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Lymphoid Organs

Q: What is the microscopic structure of a thymic lobule?

A thymic lobule (lobuli thymi) consists of a peripheral cortex and a central medulla, formed by a network of epitheliocytes and filled with lymphocytes. SubstanceLocalizationFeaturesCortex (cortex thymi)Peripheral part of the lobuleDarker, densely populated with T lymphocytesMedulla (medulla thymi)Middle part (center) of the lobuleLighter, contains fewer lymphocytes, features concentric corpuscles Both regions contain maturing or mature T cells, epithelial stroma, macrophages, as well as blood and lymphatic vessels.

Q: Which arteries supply blood to the thymus?

Blood supply to the thymus is provided by thymic branches (rr. thymici) originating from several arterial sources. These include: Internal thoracic artery (a. thoracica interna)Inferior thyroid artery (a. thyroidea inferior)Pericardiacophrenic artery (a. pericardiacophrenica)Superior phrenic artery (a. phrenica superior) An abundance of arterial blood supply sources is characteristic of childhood.

Organa lymphoidea

For medical students2 min readUpdated 2026-10-10

Lymphoid organs are specialized structures containing lymphoid tissue. Their primary function is to produce cells that mediate protective responses and establish the human immune system.

Main functionProviding protective responses and forming immunity.

HemocytopoiesisBone marrow gives rise to all blood cells and lymphocytes.

SecretionThe thymus secretes thymosin to stimulate immunity and growth.

Age-related changesMaximum relative mass of the thymus is observed in newborns.

Classification of Lymphoid Organs

Depending on their role in the maturation of immune cells, all lymphoid organs are divided into two large groups: primary and secondary.

1. Primary lymphoid organs (organa lymphoidea primaria): This group includes the foundational structures where immune cells originate:

Bone marrow (medulla ossium).
Thymus (thymus).

2. Secondary lymphoid organs (organa lymphoidea secundaria): Lymphocytes migrate here for further functional maturation:

Regional lymph nodes (nodi lymphoidei regionales).
Spleen (splen).
Tonsils: palatine (tonsillae palatini), pharyngeal (tonsilla pharyngea), tubal (tonsillae tubarii), and lingual (tonsilla lingualis).
Lymphoid nodules. They can be solitary (noduli lymphoidei solitarii), located in the walls of hollow organs of the respiratory and digestive systems, and aggregated (noduli lymphoidei aggregati), found in the wall of the ileum and the vermiform appendix.

Functions of Primary Lymphoid Organs

Primary organs play a key role in developing the body's defense system.

Bone marrow is the vital organ of hemocytopoiesis (blood formation). Its structures house stem cells that give rise to absolutely all types of blood cells (hemocytes) and lymph cells (lymphocytes).

The thymus is the central organ of immunogenesis. In addition, the gland performs an endocrine function by producing a hormone-like substance — thymosin. This secretion promotes an increase in the number of lymphocytes, globally enhances immunity, and actively participates in growth and skeletal development.

Lymphocyte differentiation is strictly compartmentalized:

T lymphocytes (thymus-dependent) are formed in the thymus through the complex transformation of stem cells originating from the bone marrow.
B lymphocytes are thought to differentiate from bone marrow cells themselves, as well as from lymphoid nodules in the appendix and ileum.

Following primary differentiation, lymphocytes migrate via the bloodstream to secondary lymphoid structures to complete their development.

Macroscopic Anatomy of the Thymus

The thymus (thymus) is an unpaired lymphoid organ with an elongated, asymmetrical shape.

In most cases, it consists of two closely apposed lobes: the right lobe (lobus dexter) and the left lobe (lobus sinister). Rarely, the organ may consist of three or four lobes. These structures are connected by a layer of loose connective tissue. The surface of the thymus is uneven and lobulated. Anatomically, the organ features a lower portion, which is the widest and forms the base, and tapering upper regions forming the apex of the gland.

Age-Related Anatomy and Topography of the Thymus

The topography and dimensions of the thymus change significantly during development.

At the seventh month of prenatal development (fetus): The thymus is located in the superior mediastinum. The organ has a cone shape, tapering superiorly. The lower margin of the thymus lies adjacent to the base of the heart (basis cordis) and large vessels. Laterally, it borders the medial surfaces of the lungs (facies medialis pulmonis), and its upper pole extends high into the neck region. During this period, the organ grows actively.

In the newborn: Thyhmus shifts, occupying the anterior part of the superior mediastinum and extending into the anterior mediastinum. The organ increases significantly in size, with its lobular structure clearly visible (two asymmetrical, tightly adjacent lobes).

Syntopy: anteriorly, the thymus lies against the posterior surface of the sternum; posteriorly, its broad base covers the anterior surface of the pericardium and great vessels (aorta, pulmonary trunk, superior vena cava); laterally, it significantly overlaps the anterior borders of the lungs.

By birth, the thymus is a large immune organ occupying a substantial volume of the retrosternal space. It is in newborns that the thymus has its maximum mass relative to body weight.

Mnemonic

T lymphocytes are named so because they are Thymus-dependent (mature in the thymus).

Frequently asked questions

What is the microscopic structure of a thymic lobule?

A thymic lobule (lobuli thymi) consists of a peripheral cortex and a central medulla, formed by a network of epitheliocytes and filled with lymphocytes.

Substance	Localization	Features
Cortex (cortex thymi)	Peripheral part of the lobule	Darker, densely populated with T lymphocytes
Medulla (medulla thymi)	Middle part (center) of the lobule	Lighter, contains fewer lymphocytes, features concentric corpuscles

Both regions contain maturing or mature T cells, epithelial stroma, macrophages, as well as blood and lymphatic vessels.

Which arteries supply blood to the thymus?

Blood supply to the thymus is provided by thymic branches (rr. thymici) originating from several arterial sources.

These include:

Internal thoracic artery (a. thoracica interna)
Inferior thyroid artery (a. thyroidea inferior)
Pericardiacophrenic artery (a. pericardiacophrenica)
Superior phrenic artery (a. phrenica superior)

An abundance of arterial blood supply sources is characteristic of childhood.

What functions does the spleen perform as a secondary lymphoid organ?

The main function of the spleen as a secondary lymphoid organ is the generation of an immune response. It executes cellular mechanisms of immunity, including antigen recognition and the function of immunocompetent cells. The spleen contains one of the largest lymphocyte populations in the body, accounting for 20–25% of the total number of cells in the immune system.

What happens to the thymus during age-related involution in adults?

During age-related involution, which begins after age 20, there is a progressive decrease in thymic mass and the replacement of its parenchyma with connective and adipose tissue.

Key changes include:

Loss of lymphoid and epithelial cells with cyst formation at sites of stromal cell death.
Breakdown of the corticomedullary architecture and weakening of the corticomedullary barrier.
Migration of atypical cells (predominantly B lymphocytes) into the thymic cortex.

In older individuals, the organ is filled with adipose tissue interspersed with islands of normal parenchyma. However, the thymus does not completely lose its function, retaining low-level hormone secretion and cell emigration.

What hormones does the thymus produce?

The thymus produces polypeptide hormones secreted by stromal epithelial cells that influence T lymphocyte maturation.

Key thymic hormones and factors include:

Thymosins (including thymosin α1) — a group of polypeptide factors.
Thymopoietins — produced by the same cells as thymosins.
Thymulin — a nonapeptide active only in complex with zinc ions.
Parathymosin — one of the main groups of thymic hormones.
Thymic humoral factor — a peptide complex whose active fraction is γ2.

Which organs are classified as primary lymphoid organs?

Primary lymphoid organs (organa lymphoidea primaria) include the bone marrow and the thymus.

Where and how are T lymphocytes formed?

T lymphocytes are formed in the thymus. They differentiate through the transformation of a subset of stem cells originating from the bone marrow.

What endocrine function does the thymus perform?

The thymus produces a hormone-like substance, thymosin, which increases the number of lymphocytes, enhances the immune response, and promotes skeletal growth processes.

What is the key feature of the thymus in newborns?

In newborns, the thymus has the maximum relative body mass compared to other life periods, a distinct lobular structure, and occupies a significant volume of the retrosternal space.

Go deeper

Regional lymph nodes: classification and topography
Microscopic structure and functions of the spleen (splen)
Solitary and aggregated lymphoid nodules of the digestive tract
Anatomy of the tonsils: palatine, pharyngeal, lingual, and tubal
Postpubertal involution of the thymus