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The lymphatic system is comprised of:

- Lymphatic vessels
- Lymphatic organs (Lymph nodes, tonsils, thymus, spleen)

Characteristic of Lymph

- Found only in closed lymphatic vessels
- Transparent, slightly yellow, watery flood, resembles blood plasma but more dilute

Function of the Lymph System

1. Drain interstitial fluid (15% total volume capillary fluid) (also collected: cell debris, bacteria, dead cells, lymphocytes)
2. Transport lipid and lipid soluble vitamins from GI tract to vascular system
3. Filtration & immunological defense

Lymphoid tissue is a ____ ________ ____________

- Specialized connective tissue
- Characterized by large # lymphocytes (T & B cells) w/ additional cells (APC)

Different types of Lymphoid Tissue

- Diffuse (exist freely, not encapsulated) -> MALT, GALT, BALT
- Dense (encapsulated) -> Lymphoid organ (nodes, tonsils, thymus, spleen)

Lymphoid Nodule

- Structural unit lymphoid tissue
- Made of clusters of lymphocytes

Diffuse Lymphoid Tissue

- Occur throughout body, under WET EPITHELIAL MEMBRANES where loose CT is INFILTRATED by LYMPHOID CELLS
- Lymphoid cells scattered randomly to make lymphoid nodules
- Transient structures

Examples of Lymph Nodes

- Parotid Lymph Nodes
- Jugular Lymph Nodes
- Pre-aortic Lymph nodes
- Axillary lymph nodes
- Para vertebral and para sternal lymph nodes
- Name based on location

What are the shape of lymph nodes?


Characteristics of Lymph

- Convex surface which receives afferent lymph vessels (several openings)
- Have hilum where blood vessels enter and efferent lymph vessels leave
- Connective tissue septa (derived from capsule), subdivide cortex into incomplete compartments
- Cortex has lymphatic nodules (primary/inactive, secondary/active)
- Paracortex zones: separates cortical area from medulla, mostly T Cells and MACROPHAGES

Picture Lymph Nodes. Can you spot the capsule, afferent vessels, medulla, cortex, paracortex, and efferent lymphatic?

- Afferent lymph vessels: several afferent openings
- Septum: extensions of capsule
- Cortex is deep to the capsule, mostly B cells

Lymph Node Light Microscope Image

Can you spot the germinal centre?

Lymphatic Nodules are made of mostly what

- B cells accompanied by macrophages

Lymphatic Nodule Activation

- Spherical dense aggregations of mostly B cells accompanied by macrophages
- Activated when introduced to antigen
- B lymphocytes PROLIFERATE in LYMPHOID NODULES -> Germinal centre (look more pink)

Difference between primary and secondary lymph nodule

Primary -> Inactive B cells, mature, not exposed to antigens, purple
Secondary -> Active B cells, immature, exposed to antigens, pink

Medullary Cords

T, B, and plasma cells in medullary region

Medullary Sinus

- Areas between medullary cords
- Part of the rich network of interconnected lymphatic sinuses

Lymph Circulation in Lymph Node

Afferent lymphatic vessel -> subcapsular sinus -> trabecular sinus -> medullary sinus -> efferent lymphatic vessels
- T & B cells added to the lymph
- Lymph is filtrated (damaged/old lymphocytes and antigens or other debris destroyed)

Lymph circulation image

- Need to know: Afferent lymph vessel, capsule, subcapsular sinus, medullary sinus, efferent lymphatic vessel, capillary bed, trabecular sinus, trabecula,

Maturation t cells in Lymph Node

- APCs activated via cytokines (contact w/ organisms)
- Activated APCs from mucosal membranes to lymph nodes
- Immature T cells made in bone marrow -> brought by blood vessels
- T cells scan peptides carried by APCs -> recognition -> activation/maturation/proliferation
- If no recognition, return to lymph flow by efferent lymph vessel


- Encapsulated lymph tissue (organ) situated @ PHARYNX
- Made of -> pharyngeal, palatine, lingual

Function Tonsils

- Produce ANTIBODIES against antigens in vicinity
- Has CRYPTS (10-20 invaginations) -> have desquamated epithelial cells (dead cells)

Can you see the Crypts, lymphoid nodules, and the capsule?

- Crypts: Collects samples of content of oral cavity
- B cells detect for antigens from lymph nodules, produce antibodies if necessary

Humoral Immunity

- Activation B cells
- Functions: Neutralization, agglutination, precipitation, activation complement proteins

The thymus arises from the

- Located below thyroid gland, above the heart

Thymus Characteristics

- Composed 2 soft pinkish gray lobes
- Surrounded by DENSE IRREGULAR CT
- Give rise to interlobular trabeculae -> divide gland into incomplete lobules
- Thymus usually gets replaced by loose CT and fat ~ 13-15

Each lobule in the thymus is composed of 2 portions

Cortex and Medulla

Thymus Cortex

- No lymphatic nodule or plasmic cells
- Composed of EPITHELIAL RETICULAR CELLS, macrophages (positive selection)

How many different types of epithelial reticular cells are there?

6. 3 in cortex, 3 in medulla

Functions ERCs

I: Surround blood vessels, separate immature T lymphocytes from antigens (no access)
II: Framework. Prevent clusters of T lymphocytes to pass information
III: Separate coffee from medulla on the cortex side
IV: Separate cortex from medulla on the medulla side
V: Act as macrophages to digest undeveloped T cells (positive selection)
VI: Dying cells, dead (form thymic (Hassall's) corpsucles))
- All ERC make different hormones (Thymosin) -> facilitate transformation of Thymocytes to T cells

Main function thymus

- Expose immature T cells to the antigens

2 Phased Elimination of Thymus

- Positive -> in cortex -> caused by macrophages (those that produce proteins that are abnormal)
- Negative -> in medulla -> caused by ERC V -> to kill autoreactive T cells
- About 98% will be killed by macrophages

Thymus Medulla

- Contain plasma cells, lymphocytes, macrophages, reticuluar cells (in negative selection)

Negative Selection

- Not 100% complete
- Some auto reactive T cells escape -> releasted into circulation
- Additional mechanisms present (regulatory T cells)
- If these fail, autoimmune diseases my arise

What is the largest lymphoid organ? Where does it arise from?

Spleen; mesoderm

What are the major lymphatic vessels connected to the spleen?

There are none.
- A capsule made of CT surrounds the organ & sends trabeculae that travel within parenchyma

The spleen is subdivided into ______ and _______ ________

Red and white pulps

What is white pulp made of?

- Lymphoid tissue organized around arteries (PALS) -> housed as T cells
- Lymphatic nodule housing B cells
- 20% of spleen

What is red pulp made of?

- Cords of cells (reticuluar cell, plasma cell, macrophage)
- Blood sinuses

Spleen Function

- White pulp (20% spleen) -> clusters lymphocytes (dense basophilic aggregates) -> site of immune surveillance & production T & B cells, antibodies
- Red pulp -> filter blood and invading micro-organisms

Marginal Zone

- Between white and red pulp
- Contains heavy concentration macrophages
- Site where blood brought to spleen is released, and functions initiated

Movement of Blood from PALS to Red Pulp

- Central artery of PALS -> Lymphoid nodule -> Branches into sinusoids -> blood goes into sinusoids and leak out and enter red pulp (open circulation)

How macrophages kill old blood cells

- Macrophages kill old blood cells (RBC live for ~120 days)
- Need open blood circulation so macrophages can phagocytose RBC
- Abnormal RBCs that are too rigid can't pass through

Splenic cords

- Path of blood that leaves open ended capillaries in marginal zone & is heading for sinuses through open ends/slits in walls of those sinuses

Can you find the sinusoid, capsule, lymph nodule, secondary sinusoids, PALS, and the central artery?


- Big spleen that can compress other organs
- Results from antigenic stimulation, obstruction blood flow, underlying functional abnormality, infiltration, viral infection
- Splenoctamy: Removing the spleen

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