
of the digestive, respiratory, and genitourinary tracts are called
mucosa-associated lymphatic tissues
(
MALT
). They can be
subdivided into
gut-associated lymphatic tissue
(
GALT
) and
bronchus-associated lymphatic tissue
(
BALT
), according to their
locations.
GALT
is found in the digestive tract, such as
Peyer
patches
in the ileum and lymphatic nodules in the appendix and
large intestine.
BALT
is found in the respiratory tracts, mostly in
bronchi and bronchioles (see Chapter 11, “Respiratory System”).
Tonsils are covered by epithelium and have an incomplete cap-
sule. Most tonsils contain lymphatic nodules but some of them
have diffuse lymphatic tissues. Tonsils are located in the oral
cavity and posterior roof of the nasopharynx. Tonsils include
lingual tonsils
,
palatine tonsils
, and a
pharyngeal tonsil
; they are
classi± ed as MALT (Fig. 10-8A,B and Table 10-1). MALT traps
bacteria and viruses, defends against infection, and provides sites
where lymphocytes meet antigens.
Lymphatic nodules
occur in
most of the secondary lymphoid organs (MALT, lymph nodes,
and spleen). Lymphatic nodules with a germinal center are called
secondary nodules
. The germinal center is evidence of prolifera-
tion of lymphocytes after they encounter antigen and become
activated. Lymphatic nodules contain various stages of B cells
and most are
lymphoblasts
(enlarged and proliferated lympho-
cytes). The
mantle zone
(peripheral to the germinal center) of the
lymphatic nodule contains tightly packed small lymphocytes.
The outside of the nodules is usually surrounded by T cells. A
lymphatic nodule without a germinal center is called a
primary
nodule
, and it contains mostly inactivated (small) B cells.
Lymph Nodes
Lymph nodes
are bean-shaped organs that are covered by a layer
of connective tissue (
capsule
). They are distributed throughout
the body. The regions that are associated with rich clusters of
lymph nodes include the neck (cervical nodes and pericranial
ring), axilla (axillary nodes), thorax (tracheal nodes), abdo-
men (deep nodes), groin (inguinal nodes), and femoral (fem-
oral nodes) regions. They play important roles in circulating
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