What are the primary lymphoid organs?

Basil Manning
What are the primary lymphoid organs?

The primary lymphoid organs or central are the organs in charge of creating a specialized microenvironment for the production of cells of the immune system and blood (hematopoiesis) and for the maturation of lymphocytes, where they acquire specific receptors that enable them to respond to an antigen.

The primary lymphoid organs are the bone marrow and the thymus. Once the cells are produced in the bone marrow and have completed their maturation process in the marrow itself or in the thymus, they are ready to be directed towards the secondary lymphoid organs..

Bone marrow. Mysid. Spanish translation and layout retouch: Basquetteur [CC0], via Wikimedia Commons

This is how vertebrate organisms have developed a ubiquitous and specialized tissue and cellular system, strategically distributed throughout the body, known as the immune system..

The classification of the organs that are part of this system has been established according to their functions.

Article index

  • 1 Bone marrow
    • 1.1 -Location
    • 1.2 -Embryology
    • 1.3 -Histology of the bone marrow
    • 1.4 -Bone marrow function
    • 1.5 Other functions
  • 2 Scam
    • 2.1 -Location
    • 2.2 -Embryology
    • 2.3 -Histology of the thymus
    • 2.4 -Function of the thymus
  • 3 References

Bone marrow


The bone marrow is considered the largest organ in the body, as it is distributed throughout the body, located in the medullary canal of long bones and flat bones, especially those of the skull..

The approximate weight of the bone marrow is 30 to 50 ml / kg of weight.


In early fetal life the function of the bone marrow is assumed first by the embryo sac and then, until birth, by the liver and spleen..

However, the spleen and liver could fulfill this role after birth in emergencies. That is, in the presence of very extensive damage to the bone marrow or situations that demand a significant increase in cell production.

-Bone marrow histology

In the bone marrow, two compartments are clearly distinguished: the vascular and the hematopoietic..

Vascular compartment

This compartment includes the arteries and veins that feed the medulla: the nutrient artery, the longitudinal central artery, the capillary artery, the venous sinuses, the longitudinal central vein, and the nutrient vein..

The venous sinuses represent the most important elements within the vascular system, since their function is essential for the functions of the medulla..

Its walls are very complex from a structural point of view. Through the venous sinuses the cells pass from the hematopoietic compartment to the vascular compartment.

Hematopoietic compartment

It is located between the vascular sinuses and limited by them, it is the source of erythrocytes, platelets, granulocytes, monocytes and lymphocytes.

Its stroma is made up of adipocytes, fibroblasts, and precursor cells..

-Bone marrow function

This organ is of utmost importance, since it is responsible for producing the formed elements of the blood (erythropoiesis, thrombopoiesis, granulopoiesis, monocytopoiesis, lymphopoiesis).

All cells are formed from a pluripotential cell called a stem cell. From there arise two types of cells called myeloid common precursor and lymphoid common precursor.

The common myeloid precursor will give rise to the megakaryocytic series (platelets), the erythroid series (erythrocytes or red blood cells) and the myeloid series (monocytes / macrophages, segmented neutrophils, segmented eosinophils, segmented basophils and dendritic myeloid cells).

While the common lymphoid precursor will give rise to T lymphocytes, B lymphocytes / plasma cells, NK lymphocytes (natural killer cells) and dendritic lymphoid cells..

In the production and differentiation processes of the precursor cells that will give rise to each of the cell series, a variety of substances are involved that make these actions possible..

These substances are: interleukins (IL): 1, 3, 6, 7,11 and factors that stimulate granulocytic and monocytic colonies..

Other functions

On the other hand, it has been proven that the bone marrow fulfills a double function on the lymphoid system. The first is to generate the immature lymphocytes called thymocytes..

These, when attracted by chemokines, are directed towards the thymus, where they complete their maturation and thus be responsible for the primary immune response at the level of peripheral lymphoid tissues..

The second is to receive recirculating lymphocytes, which makes it an important environment for the secondary immune response..

Another function of the bone marrow is to fulfill the maturation process of B lymphocytes, thanks to the release of growth factors and cytokines by the cells present in the stroma..

Self-reactive B lymphocytes are eliminated by apoptosis. Those that survive are carried by the circulation to the secondary lymphoid organs where they are activated and come into contact with some foreign antigen..



The thymus is a bilobed organ, located in the midline of the body, specifically in the anterior mediastinum, above the heart.


Embryologically speaking, it originates from the third and fourth pharyngeal pouch of the embryo. At birth, the organ is fully developed and undergoes a progressive involution throughout life..

However, despite this in very advanced ages, remains of thymic tissue with functional epithelium are still detected.

-Thymus histology

Both lobes of the thymus are surrounded by a capsule of connective tissue that interweaves within the parenchyma, in such a way that it forms septa (trabeculae) that divide the lobes into smaller segments called lobules..

Two areas are easily recognized: the cortical and the medullary.

Cortical area

Presents infiltration of lymphocytes and highly specialized epithelial cells called nurse cells.

The latter have the function of promoting the renewal and maturation of lymphoblasts or thymocytes and other thymic cells..

Further into the cortex are epithelial dendritic cells that communicate with each other through intercellular bridges, forming a large loose network in which a large number of lymphocytes are found..

Both lymphocytes and dendritic cells express gene-encoded determinants of the major histocompatibility system on their surface, which allows intimate contact between them..

In this process, T cells capable of reacting with one's own tissue are detected through a process called negative selection. Lymphocytes that are marked as undesirable are eliminated, while the others survive (tolerance).

Macrophages possibly responsible for phagocytosis and destruction of unwanted lymphocytes are found in the area bordering the medullary area..

Medullary area

It is a scarce area in intercellular substance, but rich in epithelial cells joined together by desmosomes. These cells are responsible for secreting a group of immunologically active chemical mediators, called thymic hormones..

The thymic hormones are serum thymic factor, thymopoietin, and thymosin. In this area there are also Hassall's corpuscles, structures made up of a group of hyalinized and hypertrophied epithelial cells..

It is believed that destruction of the thymic lymphocytes identified in the cortex takes place at these sites. The entire organ is enriched with blood vessels that are surrounded by epithelial cells.

The space between the epithelial cells and the blood vessels is called the perivascular space. The epithelial cells that surround the vessels serve as a selective barrier.

These prevent macromolecules from the blood from entering the gland, but allowing the passage of the different types of T lymphocytes (CD4 and CD8) into the circulation..

-Thymus function

The thymus is an important organ from the first years of life for the development of a successful immune function. This organ maintains homeostasis by controlling defense functions and permanent vigilance.

It is able to remotely control the functioning of secondary or peripheral lymphoid organ tissues through thymic hormones. They act by controlling mitosis and some cell functions of lymphocytes at these sites..

Also, the thymus is responsible for the maturation of thymocytes to mature T lymphocytes. It also controls the high rate of mitosis that occurs at this site at the cortical level..

On the other hand, the thymus is responsible for detecting lymphocytes capable of reacting against its own antigens, in order to destroy them before they enter the circulation..

In short, it can be said that the thymus is an immunoregulatory organ.


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