ORGANS OF THE IMMUNE SYSTEM AND THEIR FUNCTION

The organs of the immune system are generally divided into two major groups: the primary lymphoid organs and the secondary lymphoid organs. The bone marrow, spleen, thymus, Peyer’s patches, liver, tonsils, lymph nodes, and several mucosal-associated lymphoid tissues (MALT) are some of the major organs of the immune system in humans and other mammals. These organs of the immune system play unique and various critical roles in the development of immune responses against invading pathogens or antigens, and they are specialized types of organs or tissues that make the immune system outstanding compared to other body systems.

Understanding the Organs of the Immune System

Understanding the organs of the immune system is essential for studying how the body defends itself against diseases.

Primary lymphoid organs are the central organs of the immune system; in these organs, the maturation of immune system cells (particularly the B and T lymphocytes) primarily occurs. The primary lymphoid organs primarily serve as sources of lymphocytes for the other components of the immune system. Bone marrow and thymus are the two main organs that make up the primary lymphoid organs. In the primary lymphoid organs, hematopoiesis and the generation of lymphocytes occur, which are crucial for the immune system’s functionality.

These organs of the immune system are the central sites where lymphocytes (i.e., B and T cells) develop the characteristic features that typify the adaptive immune response, such as discrimination between self and non-self molecules. It is also at the primary lymphoid organs that lymphocytes develop their first ability to specifically recognize antigens or foreign bodies that invade the body. The lymphocytes produced in the primary lymphoid organs are initially immature or immunocompetent cells until they become committed to a particular antigen within these organs and then transform into immunocompetent cells with unique antigenic specificity.

The tonsils are crucial organs of the immune system that help protect against pathogens entering through the mouth.

Tonsils are found at the base of the tongue and at the side of the back of the mouth where they defend against pathogens that enter the body through the mouth; the adenoids are located in the nasopharyngeal roof where they defend against antigens that enter the body via the nasal passage or opening; the thymus is the organ where T cell maturation occurs after its production in the bone marrow and it is located above the heart.

The spleen is another important organ of the immune system located in the left abdominal cavity, filtering blood to trap blood-borne pathogens.

The bone marrow is where B cell development and maturation occur, as well as the development of other hematopoietic stem cells such as RBCs and WBCs; lymph nodes are found all over the body, especially along the junctions of the lymphatic vessels, where they provide a microenvironment for trapping, processing, and presenting antigens to the immune system.

The organs of the immune system play a vital role in the body’s defense mechanisms.

The spleen is located at the left abdominal cavity and below the pancreas where they filter the blood to trap blood-borne pathogens (Figure 1); the peyers patches are located within the gastrointestinal tract (GIT) and even in the appendix where they provide immunological response; and the lymphatic vessels is primarily responsible for the distribution of antigens that entered the tissues to other lymphoid organs for specific immunological response.    

e=”background-color: #fee894″>In birds, B cell development occurs in the Bursa of Fabricius. After their initial development in the bone marrow, lymphocytes leave to complete their maturation in the peripheral lymphoid organs (i.e., the secondary lymphoid organs).

While the development and maturation of B cells occur in the bone marrow, T cells mature in the thymus. A lymphocyte becomes immunocompetent only when it has matured within a primary lymphoid organ (e.g., thymus and bone marrow); immune system cells that are immunocompetent are B and T cells that have the ability to mount an immune response against particular antigens or pathogens in the body of an animal or mammal. Immune system cells become immunocompetent after their interaction with antigens.

During clonal selection, lymphocytes are educated to attack only antigens and not self-molecules; only lymphocytes with antigenic specificity for foreign bodies are allowed to proliferate, while those triggering detrimental autoimmune responses are eliminated. The bone marrow is the origin of all immune system cells, also producing other important cells of the body.

The bone marrow is a yellowish soft tissue found in all the major bones of the body (especially the long bones); it is the primary site where all the cells of the immune system derive from during hematopoiesis. Aside from B cells that develop and mature in the bone marrow, other cells of the immune system, including progenitor T cells, mast cells, dendritic cells, NK cells, erythrocytes, platelets, and granulocytes, also develop from the bone marrow.

For instance, B cells differentiate into antibody-secreting plasma cells and memory B cells (the effector cells of antibody-mediated immunity), while T cells differentiate into effector T cells and memory T cells, responsible for cell-mediated immunity (CMI). These secondary lymphoid organs provide an environment for effector lymphocytes to interact with antigens.

In neonates or foetus in utero, B cell development initially occurs in the fetal liver, fetal bone marrow or yolk sac before continuation in the adult bone marrow. In birds, B cell development and maturation occurs in the Bursa of Fabricius. After their initial development and maturation in the bone marrow, the lymphocytes leave the bone marrow to continue their maturation in the peripheral lymphoid organs (i.e. the secondary lymphoid organs).

It is noteworthy that the B cells produced during hematopoiesis of HSCs in the bone marrow are B lymphocytes that primarily attack non-self-molecules (i.e., antigens or pathogens). However, in some circumstances, B cells with specificity for self-molecules may be produced; in such scenarios, the clonal selection process that frequently occurs in the bone marrow during hematopoiesis ensures that self-reactive B lymphocytes are immediately eliminated. This process also applies during T cell maturation in the thymus, ensuring that only T cells recognizing non-self molecules are produced and propagated.

During these processes of clonal selection of T and B cells with specificity for only non-self-molecules, the lymphocytes are educated to attack only antigens and not self-molecules; and only the population of lymphocytes with antigenic specificity for foreign bodies are allowed to proliferate while those that evoke detrimental autoimmune response in the host are immediately eliminated.The bone marrow as earlier stated is the site where all the cells of the immune system originate from; and it is also responsible for the production of other important cells of the body such as platelets and RBCs.

The thymus is a bilobed organ located above the heart; it serves as the main site for the maturation of immature or progenitor T cells that come from the bone marrow. It consists of two main compartments: the medulla (inner part) and cortex (outer part), both containing thymocytes. The thymus’s major biological function is to ensure the continuous production and selection of immunocompetent T lymphocytes that successfully process antigenic peptides presented by MHC molecules.

Secondary lymphoid organs are specialized organs where lymphocytes continue their maturation after their initial production in primary lymphoid organs (e.g., bone marrow and thymus). Unlike primary lymphoid organs, secondary lymphoid organs are sites where lymphocytes (i.e., B and T cells) proliferate and differentiate into effector cells that neutralize invading pathogens or antigens.

For example, the B cells proliferate or differentiate into antibody-secreting plasma cells and memory B cells (which are the effector cells of the antibody-mediated immunity) while the T cells differentiate into effector T cells and memory T cells which are mainly responsible for cell-mediated immunity (CMI). Secondary lymphoid organs or tissues are mainly located along the vessels of the lymphatic system; and secondary lymphoid organs provide environment where effector lymphocytes interact with antigens or foreign bodies.

The lymphatic system is a series of vessels responsible for transporting lymph fluids (i.e., pale biological liquids containing WBCs) from tissues via lymph nodes into the bloodstream. The interaction of lymphocytes with antigens in secondary lymphoid organs leads to the differentiation of clones into effector cells with unique antigenic specificity. Examples of secondary lymphoid organs include lymph nodes, spleen, and several mucosal-associated lymphoid tissues (MALT).

Secondary lymphoid organs also present antigens to immune system cells for immunological response. The lymph nodes and spleen are the most important secondary lymphoid organs due to their fundamental roles in processing and presenting antigens to effector lymphocytes and other immune system cells.

Lymph nodes are encapsulated, bean-shaped lymphoid structures found throughout the body system (Figure 2); they comprise lymphocytes (i.e., B and T cells), macrophages, and dendritic cells, forming a mesh of the immune system. Lymph nodes serve as sites for filtering antigens from lymph fluid before presentation to immunocompetent lymphocytes and other immune system cells; they are richly supplied by lymphatic vessels that ensure continuous lymph fluid supply.

Note: while lymph nodes trap antigens from lymph, the spleen traps antigens in the blood. The mucosal-associated lymphoid tissues (MALT) are secondary lymphoid organs lining mucous membranes of the GIT, urogenital tract, respiratory tract, and other mucous membrane surfaces in the body, defending these surfaces against pathogenic microorganisms or antigens.

The germinal center of secondary follicles is densely populated with B lymphocytes. Lymph nodes mount immune responses to antigenic molecules in lymph fluids and are mainly located at the junctions of lymphatic vessels throughout the body. Phagocytic cells and dendritic cells within the lymph nodes trap particulate antigens or pathogens entering via lymph; lymphocytes then respond to mount an immune response against trapped antigens.

The cortex, paracortex, and medulla are the three main sections of the lymph node (Figure 2). The cortex is the outermost part, rich in macrophages, lymphocytes (particularly B cells), and dendritic cells. The paracortex underlies the cortex and is rich in T lymphocytes, with dendritic cells also found in this area. The medulla is the innermost part of the lymph node, sparsely populated with antibody-secreting plasma cells and other cells of lymphoid origin.

Lymph nodes trap foreign bodies or antigens before returning lymph fluids to systemic circulation. Activated lymphocytes in lymph nodes proliferate and differentiate into numerous effector cells following antigen invasion. After antigen interaction with macrophages or dendritic cells in lymph nodes, the antigen is processed and presented to immunocompetent B and T cells for instigating appropriate immune responses against pathogens.

Lymph fluids containing antigens or pathogens enter lymph nodes via the afferent lymph duct, while the efferent lymphatics are the passageways through which immunoglobulins and lymphocytes leave the lymph nodes to the bloodstream (Figure 2). The spleen is an oval-shaped organ located in the upper left abdominal cavity; it is the site where antigens or pathogens are trapped from blood circulation, and outworn RBCs are destroyed.

The spleen, unlike lymph nodes, primarily filters blood that passes through it, trapping antigens and presenting them to lymphocytes in peripheral lymphoid tissues. It also traps antigens from local tissues aside from those carried through blood. Lymphocytes and blood-borne antigens are carried into the spleen via blood vessels (e.g., spleen arteries); foreign bodies are presented to lymphocytes for appropriate immune responses.

Note: While the lymph nodes only trap antigens from the lymph the spleen traps antigens in the blood.The mucosal-associated lymphoid tissues (MALT) are secondary lymphoid organs or tissues that line the mucous membranes of the GIT, urogenital tract, respiratory tract and other mucous membrane surfaces that are found in the body. The primary function of MALT is to defend the mucous membranes surfaces of the body against invasion and colonization by pathogenic microorganisms or antigens.

MALT produces numerous plasma cells that create antibodies with unique antigenic specificity. Typical examples of tissues making up MALT include Peyer’s patch (lining the GIT) and tonsils (located at the tongue’s base), which express several immunological functions at their sites.

REFERENCES

Abbas A.K, Lichtman A.H and Pillai S (2010). Cellular and Molecular Immunology. Sixth edition. Saunders Elsevier Inc, USA.

Actor J (2014). Introductory Immunology. First edition. Academic Press, USA.

Alberts B, Bray D, Johnson A, Lewis J, Raff M, Roberts K and Walter P (1998). Essential Cell Biology: An Introduction to the Molecular Biology of the Cell. Third edition. Garland Publishing Inc., New York.

Bach F and Sachs D (1987). Transplantation immunology. N. Engl. J. Med. 317(8):402-409.

Barrett   J.T (1998).  Microbiology and Immunology Concepts.  Philadelphia,   PA:  Lippincott-Raven Publishers. USA.

Jaypal V (2007). Fundamentals of Medical Immunology. First edition. Jaypee Brothers Medical Publishers (P) Ltd, New Delhi, India.

John T.J and Samuel R (2000). Herd Immunity and Herd Effect: New Insights and Definitions. European Journal of Epidemiology, 16:601-606.

Levinson W (2010). Review of Medical Microbiology and Immunology. Twelfth edition. The McGraw-Hill Companies, USA.

Roitt I, Brostoff J and Male D (2001). Immunology. Sixth edition. Harcourt Publishers Limited, Spain.

Zon LI (1995). Developmental biology of hematopoiesis. Blood, 86(8): 2876–91.