Lesson # 9The Lymphoid System-2

Chapter 22

Objectives:

1- To list the major lymph organs and the functions of each.

The Lymphatic System

- Lymphatic capillaries

- Lymphatic collecting vessels

- Lymphatic trunks

Aggregates of lymphocytes in the connective tissue of mucous membrane and various organs.Mucosa-associated lymphatic tissue (MALT)

1- Peyer patches in the distal portion of the small intestine.

They have well –defined anatomical sites and at least partial connective tissue capsules.

2- Thymus

1- Lymph nodes

3- Spleen

It is the recovered fluid. Lymph is usually a clear colorless fluid, similar to blood plasma but low in protein.

Lymphatic Vessels

Diffuse Lymphoid

Tissues

Encapsulated Lymphoid

OrgansLymph

- Collecting duct

3- Tonsils (palatine tonsils and pharyngeal tonsils or adenoids).

2- Vermiform appendix

Lymphoid Tissues Lymphoid tissues are areolar connective tissues with concentrations of lymphocytes.

The lymphatic nodules or follicles have no fibrous capsule surrounding them.

When lymphocytes are densely packed they form lymphoid or lymphatic nodules or follicles.

Germinal centerIt contains dividing lymphocytes.

Lymphatic nodule or

follicle

It is the collection of lymphoid tissues that protect the epithelia of the respiratory, digestive, urinary, and reproductive systems.

Clusters of lymphoid nodules deep to the epithelial lining of the intestine are known as Peyer’s patches.

MALT(Mucosa-Associated Lymphatic Tissue)

The appendix vermiform is other example of MALT. It walls contain a mass of fused lymphoid nodules.

Tonsils

Functions

They are large concentration of lymphoid nodules in the walls of the pharynx.

Pharyngeal tonsil or adenoid (single)

Palatine tonsils (two)

Lingual tonsils (two)

They guard against ingested or inhaled pathogens.

Lymphatic organs have well-defined anatomical sites and have connective tissue capsule that separates the lymphatic tissue from neighboring tissues.

Lymphoid Organs

- Thymus- Lymph nodes

- Spleen

Lymph nodesThey are the smallest and most numerous lymphatic organs (about 450 in typical young adult).

Thymus

Spleen

Lymph nodes

They range in diameter from 1 mm to 25 mm (about 1 inch)

Cervical lymph nodes

Axillary lymph nodes

Lumbar lymph nodes

Pelvic lymph nodes

Inguinal lymph nodes

Lymph nodes are wide-spread but specially concentrated in some locations.

Lymph nodes

Lymphvessel

Lymphnodes

Hilum

Medullary sinus

Outer cortex (B cells)

Afferentvessel

Medullary cord(B cells and

plasma cells)Capsule

Dense connective tissue

Deep cortex(T cells)

Subcapsularspace

Medulla

Trabeculae

Efferentvessel

Lymph nodeartery and vein

Lymph nodes are shaped like kidney beans.

Cortex

Capillary

Capsule

Subcapsularspace

Outercortex

Germinalcenter

DividingB cell

Dendritic cells

Nuclei of B cells

Lymph nodesHilum

Medullary sinus

Outer cortex (B cells) Afferent

vessel

Medullary cord(B cells and

plasma cells)Capsule

Dense connective tissue

Deep cortex(T cells)

Subcapsularspace

Medulla

Trabeculae

Efferentvessel

Lymph nodeartery and vein

Lymph Flow

Cortex

Dendritic cells and macrophages that start the immune response.

1

2

3

4

5

6

7

The medulla contains B cells and plasma cells responsible for the production of antibodies (humoral immunity).

Lymph Node Functions:

1- They filter and purify the lymph before return to venous circulation (they remove 99% of cell debris, bacteria, viruses, fungi).

2- They provide an early warning system against infection in peripheral tissue, responding before infections reach vital organs of trunk.

3- The first step in immune response takes place in the lymph nodes (extracted antigens are “presented” to lymphocytes).

The ThymusThe thymus is located in the mediastinum.It atrophies after puberty.

The thymus is divided into two thymic lobes.

Rightlobe

Leftlobe

Septa

Lobule

Septa divide lobes into smaller lobules.Thymus

Lobule

Lobule

The thymus gland LM 50

Cortex

Septa

Medulla

Dividing lymphocyte

s

Maturelymphocyte

s

LymphocytesCortexSeptaMedulla

Lobule

Lobule

The thymus gland LM 50A thymic corpuscle LM 550

Reticularcells

Thymiccorpuscle

Mature T cells leave thymus by medullary blood vessels.

T lymphocytes divide in the cortex and mature while they migrate to the medulla.

The thymus secretes thymic hormones that stimulate T cell differentiation.

Spleen

Rib

Pancreas

Aorta

Parietal peritoneum

Visceral peritoneum

Stomach

Diaphragm

Gastric area

Diaphragmatic surface

Renal area

Kidneys

Liver Gastrosplenic ligament

SPLEENHilum

The SpleenIt is the body’s largest lymphatic organ.

The spleen LM 50

White pulp ofsplenic nodule

Capsule

Red pulp

Trabecularartery

Central artery insplenic nodule

White pulp isdominated by lymphocytes.

Red pulp contains a large number of red blood cells.

Functions:1- Removal of abnormal and worn out blood cells.2- Storage of iron recycled from red blood cells.3- Initiation of immune responses by B cells and T cells.

Histology of the Spleen