Lymphatic System: Definition, Types and Structure

It is one of two circulation systems of fluid around the body (the other is blood circulation).

A distinction is made between the primary and secondary lymphatic organs in humans. The lymphocytes mature to become immunocompetent cells and migrate to the secondary lymphatic organs in the primary lymphatic organs. There they are responsible for mounting the specific defenses of the body.

The lymphatic system consists of a lymphatic fluid (called “lymph”) that flows through a system of lymphatic vessels.

It includes lymphatic capillaries, other lymphatic vessels of various sizes (which can be compared to blood vessels), and lymph nodes (encapsulated masses of B-cells and T-cells).

The blood system and the lymphatic system are interconnected.

The clear fluid is known as “lymph” when it flows through the vessels and initially passes as “interstitial fluid” contained in the spaces between the tissue cells, filtered from the blood.

Ultimately, after traveling through the lymphatic vessels, the same lymph returns to the blood system – at the jugular and subclavian veins (on the right and left sides of the body).


Key points

  • The lymphoid tissue can be primary or secondary depending on its stage of development and maturation of lymphocytes.
  • The secondary lymphoid tissues consist of lymph nodes, tonsils, Peyer’s patches, spleen, adenoids, skin, and lymphoid tissue associated with mucosa. They are responsible for maintaining mature lymphocytes and initiating an adaptive immune response.
  • The thymus and the bone marrow constitute the primary lymphoid tissues that are the sites of the generation and maturation of lymphocytes.
  • Lymphoid tissue develops from the venous endothelial tissues after the fifth week of gestation, beginning at the end of the lymphatic system (subclavian vein and lymphatic ducts) and extending outward.

The tissues that make up the lymphatic system

Several different types of tissues form the structures of the lymphatic system.

They are classified according to the parts of the lymphatic system that they form:

(A) Primary lymphatic organs.

The places where the stem cells divide and mature into B cells and T cells:

The thymus: this is a two-lobed organ located in the chest. The thymus tissue consists of T cells, macrophages, and epithelial cells. A layer of connective tissue encapsulates each lobe.

T cells mature from stem cells produced in the bone marrow and transported to the thymus.

They migrate out of the thymus, colonize the secondary lymphatic organs, and are active there as immunocompetent cells for the body’s defense against infections.

The thymus is derived from the anterior intestine of the third and fourth pharyngeal pouches. Its stroma arises from epithelial cells of ectodermal and endodermal origin.

Bone marrow

The production of blood cells in the bone marrow begins approximately 4-5 months after conception. The stem cells migrate from the liver to the bone marrow, where the “microenvironment” is decisive for developing stem cells.

This stroma consists of endothelial cells, fat cells, osteoblasts, and fibrocytes. Here, among others, the B lymphocytes mature.

Macrophages also colonize the stroma but come from hematopoietic stem cells. This creates an environment that, according to the needs, stimulates the proliferation and differentiation of the precursor cells.

As soon as these cells mature, they proceed through the openings in the sinusoids from the bone marrow into the bloodstream.

(B) Secondary lymphatic organs and tissues.

Secondary lymphoid tissues are arranged as a series of filters that control the content of extracellular fluids, i.e., lymph, tissue fluid, and blood.

The lymphoid tissue that filters each of these fluids is arranged differently. Secondary lymphoid tissues are also where the lymphocytes are activated.

The locations in which most immune responses occur:

The lymph nodes – These are small bean-shaped organs located throughout the body and consist of B cells that become plasma cells – which secrete antibodies, T cells, and macrophages. A capsule of dense connective tissue covers each node.

The Spleen: It is the enormous mass of lymphatic tissue in the human body. The outer covering of the spleen is formed by dense connective tissue. The spleen consists of two types of tissue, called white pulp and red pulp.

White pulp: lymphatic tissue consisting mainly of lymphocytes and macrophages.

Red pulp: Blood sinuses and splenic tissue cords consisting of lymphocytes, macrophages, erythrocytes, granulocytes, and plasma cells.

Lymph Nodes: Lymph nodes differ from lymph nodes in that the lymph nodes are not surrounded by capsules (dense connective tissue).

(C) Lymph (fluid).

The fluid “lymph” can be described as tissue in its own right – in the same way that fluid “blood” can be described as “blood tissue.”

Lymph is a clear fluid similar to plasma but contains less protein.

It flows through the body’s lymphatic vessels and includes chemicals and cells whose composition varies according to the location within the body.

Despite being fluid, lymph is classified as connective tissue.

Where in the body are the lymphatic tissues?

As explained above, there are lymphatic tissues throughout the body.

However, the structure of the lymphatic tissues varies according to the particular type of tissue or lymphatic organ of which they are part of (a), (b), or (c), and possibly also their location in the body, especially in the case of the Lymph (Fluid).

The structure of lymphatic tissues

Lymph is a clear fluid similar to plasma (from the blood) but contains less protein.

The functions of lymphatic tissues.

The lymphatic tissues throughout the body function in conjunction with others to perform the parts of the lymphatic system.

The lymphatic system is part of the body’s immune system. The functions of the lymphatic system include:

  • Drainage of interstitial fluid.
  • Transport of dietary lipids.
  • Protect the body against infections.