Secondary lymphoid organs initiate immune responses by allowing lymphocytes and antigen presenting cells to interact. These organs include lymph nodes, the spleen, tonsils, and mucosa-associated lymphoid tissue found in the digestive and respiratory tracts. They are connected by both the circulatory and lymphatic systems which transport immune cells and antigens between tissues and lymphoid organs. Within secondary lymphoid organs, T cell and B cell zones allow interactions that initiate adaptive immune responses to infections and foreign substances that enter the body.

1. Secondary lymphoid organs—where the
immune response is initiated
• Lymphocytes and myeloid cells develop to maturity
in the primary lymphoid system:
– T lymphocytes in the thymus, and
– B cells, monocytes, dendritic cells, and granulocytes in the
bone marrow.
• They encounter antigen and initiate an immune
response in the microenvironments of secondary
lymphoid organs (SLOs).

2. Lymph nodes and the spleen
• most highly organized of the secondary lymphoid organs
• compartmentalized from the rest of the body by a fibrous
capsule.
Mucosa-associated lymphoid tissue (MALT)
• some what less organized system of secondary lymphoid tissue
• found associated with the linings of multiple organ systems,
including the gastrointestinal (GI) and respiratory tracts.
• Includes tonsils, Peyer’s patches (in the small intestine), and
the appendix, as well as numerous lymphoid follicles
Secondary lymphoid organs are distributed throughout the body
and share some anatomical features

3. • Lymphoid follicles are
– present
• within the lamina propria of the intestines and
• in the mucous membranes lining the upper airways, bronchi,
and genitourinary tract.
– highly organized microenvironments that are
responsible for the development and selection of B
cells that produce high-affinity antibodies.
• Although secondary lymphoid organs vary in their
location and degree of organization, they share key
features:
– include anatomically distinct regions of T-cell and B-
cell activity, and
– all develop lymphoid follicles

4. The human lymphoid system. The
primary organs (bone marrow and
thymus) are shown in red; secondary
organs and tissues, in blue. These
structurally and functionally diverse
lymphoid organs and tissues are
interconnected by the blood vessels (not
shown) and lymphatic vessels (purple).
Most of the body’s lymphatics
eventually drain into the thoracic duct,
which empties into the left subclavian
vein.
However, the vessels draining the right
arm and right side of the head (shaded
blue) converge to form the right
lymphatic duct, which empties into the
right subclavian vein.

5. • The immune cells
-most mobile cells in a body
-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
-lined by endothelial cells that are very responsive to
inflammatory signals.
Lymphoid organs are connected to each other and to infected tissue
by two different circulatory systems: blood and lymphatics

6. • 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.

7. • network of thin walled vessels
• play a major role in
– immune cell trafficking
– the travel of antigen and antigen-presenting cells to
secondary lymphoid organs and
– the exit of lymphocytes from lymph nodes
• Lymph vessels are filled with a protein-rich fluid
(lymph)
• Lymph is derived from the fluid component of
blood (plasma) that seeps through the thin walls of
capillaries into the surrounding tissue.
The lymphatic system

8. • This fluid, called interstitial fluid, permeates all tissues
and bathes all cells.
• Much of the fluid is returned to the blood through the
walls of venules.
• If this fluid were not returned to the circulation, the
tissue would swell, causing edema that would
eventually become life threatening.
• The remainder of the interstitial fluid enters the
delicate network of primary lymphatic vessels.
• The walls of the primary vessels consist of a single layer
of loosely apposed endothelial cells.
• 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

9. the lymphatic vessels in more detail

10. the relationship between blood and lymphatic capillaries in tissue. The
lymphatic capillaries pick up interstitial fluid, a particulate and soluble
proteins, as well as immune cells from the tissue surrounding the blood
capillaries (see arrows)

11. • 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 (the right arm and right side of the
head) 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.

12. The human lymphoid system. The
primary organs (bone marrow and
thymus) are shown in red; secondary
organs and tissues, in blue. These
structurally and functionally diverse
lymphoid organs and tissues are
interconnected by the blood vessels (not
shown) and lymphatic vessels (purple).
Most of the body’s lymphatics
eventually drain into the thoracic duct,
which empties into the left subclavian
vein.
However, the vessels draining the right
arm and right side of the head (shaded
blue) converge to form the right
lymphatic duct, which empties into the
right subclavian vein.

13. • The heart does not pump the lymph through the
lymphatic system; instead, the slow, low-pressure flow
of lymph is achieved by the movements of the
surrounding muscles. 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) and
– is carried to various organized lymphoid tissues such as
lymph nodes, which trap the foreign antigen.

14. • 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.

15. • 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
• Chemokines 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.