Lymphoid tissue

By
Dr. Mohammed Hussein

The cells of the immune system

Are distributed throughout the body in the blood, lymph, and epithelial and connective tissues.

Are arranged in small spherical nodules called lymphoid nodules found in connective tissues and inside several organs.

Are organized as differently sized organs called lymphoid organs (the lymph nodes, the spleen, the thymus, and the bone marrow).

Lymphoid tissue

Lymphoid tissue
• Is a type of C.T characterized by a rich supply of lymphocytes
• Because lymphocytes have very little cytoplasm, lymphoid tissue stains dark blue in H&E-stained sections
• Lymphoid tissues are basically made up of free cells; as a result, they typically have a rich network of reticular fibrils that supports the cells

Lymphoid Nodules

• In the nodular lymphoid tissue, groups of lymphocytes are arranged as spheres, called lymphoid nodules or lymphoid follicles, that primarily contain B lymphocytes.
• When lymphoid nodules become activated, these lymphocytes proliferate in the central portion of the nodule, which then stains lighter and is called a germinative center.

Primary nodule

Secondary nodule

Lymphoid organs

MALT
Thymus
Lymph Node
Spleen

MALT

Lymphoid nodules found in the MUCOSA of digestive system , respiratory system, reproductive system, and urinary system.
Are collectively known as


Mucosa-associated lymphoid tissue (MALT), GALT, BALT
May be considered as a lymphoid organ

BALT

The tonsils is a lymphoid tissue that lies beneath and in contact with the epithelium of the initial portion of the aerodigestive tract .
Palatine
Pharyngeal
Lingual
Tonsils

Waldeyer’s ring

palatine
pharyngeal
lingual
palatine


Tonsils
Location
Capsule
Lining
Crypt
Lymphoid tissues
Palatine
lateral walls of the oropharynx
A band of dense C.T. separating the lymphoid tissue from subjacent structures.
Stratified squamous epithelium ( non-keratinized)
10–20 epithelial invaginations that penetrate the tonsil deeply
contains
numerous lymphatic
nodules, most having
a germinal center
Pharyngeal
Superior-posterior portion of the pharynx
Thinner than the capsule of the palatine tonsils
Ciliated pseudostratified columnar epithelium
No
Mostly diffuse
lymphoid tissues
and some lymphatic
nodules
Lingual
At the base of the tongue
No
Stratified squamous epithelium
( non-keratinized)
Single crypt
Rows of lymphatic
nodules supported
by connective tissue septa


• Crypt
• Epithelium
• Lymphoid tissue
• Lymphoid nodules
• Germinative center
• Capsule

(H&E) stain. Low magnification

1
2
4
3
5
6

thymus

• Located in the mediastinum.
• Has a dual embryonic origin.
• Has a C.T capsule that penetrates the parenchyma and divides it into incomplete lobules.
• Each lobule has :
Peripheral dark zone called the
cortex
Central light zone called the
medulla

cortex

Composed of :
• Extensive population of T cell precursors (thymocytes).
• Dispersed epithelial reticular cells.
• Macrophages.
A subpopulation of epithelial reticular cells present in the cortex consists of thymic nurse cells (TNCS), which contain many (20–100) maturing lymphocytes in their cytoplasm.

Medulla

Composed of :
• Differentiated T lymphocytes.
• Epithelial reticular cells.
• Thymic corpuscles or Hassall corpuscles.
Which are characteristic of this region, although their function is unknown.
These corpuscles contain flattened epithelial reticular cells that are arranged concentrically and are filled with keratin filaments. They sometimes calcify.


Blood – Thymus Barrier
• Capillary endothelium and its basal lamina.
• Perivascular C.T space occupied by macrophages.
• Epithelial reticular cells with their basal lamina.

Lymph Nodes

Series of in-line filters that are important in the body's defense against microorganisms and the spread of tumor cells.

Lymph, derived from tissue fluid, is filtered by at least one node before returning to the circulation.

Structure

Elongated or kidney-shaped organs that have:
Convex Surface
(that is the entrance site of lymphatic vessels)
Hilum
a concave depression,, through which arteries and nerves enter and veins and lymphatic vessels leave the organ.
Capsule
surrounds the lymph node, sending trabeculae into its interior.

Regions of Lymph Nodes

Capsule

Subcapsular Sinus
Lymphoid Nodule
Intermediate Sinus
Medullary cords
Medullary sinus

Outer cortex

Inner cortex
Medulla

H&E stain

Medium magnification

The Inner Cortex ( Paracortical Region )

Does not have precise boundaries with the outer cortex and contains few, if any, nodules but many T lymphocytes.
Thymus Dependent Area

High Endothelial Venules (HEVs)

Found in the paracortex
HEVs are postcapillary venules
Have a cuboidal cell lining
Allow lymphocytes to pass through their walls to enter the lymphatic tissue.

The Medulla

Medullary

cord
Medullary sinus

Lymph Circulation

Afferent Lymphatic Vessel
Subcapsular Sinus
Intermediate Sinus
Medullary Sinus
Efferent Lymphatic Vessel

Role of Lymph Nodes in Immune Response

The lymph(contain Antigens) Lymph node
In L.N. Antigens Presented to B and T cells
B lymphocytes Plasma cells
Plasma cells Antibodies Lymph
Lymph Veins Blood circulation
Antibodies Entire body


Recirculation of Lymphocytes

The continuous recirculation of lymphocytes enables most parts of the body to be constantly monitored, increasing the opportunity for lymphocytes antigens that have migrated to lymph nodes

SPLEEN

The largest lymphoid organ in the body

The only one interposed in the blood circulation

• Capsule ( surrounding the spleen)
• Trabeculae
• Parenchyma( Splenic Pulp)
• Incomplete compartments
Sends
Dividing
In to
General structure

Splenic Pulp = Splenic Parenchyma

The spleen is composed of a network of reticular tissue that contains:
reticular cells, lymphocytes and other blood cells, macrophages, and APCs.
Splenic pulp White pulp
Red pulp
These names derive from the fact that:
on the surface of a cut through an unfixed spleen, white spots (lymphoid nodules) are observed within a dark red tissue that is rich in blood.

Circulation

Splenic A.

at the hilum
Trabecular V.
Envloped by a sheath of T lymphocytes ( PALS) which part of White pulp

Central A or White pulp A

PALS receive large collections of lymphocytes (mostly B cells)
Lymphoid nodule

the artery now turned into an Arteriole, occupies an Eccentric position but is still called the Central artery
Radial branches that supply the surrounding lymphoid tissue


After leaving the white pulp, subdivides to form straight Penicillar arterioles
Splenic sinusoids

Trabecular A.

Splenic V.
at the hilum

Germinative Center

Central Artery

Red Pulp

Splenic (Bilroth’s) Cords
Splenic Sinusoids

Sinusoid

Cord

Functions of the Spleen

• Filtration function
Removal of old or abnormal RBCs, Removal of abnormal WBCs, Removal of normal and abnormal platelets and cellular debris
• Immunological function
• Opsonisation: while opsonised bacteria can be removed from the circulation by entire lymphoid system, the spleen is well suited for removing poorly opsonised or encapsulated pathogens.
• Antibody synthesis occurs chiefly in the white pulp.
• Protection from infection

Thank you for your attention