The Immune System: Secondary lymphoid tissues Lecture 21

Secondary lymphoid tissues (including lymphatic vessels):

Site of immune system activation and defense

Remember that all secondary lymphoid tissues are enriched in

Lymphocytes, and sometimes those lymphocytes aggregate to form lymphoid nodules

MALT

Secondary lymphoid tissue located within the mucosa of the respiratory, digestive, or urogenital systems

Permanent places of transient MALT

• Peyer’s patches in the small intestine

• The appendix (large intestine)

• The tonsils (nasopharynx)

Lymph nodes

Encapsulated immune organs distributed along lymphatic vessels

The lymphatic system is

A system of vessels that circulates interstitial fluids throughout the body

The lymphatic system is mostly made up of

Water, but also a transport system for cellular debris, signal molecules, antigens, pathogens, and immune cells

Lymph nodes are

Encapsulated secondary lymphoid organs distributed regularly along lymphatic vessels; they harbor active populations of leukocytes, and filter the lymphatic fluid

• 400-450 total

Lymphatic vessels

Vessels composed of single layer of simple squamous endothelial cells with incomplete basal lamina and little additional supportive tissue; no distinct tunica structure

• Numerous valves

Lymph nodes have important sites of lymphocytes activation

Pathogens, antigens, APCs, and cytokines are drained from solid tissues of the body and travel in interstitial fluid to nearest lymph node

• There, lymphocytes will become activated and mount an adaptive immune response

Lymph node structure

• Capsule (CT), connected to afferent lymphatic vessels

• Outer cortex – lymph fluid entry and filtration, macrophages, and

lymphoid nodules full of lymphocytes

• Medulla – re-collection and final filtration of lymph fluid, location of differentiated plasma cells

• Filtered lymph fluid exits through efferent lymphatic vessel

Incoming lymph fluid contains

APCs, antigens, pathogens, and cytokines from tissues of the body

1. In the cortex macrophages can ______ pathogens and debris

2. Lymphocytes in lymphoid nodules an be activated by ________

1. Phagocytose

2. Antigen presentation

In the medulla lymph fluid recollects

• Plasma cells can deposit antibodies into that fluid so that they can exit the lymph node and circulate through the body

The cortex of the lymph node contains

Lymphoid nodules

How does the medulla of the lymph node differ from the lymphatic fluid

• The medulla is less densely populated, with alternating solid cords of tissue and open sinuses

• Lymphatic fluid recollects before existing the lymph node

Capsule of the lymph node contains what kind of tissue

Dense irregular CT (little but of adipose tissue as well)

Cortex of the lymph node contains

• Subcapsular sinuses: incoming lymph accumulates here prior to filtration

• Lymphoid nodules

Lymph node medulla contains

• Medullary cords: Solid reticular tissue, including lymphocytes, macrophages, and plasma cells

• Sinuses: open cavities where lymph fluid recollects before exiting the lymph node

Spleen

• Only lymphoid organ that specifically filters blood

• Additional non- immune function: destruction and recycling of old RBCs

Spleen structure

• CT capsule and invading trabeculae

• Interior contains intermingled “white pulp” and “red pulp

Red and white pulp of spleen

• White pulp: lymphoid tissue – more basophilic (lymphocytes)

• Red pulp: RBC recycling; more abundant; less basophilic

Note: named after appearance in fresh tissue, not in fixed, stained slides!

White pulp and red pulp staining of within spleen

White pulp is more purple than red pulp in H&E because it has greater enrichment of basophilic lymphocytes, whereas red pulp has more acidophilic RBCs

Spleen structure and function steps

1. Includes unique open-ended arteries (often sheathed by lymphoid tissue aka white pulp) that empty blood into solid tissue of the spleen (the red pulp)

2. Blood filters through red pulp, which also contains macrophages to recycle old or Damaged RBCs

3. Healthy blood cells re-aggregate inside porous structures called “splenic sinuses”, which lead to veins that carry filtered blood back into body circulation

White pulp of the spleen: structure and function

All aggregations of lymphocytes in the spleen are classified as white pulp

• Most white pulp is seen forming a sheath around splenic arterioles – “peri-arterial lymphatic sheath” or “PALS”

• Important center of immune response to blood-borne pathogens: macrophages phagocytose blood-borne pathogens, and lymphocytes can be activated by blood-borne antigens, APCs, and/or cytokines

Red pulp of the spleen: structure and function

• Red pulp contains cords of tissue (cords of Billroth) that are rich in macrophages and RBCs

• Blood exits open-ended arteries and filters through red pulp, allowing macrophages to phagocytose old or damaged RBCs

• Healthy RBCs need to re-enter circulation and leave the spleen; they do so by entering structures called sinusoids

Red pulp: Healthy RBCs need to re-enter circulation and leave the spleen; they do so by entering structures called sinusoids: What are the these sinusoids?

• Sinusoidal capillaries lined with stave cells, which create narrow openings in the endothelial wall

• Only healthy RBCs are flexible enough to squeeze through openings and enter sinusoid; old or damaged RBCs are thus trapped in the red pulp

• Sinusoids are connected to efferent veins that carry healthy RBCs back into circulation

Red pulp contains _____ engulfing damaged RBC

Macrophages