Lymphatic Tissues and Organs

LYMPHATIC TISSUES AND ORGANS

Lymphatic tissues and organs are classified into two types:

PRIMARY LYMPHATIC ORGANS

Primary lymphatic organs are responsible for the formation and maturation of lymphocytes.

Primary Lymphatic Organs:

Stem cells in bone marrow produce B-cells and immature T-cells. Immature T-cells migrate to the thymus for maturation.

Thymus

The thymus is a lymphoid organ located in the thoracic cavity behind the sternum and in front of the heart.

Key features:

• Large in infants
• Maximum size at 10–12 years (about 40 g)
• Gradually shrinks after puberty

The thymus has two lobes, each covered by a connective tissue capsule. The capsule extends inward as trabeculae, dividing the organ into lobules.

Each lobule has:

• Outer dark cortex
• Inner light medulla

The thymus is made of glandular epithelium and hematopoietic connective tissue. It is the site where T-cells mature, multiply, and gain immune competence.

SECONDARY LYMPHATIC ORGANS

Secondary lymphatic organs are sites where lymphocytes become activated.

Secondary Lymphatic Organs

• Lymph nodes
• Spleen

These organs are surrounded by a capsule.

Lymphatic nodules are not considered secondary organs because they lack a capsule and are found along mucous membranes, protecting respiratory, digestive, urinary, and reproductive tracts.

Lymph Node

Lymph nodes are small, bean-shaped structures, measuring 1–25 mm in length. They are found in the neck, armpits, groin, and other regions.

Each lymph node is covered by a dense connective tissue capsule. Lymph nodes often swell during infections, indicating immune activity.

Structure of Lymph Node

• Outer capsule of fibrous tissue
• Internal stroma divided into cortex and medulla
• Trabeculae extend inward
• Cortex contains lymph follicles
• Germinal centers are sites of B-cell proliferation
• Medulla contains plasma cells and macrophages
• Hilum allows entry and exit of vessels

Afferent vessels bring lymph into the node, and efferent vessels carry filtered lymph away.

Functions of Lymph Node

• Filters lymph and removes pathogens
• Exposes antigens to immune cells
• Activates B-cells and T-cells
• Produces lymphocytes
• Removes dead cells and cancer cells
• Produces plasma proteins like globulins

Spleen

The spleen is a dark purple, bean-shaped organ, about 12 cm long and weighing approximately 200 g. It lies in the left upper abdomen, beneath the rib cage.

It filters blood, stores blood cells, and produces immune cells. Damage to the spleen can be life-threatening.

Structure of Spleen

The spleen has:

• Diaphragmatic surface (contacts diaphragm)
• Visceral surface (contacts stomach)

It is covered by peritoneum and a fibrous capsule. Trabeculae divide the organ internally.

The splenic tissue contains Malpighian corpuscles, which are lymphoid structures.

The spleen is supplied by the splenic artery and drained by the splenic vein.

White Pulp

White pulp consists of lymphatic tissue surrounding arteries. It contains lymphoid follicles rich in B-cells and follicular dendritic cells.

White pulp appears as white dots on a cut surface of the spleen and is responsible for immune functions.

Red Pulp

Red pulp consists of blood-filled sinusoids and splenic cords. It contains macrophages that destroy old and damaged red blood cells.

Functions of Spleen

• Produces blood cells during foetal life
• Filters blood and removes old RBCs
• Produces antibodies and WBCs
• Destroys pathogens by phagocytosis
• Maintains fluid balance
• Acts as a blood reservoir