Primary vs Secondary Lymphoid Organs Explained

Primary Lymphoid Organs are the centers of the Immune System where Lymphocyte development and Maturation occur. That is to say, they are responsible for the Lymphocyte Proliferation, Differentiation and Maturation. The initial Cells in Primary Lymphoid Organs are therefore the undifferentiated Stem Cells in the Lymphoids. Such sites provide the Environment in which the Stem Cells grow into either T Cells or B Cells.

In addition, Bone Marrow is a type of Primary Lymphoid Organ, where Lymphoid stem Cells proliferate and differentiate. Therefore, these stem Cells are differentiated into both T Lymphocytes and B Lymphocytes in the bone marrow. Immature T Cells then travel through the blood to the thymus, which is another Primary Lymphoid Organ for maturation. But B Lymphocyte maturation occurs in the bone marrow itself.

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What are the Secondary Lymphoid Organs?

Secondary Lymphoid Organs are the locations of the Immune System, where Lymphocytes are functionally specialized by allowing them to come into contact with different antigens. Such Organs therefore form an array of filters in order to collect antigens, which always track the contents of the extraCellular fluid including the blood, Tissue fluid, and lymph. Some examples of Secondary Lymphoid Organs are lymph nodes, tonsils, spleen, Peyer's patches, mucosa - associated Lymphoid Tissue (MALT), etc.

In addition, about 100 - 200 lymph nodes are present in the body, encapsulating the Lymphoid Tissue. We are mostly concentrated in the back, thorax, abdomen, and pelvis. They have Lymphocytes T and B, as well as antigen-capture macrophages. MALT is also responsible for filtering the fluid in Tissues. The Tissue is about 1 mm tiny in diameter. This exists in the digestive tract, oral passage, nasopharyngeal tract, thyroid, breast, lung, salivary glands, hair, and skin. Often, three different MALT cases are tonsils, Peyer's spots, and appendix. Significantly, all of the above Secondary Lymphoid Organs contain T and B Lymphocytes as well as antigen-capture macrophages.

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Similarities Between Primary and Secondary Lymphoid Organs

• Two structures in the body which assist the Immune System are Primary and Secondary Lymphoid Organs.

• They provide a stationary structure for producing, maturing and interacting migratory Immune Cells with antigens

Difference Between Primary and Secondary Lymphoid Organs

Definition

Primary Lymphoid Organs refer to Immune System Organs in which Lymphocytes form and mature while Secondary Immune Organs refer to Immune System Organs that maintain mature naive Lymphocytes and initiate an adaptive Immune response. This is the major difference between Lymphoid Organs of Primary and Secondary origin.

Role

Their function represents a significant difference between Primary and Secondary Lymphoid Organs. Primary Lymphoid Organs allow Lymphoid stem Cells to proliferate, differentiate, and mature while Secondary Lymphoid Organs encourage functional Lymphoid Cells to grow.

Types of Lymphocytes

In addition, Primary Lymphoid Organs contain only T Cells or B Cells, while Secondary Lymphoid Organs contain T Cells and B Cells , respectively.

Contact with Antigens

A further difference between Primary and Secondary Lymphoid Organs is contact with antigens. Primary Lymphoid Organs have no antigen contact whilst Secondary Lymphoid Organs have antigen contact.

With Age

Primary Lymphoid Organs suffer age - related atrophy while Secondary Lymphoid Organs increase size by age. This is another difference between Lymphoid Primary and Secondary

Primary Lymphoid Organs Versus Secondary Lymphoid Organs

Development  of Lymphatic Tissues

The Lymphatic Tissues begin their development by the end of the fifth week of gestation in the embryo. Lymph vessels develop from lymph sacs which in turn arise from the developing veins (formed from the mesoderm).

The very initial lymph sacs that develop during the Lymphatic System formation are the paired jugular lymph sacs. They occur at the junction of the internal jugular and subclavian veins. These Lymphatic capillary plexuses then spread to the thorax, upper limbs, neck, and head. Some of these plexuses enlarge and form Lymphatic vessels in their respective sites in the body.

Other than the anterior part of the sac (from which the cisterna chyli shall develop), all lymph sacs are occupied by mesenchymal Cells and later transformed into clusters of lymph nodes.

The spleen, a Secondary Lymphoid Organ, develops from mesenchymal Cells between layers of the dorsal mesentery of the stomach. The thymus gland develops as an outgrowth of the third pharyngeal pouch.

Immune Function of the Lymphoid Tissues

The Lymphatic System plays a major role in the body's Immune System through the Lymphatic Tissues found at specific regions. They act as the Primary site for white blood cells to initiate  adaptive Immune System response, such as for the T-Cells and B-Cells. These Cells present in the Lymphatic System will react with the antigens found either by the Cells directly or through other associated Cells. When an antigen is identified, a series of immunological cascade reactions begin which then cause the activation and recruitment of more and more Lymphocyte Cells, along with the production of antibodies and cytokines and the deployment of other immunological Cells such as macrophages.

Clinical significance of Lymphoid Organs

The study of Lymphatic drainage of various Organs is important diagnostic tool in the diagnosis, prognosis, and treatment of cancer. Since the System lies in close affinity to many Tissues of the body, is majorly associated with the transport of cancerous Cells between the various parts of the body. This process is known as metastasis in cancer biology. The lymph nodes can trap these cancer Cells which when not fully destroyed,  become sites of Secondary tumours in the patient's body.

Other Lymphoid Tissues of the Lymphatic System

It will be interesting to know that other than the Primary and Secondary Lymphoid Organs, there also exists a third "tertiary Lymphoid Organ". These refer to structures that originate in node-like form (akin to a lymph node) in peripheral Tissues mainly at sites of chronic inflammation, such as during chronic infection or when a patient undergoes Organ transplant and faces graft rejection by the body. They can also develop during some cancers and autoImmune and autoImmune-related diseases and are abnormal in their activities. Tertiary Lymphoid Organs are regulated in a different manner from the normal process. These Lymphoid Tissues are formed during ontogeny, are found to be dependent on cytokines and hematopoietic Cells, but still functions like other Lymphoid Tissues as they drain interstitial fluid and transport lymph Cells in response to the same chemical messengers related to the antigen-antibody reaction. They ideally contain very few Lymphocytes, and associate with an Immune response only when challenged with antigens that result in inflammation.