What is the Difference Between Primary and Secondary Lymphoid Organs

The main difference between primary and secondary lymphoid organs is that primary lymphoid organs allow the lymphoid stem cells to proliferate, differentiate, and mature whereas secondary lymphoid organs allow lymphoid cells to become functional. Furthermore, antigens are not allowed to enter into the primary lymphoid organs while antigens are allowed to enter into the secondary organs, stimulating lymphoid cells. Moreover, bone marrow and thymus are the primary lymphoid organs while spleen, tonsils, Peyer’s patches, and lymph nodes are secondary lymphoid organs.

Primary and secondary lymphoid organs are two types of structures in the immune system, providing a stationary framework where migratory immune cells generate, mature, and interact with antigens.

Key Areas Covered

1. What are Primary Lymphoid Organs
     – Definition, Types, Function
2. What are Secondary Lymphoid Organs
     – Definition, Types, Function
3. What are the Similarities Between Primary and Secondary Lymphoid Organs
     – Outline of Common Features
4. What is the Difference Between Primary and Secondary Lymphoid Organs
     – Comparison of Key Differences

Key Terms

Differentiation, Lymphoid Stem Cells, Maturation, Primary Lymphoid Organs, Proliferation, Secondary Lymphoid Organs, Specialization

What are Primary Lymphoid Organs

Primary lymphoid organs are the sites of the immune system where the formation and maturation of lymphocytes occur. In other words, they are responsible for the proliferation, differentiation, and maturation of lymphocytes. Therefore, the initial cells in primary lymphoid organs are the undifferentiated lymphoid stem cells. These sites provide the environment for the stem cells to develop either into T cells and B cells.

Figure 1: Lymphoid Stem Cell Differentiation in Bone Marrow

Furthermore, bone marrow is a type of primary lymphoid organ where the proliferation and differentiation of lymphoid stem cells take place. Therefore, the differentiation of these stem cells into both T lymphocytes and B lymphocytes occurs in the bone marrow. Subsequently, immature T cells travel through the blood to the thymus, which is another primary lymphoid organ for their maturation. However, the maturation of B lymphocytes occurs in the bone marrow itself.

What are the Secondary Lymphoid Organs

Secondary lymphoid organs are the sites of the immune system where the functional specialization of lymphocytes occurs by allowing them to be in contact with different antigens. Therefore, in order to collect antigens, these organs form an array of filters, which always monitor the contents of the extracellular fluid including the blood, tissue fluid, and lymph. Lymph nodes, tonsils, spleen, Peyer’s patches, mucosa-associated lymphoid tissue (MALT), etc. are some examples of secondary lymphoid organs.

Figure 1: Lymph Nodes

Moreover, around 100-200 lymph nodes occur in the body, encapsulating lymphoid tissue. Most of them are concentrated into the neck, thorax, abdomen, and the pelvis. They contain T and B lymphocytes as well as macrophages to capture antigens. Also, MALT is responsible for filtering the tissue fluid. The tissue has a small diameter of around 1 mm. It occurs in the gastrointestinal tract, oral passage, nasopharyngeal tract, thyroid, breast, lung, salivary glands, eye, and skin. Also, tonsils, Peyer’s patches, and appendix are three different situations of MALT. Significantly, all of the above secondary lymphoid organs contain T and B lymphocytes as well as macrophages to capture antigens.

Similarities Between Primary and Secondary Lymphoid Organs

• Primary and secondary lymphoid organs are two structures in the body which assist the immune system.
• They provide a stationary framework where migratory immune cells generate, mature, and interact with antigens.

Difference Between Primary and Secondary Lymphoid Organs

Definition

Primary lymphoid organs refer to the organs of the immune system where lymphocytes are formed and mature while secondary immune organs refer to the organs of the immune system which maintain mature naive lymphocytes and initiate an adaptive immune response. This is the main difference between primary and secondary lymphoid organs.

Role

Their role is a major difference between primary and secondary lymphoid organs. Primary lymphoid organs allow lymphoid stem cells to proliferate, differentiate, and mature whereas secondary lymphoid organs allow lymphoid cells to become functional.

Types of Lymphocytes

Furthermore, primary lymphoid organs contain either T cells or B cells while secondary lymphoid organs contain both T cells and B cells.

Contact with Antigens

Contact with antigens is another difference between primary and secondary lymphoid organs. Primary lymphoid organs have no contact with antigens while secondary lymphoid organs have contact with antigens.

With Age

Primary lymphoid organs undergo atrophy with age while secondary lymphoid organs increase size with age. This is another difference between primary and secondary lymphoid organs. 

Conclusion

Primary lymphoid organs are a type of organs in the immune system, allowing lymphoid stem cells to proliferate, differentiate, and mature. Some examples of them in the human body are the thymus and bone marrow. On the other hand, secondary lymphoid organs are another type of organs in the immune system, which allow lymphoid cells to become specialized cells by making contact with antigens. Therefore, the main difference between primary and secondary lymphoid organs is their function in the immune cell maturation.

References:

1. Peckham, Michelle, et al. “The Leeds Histology Guide.” Histology Guide, 1 Jan. 1970, Available Here

Image Courtesy:

1. “Stemcelldifferentiaion” By US gov – (Public Domain) via Commons Wikimedia 
2. “Diagram showing the lymph nodes lymphoma most commonly develops in CRUK 311” By Cancer Research UK – Original email from CRUK (CC BY-SA 4.0) via Commons Wikimedia