Secondary lymphoid organs include the lymph nodes, spleen, and mucosal associated lymphoid tissues. These organs trap antigens and provide sites for lymphocytes to interact with antigens. The lymph nodes are highly organized and compartmentalized, containing B cell follicles and T cell zones. They filter lymph and antigens from tissues and initiate adaptive immune responses. The spleen filters blood and mounts responses against blood-borne pathogens, containing red pulp, white pulp, and marginal zones that trap antigens and present them to lymphocytes. Secondary lymphoid organs are critical for activating antigen-specific adaptive immune responses.

1. SECONDARY LYMPHOID ORGANS
Dr Sitaram Swain

2. Organs of the Immune System
A number of morphologically and functionally diverse organs and tissues have various
functions in the development of immune responses. These can be distinguished by
function as the primary and secondary lymphoid organs. The thymus and bone marrow
are the primary (or central) lymphoid organs, where maturation of lymphocytes takes
place.
The lymph nodes, spleen, and various mucosal associated lymphoid tissues (MALT)
such as gut-associated lymphoid tissue (GALT) are the secondary (or peripheral)
lymphoid organs, which trap antigen and provide sites for mature lymphocytes to interact
with that antigen.
In addition, tertiary lymphoid tissues, which normally contain fewer lymphoid cells than
secondary lymphoid organs, can import lymphoid cells during an inflammatory response.
Most prominent of these are cutaneous-associated lymphoid tissues.
Once mature lymphocytes have been generated in the primary lymphoid organs, they
circulate in the blood and lymphatic system, a network of vessels that collect fluid that
has escaped into the tissues from capillaries of the circulatory system and ultimately
return it to the blood.

3. Secondary Lymphoid Organs
Lymphocytes and myeloid cells develop to maturity in the
primary lymphoid system and they encounter antigen and
initiate an immune response in the microenvironments of
secondary lymphoid organs (SLOs).
Secondary Lymphoid Organs are distributed throughout the
body and share some anatomical features. Lymph nodes and
the spleen are the most highly organized of the secondary
lymphoid organs and are compartmentalized from the rest of
the body by a fibrous capsule.
A some-what less organized system of secondary lymphoid
tissue, collectively referred to as mucosa-associated
lymphoid tissue (MALT), is found associated with the linings of
multiple organ systems, including the gastrointestinal (GI) and
respiratory tracts.

4. MALT includes tonsils, Peyer’s patches (in the small intestine), and the appendix,
as well as numerous lymphoid follicles within the lamina propria of the intestines
and in the mucous membranes lining the upper airways, bronchi, and
genitourinary tract.
All SLOs include anatomically distinct regions of T-cell and B-cell activity, and all
develop lymphoid follicles, which are highly organized microenvironments that
are responsible for the development and selection of B cells that produce high-
affinity antibodies.

5. The immune cells are the most mobile cells in a body and use two different systems
to traffic through tissues: the blood system and the lymphatic system. The blood
has access to virtually every organ and tissue and is lined by endothelial cells that
are very responsive to inflammatory signals.
Hematopoietic cells can transit through the blood system—away from the heart via
active pumping networks (arteries) and back to the heart via passive valve-based
systems (veins) within minutes.
Most lymphocytes enter secondary lymphoid organs via specialized blood vessels,
and leave via the lymphatic system. The lymphatic system is a network of thin
walled vessels that play a major role in immune cell trafficking, including travel of
antigen and antigen-presenting cells to secondary lymphoid organs and the exit of
lymphocytes from lymph nodes.

6. Lymph vessels are filled with a protein-rich fluid (lymph) derived from the fluid component of
blood (plasma) that seeps through the thin walls of capillaries into the surrounding tissue. In
an adult, depending on size and activity, seepage can add up to 2.9 liters or more during a 24-
hour period.
This fluid, called interstitial fluid, permeates all tissues and bathes all cells. If this fluid were
not returned to the circulation, the tissue would swell, causing edema that would eventually
become life threatening. We are not afflicted with such catastrophic edema because much of
the fluid is returned to the blood through the walls of venules.
The remainder of the interstitial fluid enters the delicate network of primary lymphatic
vessels. The porous architecture of the primary vessels allows fluids and even cells to enter
the lymphatic network. Within these vessels, the fluid, now called lymph, flows into a series of
progressively larger collecting vessels called lymphatic vessels.

7. All cells and fluid circulating in the lymph are ultimately returned to the blood
system. The largest lymphatic vessel, the thoracic duct, empties into the left
subclavian vein.
It collects lymph from all of the body except the right arm and right side of the head.
Lymph from these areas is collected into the right lymphatic duct, which drains into
the right subclavian vein . By returning fluid lost from the blood, the lymphatic
system ensures steady-state levels of fluid within the circulatory system.
Therefore, activity enhances lymph circulation. Importantly, a series of one-way
valves along the lymphatic vessels ensures that lymph flows in only one direction.
When a foreign antigen gains entrance to the tissues, it is picked up by the
lymphatic system (which drains all the tissues of the body) .

9. Antigen-presenting cells that engulf and process the antigen also can gain access to lymph. In fact,
as lymph passes from the tissues to lymphatic vessels, it becomes progressively enriched in
specific leukocytes, including lymphocytes, dendritic cells, and macrophages.
Thus, the lymphatic system also serves as a means of transporting white blood cells and antigen
from the connective tissues to organized lymphoid tissues, where the lymphocytes can interact with
the trapped antigen and undergo activation. Most secondary lymphoid tissues are situated along the
vessels of the lymphatic system.
The spleen is an exception and is served only by blood vessels. All immune cells that traffic through
tissues, blood, and lymph nodes are guided by small molecules known as chemokines.
These proteins are secreted by stromal cells, antigen presenting cells, lymphocytes, and
granulocytes, and form gradients that act as attractants and guides for other immune cells, which
express an equally diverse set of receptors for these chemokines. The interaction between specific
chemokines and cells expressing specific chemokine receptors allows for a highly refined
organization of immune cell movements.

11. Lymph nodes
Lymph nodes are the most specialized SLOs. Unlike the spleen, which also regulates red blood
cell flow and fate, lymph nodes are fully committed to regulating an immune response.
They are encapsulated, bean-shaped structures that include networks of stromal cells packed
with lymphocytes, macrophages, and dendritic cells. Connected to both blood vessels and
lymphatic vessels, lymph nodes are the first organized lymphoid structure to encounter
antigens that enter the tissue spaces.
The lymph node provides ideal microenvironments for encounters between antigen and
lymphocytes and productive, organized cellular and humoral immune responses. Structurally,
a lymph node can be divided into three roughly concentric regions: the cortex, the paracortex,
and the medulla, each of which supports a distinct microenvironment.

13. The outermost layer, the cortex, contains lymphocytes (mostly B cells), macrophages, and
follicular dendritic cells arranged in follicles. Beneath the cortex is the paracortex, which is
populated largely by T lymphocytes and also contains dendritic cells that migrated from
tissues to the node. The medulla is the innermost layer, and the site where lymphocytes exit
(egress) the lymph node through the outgoing (efferent) lymphatics.
Antigen travels from infected tissue to the cortex of the lymph node via the incoming (afferent)
lymphatic vessels, which pierce the capsule of a lymph node at numerous sites and empty
lymph into the subcapsular sinus.
It enters either in particulate form or is processed and presented as peptides on the surface of
migrating antigen presenting cells.

15. Particulate antigen can be trapped by resident antigen-presenting cells in the subcapsular
sinus or cortex, and it can be passed to other antigen-presenting cells, including B
lymphocytes.
Alternatively, particulate antigen can be processed and presented as peptide-MHC
complexes on cell surfaces of resident dendritic cells that are already in the T-cell-rich
paracortex.
T Cells in the Lymph Node It takes every naïve T lymphocyte about 16 to 24 hours to browse
all the MHC-peptide combinations presented by the antigen-presenting cells in a single
lymph node. Naïve lymphocytes enter the cortex of the lymph node by passing between the
specialized endothelial cells of high endothelial venules (HEV), so-called because they are
lined with unusually tall endothelial cells that give them a thickened appearance.

16. Once naïve T cells enter the lymph node, they browse MHC-peptide
antigen complexes on the surfaces of the dendritic cells present in the
paracortex.
The paracortex is traversed by a web of processes that arise from
stromal cells called fibroblast reticular cells (FRCs). This is referred to
as the fibroblast reticular cell conduit system (FRCC) and guides T-cell
movements via associated adhesion molecules and chemokines.
Antigen-presenting cells also appear to wrap themselves around the
conduits, giving circulating T cells ample opportunity to browse their
surfaces as they are guided down the network. The presence of this
specialized network elegantly enhances the probability that T cells will
meet their specific MHC-peptide combination.

17. T cells that browse the lymph node but do not exit via the blood, but
via the efferent lymphatics in the medulla of the lymph node.
T cells whose TCRs do bind to an MHC-peptide complex on an
antigen-presenting cell that they encounter in the lymph node will
stop migrating and take up residence in the node for several days.
Here it will proliferate and, depending on cues from the antigen-
presenting cell itself, its progeny will differentiate into effector cells
with a variety of functions.
CD8 T cells gain the ability to kill target cells. CD4 T cells can
differentiate into several different kinds of effector cells, those that
can further activate macrophages, CD8 T cells, and B cells.

18. B Cells in the Lymph Node
The lymph node is also the site where B cells are activated and
differentiate into high-affinity antibody-secreting plasma cells.
B cell activation requires both antigen engagement by the B-cell receptor
(BCR) and direct contact with an activated CD4 TH cell.
Like T cells, B cells circulate through the blood and lymph and visit the
lymph nodes on a daily basis, entering via the HEV.
They respond to specific signals and chemokines that draw them not to the
paracortex but to the lymph node follicle.
Although they may initially take advantage of the FRCC for guidance, they
ultimately depend upon follicular dendritic cells (FDCs) for guidance .
FDCs are centrally important in maintaining follicular and germinal center
structure and “presenting” antigen to differentiating B cells.
B cells differ from T cells in that their receptors can recognize free
antigen. A B cell will typically meet its antigen in the follicle.

19. If its BCR binds to antigen, the B cell becomes partially activated
and engulfs and processes that antigen. Also B cells are
specialized antigen-presenting cells that present processed
peptide-MHC complexes on their surface to CD4 TH cells.
B cells that have successfully engaged and processed antigen
change their migration patterns and move to the T-cell-rich
paracortex, where they increase their chances of encountering an
activated CD4 TH cell that will recognize the MHC-antigen complex
they present. When they successfully engage this TH cell,
becoming fully activated and receiving signals that induce B cell
proliferation .
Some activated B cells differentiate directly into an antibody-
producing cell (plasma cell) but others re-enter the follicle to
establish a germinal center. A follicle that develops a germinal
center is sometimes referred to as a secondary follicle; a follicle
without a germinal center is sometimes referred to as a primary
follicle.

20. Germinal centers are remarkable substructures that facilitate
the generation of B cells with increased receptor affinities. In
the germinal center, an antigen-specific B cell clone will
proliferate and undergo somatic hypermutation of the genes
coding for their antigen receptors.
Those receptors that retain the ability to bind antigen with the
highest affinity survive and differentiate into plasma cells that
travel to the medulla of the lymph node.
The initial activation of B cells and establishment of germinal
center take place within 4 to 7 days of the initial infection, but
germinal centers remain active for 3 weeks or more.
Lymph nodes swell visibly and sometimes painfully,
particularly during those first few days after infection. This
swelling is due both to an increase in number of lymphocytes
induced to migrate into node as well as proliferation of
antigen-specific T and B lymphocytes within the lobe.

21. The Generation of Memory T and B Cells in the Lymph
Node The interactions between TH cells and APCs, and
between activated TH cells and activated B cells,
results not only in the proliferation of antigen-specific
Lymphocytes and their functional differentiation, but
also in the generation of memory T and B cells.
Memory T and B cells can take up residence in
secondary lymphoid tissues or can exit the lymph node
and travel to and among tissues that first encountered
the pathogen.
Memory T cells that reside in secondary lymphoid
organs are referred to as central memory cells and
are distinct in phenotype and functional potential from
effector memory T cells that circulate among tissues.
Memory cell phenotype, locale, and activation
requirements are an active area of investigation .

24. The spleen is a fist-sized organ just behind the stomach that
collects antigen from the blood.
It also collects and disposes of senescent red blood cells.
The bulk of the spleen is composed of red pulp, which is the site
of red blood cell disposal.
The lymphocytes surround the arterioles entering the organ,
forming areas of white pulp, the inner region of which is divided
into a periarteriolar lymphoid sheath (PALS), containing mainly T
cells, and a flanking B-cell corona.

26. The Spleen Organizes the Immune Response Against Blood-Borne Pathogens The spleen,
situated high in the left side of the abdominal cavity, is a large, ovoid secondary
lymphoid organ that plays a major role in mounting immune responses to antigens in the
bloodstream. Whereas lymph nodes are specialized for encounters between
lymphocytes and antigen drained from local tissues, the spleen specializes in filtering
blood and trapping blood-borne antigens; thus, it is particularly important in the
response to systemic infections. Unlike the lymph nodes, the spleen is not supplied by
lymphatic vessels. Instead, blood-borne antigens and lymphocytes are carried into the
spleen through the splenic artery and out via the splenic vein.
Spleen is surrounded by a capsule from which a number of projections (trabeculae)
extend, providing structural support. Two main microenvironmental compartments can
be distinguished in splenic tissue: the red pulp and white pulp, which are separated by a
specialized region called the marginal zone .
The splenic red pulp consists of a network of sinusoids populated by red blood cells,
macrophages, and some lymphocytes. It is the site where old and defective red blood
cells are destroyed and removed; many of the macrophages within the red pulp contain
engulfed red blood cells or iron-containing pigments from degraded hemoglobin.
It is also the site where pathogens first gain access to the lymphoid-rich regions of the
spleen, known as the white pulp. The splenic white pulp surrounds the branches of the
splenic artery, and consists of the periarteriolar lymphoid sheath (PALS) populated by T
lymphocytes as well as B-cell follicles. As in lymph nodes, germinal centers are generated
within these follicles during an immune response.
The marginal zone, which borders the white pulp, is populated by unique and specialized
macrophages and B cells, which are the first line of defense against certain blood-borne
pathogens. Blood-borne antigens and lymphocytes enter the spleen through the splenic
artery, and interact first with cells at the marginal zone.

27. In the marginal zone, antigen is trapped and processed by
dendritic cells, which travel to the PALS. Resident marginal
zone B cells also bind antigen via complement receptors
and convey it to the follicles.
Migrating B and T lymphocytes in the blood enter sinuses in
the marginal zone and migrate to the follicles and the
PALS, respectively. The events that initiate the adaptive
immune response in the spleen are analogous to those that
occur in the lymph node.
Circulating naïve B cells encounter antigen in the follicles,
and circulating naïve CD8 and CD4 T cells meet antigen as
MHC-peptide complexes on the surface of dendritic cells in
the T-cell zone (PALS). Once activated, CD4 TH cells then
provide help to B cells and CD8 T cells that have also
encountered antigen. Some activated B cells, together with
some TH cells, migrate back into follicles and generate
germinal centers.

28. However, given that T cells, dendritic cells, and B cells find a way to interact
efficiently within the spleen to initiate an immune response.
Although animals can lead a relatively healthy life without a spleen, its loss does
have consequences. In children, in particular, splenectomy (the surgical removal of
a spleen) can lead to overwhelming post-splenectomy infection (OPSI) syndrome
characterized by systemic bacterial infections (sepsis) caused by primarily
Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae.
Although fewer adverse effects are experienced by adults, splenectomy can still
lead to an increased vulnerability to blood-borne bacterial infections, underscoring
the role the spleen plays in our immune response to pathogens that enter the
circulation.
It is also important to recognize that the spleen has other functions (e.g., in iron
metabolism, thrombocyte storage, hematopoiesis) that will also be compromised if it
is removed.

29. Lymph nodes and the spleen are not the only organs that
develop secondary lymphoid microenvironments. T and B-
cell zones and lymphoid follicles are also found in mucosal
membranes that line the digestive, respiratory, and
urogenital systems, as well as in the skin.
Mucosal membranes have a combined surface area of about
400 m2 (nearly the size of a basketball court) and are the
major sites of entry for most pathogens.
These vulnerable membrane surfaces are defended by a
group of organized lymphoid tissues known collectively as
mucosa-associated lymphoid tissue (MALT).
Lymphoid tissue associated with different mucosal areas
like, the respiratory epithelium is referred to as bronchus-
associated lymphoid tissue (BALT) or nasal-associated
lymphoid tissue (NALT), & that associated with the intestinal
epithelium is gut-associated lymphoid tissue (GALT).

31. The structure of GALT is well described and ranges from loose, barely organized clusters of
lymphoid cells in the lamina propria of intestinal villi to well-organized structures such as the
tonsils and adenoids (Waldeyer’s tonsil ring), the appendix, and Peyer’s patches, which are
found within the intestinal lining and contain well-defined follicles and T-cell zones.
Outer mucosal epithelial layer contains intraepithelial lymphocytes (IELs), which are mostly T cells.
lamina propria, which lies under the epithelial layer, contains large numbers of B cells, plasma cells,
activated T cells, and macrophages in loose clusters. Microscopy has revealed more than 15,000
lymphoid follicles within the intestinal lamina propria of a healthy child.
Peyer’s patches, nodules of 30 to 40 lymphoid follicles, extend into the muscle layers that are just
below the lamina propria. Like lymphoid follicles in other sites, those that compose Peyer’s patches
can develop into secondary follicles with germinal centers. The overall functional importance of
MALT in the body’s defense is underscored by its large population of antibody-producing plasma
cells, whose number exceeds that of plasma cells in the spleen, lymph nodes, and bone marrow
combined. The vesicles then fuse with the pocket membrane, delivering antigens to clusters of
lymphocytes and antigen-presenting cells, the most important of which are dendritic cells, contained
within the pocket.

33. Some cellular structures and activities are unique to MALT. Epithelial
cells of mucous membranes play an important role in delivering
small samples of foreign antigen from the respiratory, digestive, and
urogenital tracts to the underlying mucosa-associated lymphoid
tissue. In the digestive tract, specialized M cells transport antigen
across the epithelium. The structure of M cells is striking: they are
flattened epithelial cells lacking the microvilli that characterize the
rest of mucosal epithelium.

36. They have a deep invagination, or pocket, in the basolateral plasma
membrane, which is filled with a cluster of B cells, T cells, and
macrophages. Antigens in the intestinal lumen are endocytosed into
vesicles that are transported from the luminal membrane to the
underlying pocket membrane.
Antigen transported across the mucous membrane by M cells
ultimately leads to the activation of B cells that differentiate and then
secrete IgA. This class of antibody is concentrated in secretions (e.g.,
milk) and is an important tool used by the body to combat many types
of infection at mucosal sites.

37. The skin is the largest organ in the body and a critical anatomic barrier against pathogens.
It also plays an important role in nonspecific (innate) defenses. The epidermal (outer) layer
of the skin is composed largely of epithelial cells called keratinocytes. These cells secrete
a number of cytokines that may function to induce a local inflammatory reaction.
Langerhans cells, skin-resident dendritic cells that internalize antigen by phagocytosis or
endocytosis. These Langerhans cells undergo maturation and migrate from the epidermis
to regional lymph nodes, where they function as potent activators of naïve T cells. In
addition to Langerhans cells, the epidermis also contains intraepidermal
lymphocytes, which are predominantly T cells; that they play a role in combating
infections that enter through the skin.The underlying dermal layer of skin also contains
scattered lymphocytes, dendritic cells, monocytes, macrophages, and may include
hematopoietic stem cells.

39. Most skin lymphocytes appear to be either previously
activated cells or memory cells, many of which traffic to and
from local, draining lymph nodes that coordinate the
responses to pathogens that have breached the skin barrier.
Tertiary Lymphoid Tissues Also Organize and Maintain an
Immune Response Tissues that are the sites of infection are
referred to as tertiary lymphoid tissue. Lymphocytes
activated by antigen in secondary lymphoid tissue can return
to these organs (e.g., lung, liver, brain) as effector cells and
can also reside there as memory cells.
It also appears as if tertiary lymphoid tissues can generate
defined microenvironments that organize the returning
lymphoid cells. Investigators have recently found that the
brain, for instance, establishes reticular systems that guide
lymphocytes responding to chronic infection with the
protozoan that causes toxoplasmosis.

40. The gut-associated lymphoid tissues (GALT), which include the tonsils,
adenoids, and appendix, and specialized structures called Peyer's patches in
the small intestine, collect antigen from the epithelial surfaces of the
gastrointestinal tract.
In Peyer's patches, which are the most important and highly organized of
these tissues, the antigen is collected by specialized epithelial cells called
multi-fenestrated or M cells. The lymphocytes form a follicle consisting of a
large central dome of B lymphocytes surrounded by smaller numbers of T
lymphocytes .

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