Lymphoid organs play major roles in immune response

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Lymphoid organs

Lymphatic cells organize into lymphoid tissues and organs of various sizes and structural complexity. These tissues and organs broadly divide into two groups: primary lymphoid organs and secondary lymphoid organs. Lymph filters through many of these lymphoid organs and structures as it travels through lymphatic vessels.

The lymphoid organs that play principal roles in immunity and immune responses are bone marrow, thymus, spleen, and lymph nodes (however, we have few more lymphoid organs). These tissues and organs both produce and store cells of the immune system that help fight infections and diseases (only primary lymphoid organs produce lymphocytes).

Moreover, lymph a clear fluid that flows in the lymphatic vessels carries the immune cells to different parts of the body. As lymph flows through lymphatic vessels, it passes through a good number of lymphoid organs, which filter lymph and ‘scans’ it for foreign antigens.

Lymph and the formation of lymph

Lymph is a pale fluid that flows in lymphatic vessels. Plasma fluid (containing dissolved substances) and white blood cells leak from capillaries, through pores on the capillary walls, into tissue spaces. About 90% of this fluid (interstitial fluid) returns to blood through the venules. The remaining 10% drains into lymphatic vessels and flows as lymph. Lymphatic vessels convey lymph from tissues to venous bloodstream. It re-enters venous blood through the lymphatic ducts.

Among the cellular constituent of lymph, the prominent ones are lymphocytes and macrophages.

Primary lymphoid organs (central lymphoid organs)

The two primary lymphoid organs are bone marrow and thymus.

Bone marrow: bone marrow (the yellowish tissue in the center of our bones) produces white blood cells. This is the reason why patients with bone marrow diseases are immuno-suppressed.

Lymphoid stem cells, which form both B and T-lymphocytes, arise from the bone marrow. B-lymphocytes mature in bone marrow, whereas T-lymphocyte precursors migrate to thymus and mature in thymus gland.

Thymus: thymus is a small gland that lies both behind the breastbone, and in between the lungs. It is the site of maturation of T-lymphocytes. Precursors of T-lymphocytes arise from bone marrow and migrate to thymus where they acquire antigen-specific receptors (TCRs). B-lymphocytes mature in bone marrow.

Thus, primary lymphoid organs are sites where B and T-lymphocytes develop and mature.

Put together, as much as 109 lymphocytes leave bone marrow and thymus into general circulation every day. However, our primary lymphoid organs undergo atrophy as we age. Most bones lose their ability to produce blood cells. Thymus reaches peak activity at puberty and begins to show functional decline shortly after puberty. Fat gradually replaces the thymus gland, such that at age 70, fat tissue replaces almost all of thymus gland.

Secondary lymphoid organs (peripheral lymphoid organs)

Spleen, lymph nodes, and small aggregates of lymphoid tissues Peyer’s patches, tonsils are all secondary lymphoid organs (peripheral lymphoid organs). B and T-lymphocytes become active in secondary lymphoid organs (peripheral lymphoid organs); they become active when they encounter their specific antigen in one of the secondary lymphoid organs.

Foreign antigens that enter from the periphery encounter their specific lymphocytes in lymph nodes whereas blood-borne antigens encounter their specific lymphocytes in the spleen, most likely.

In contrast to primary lymphoid organs, which show functional decline with age, secondary lymphoid organs mostly increase in size with age. Lymph nodes particularly may become larger and feel sore whenever, you are infected.

Spleen: As blood flows through the spleen, spleen filters the blood and traps microbes that are present in blood. The spleen and lymph node are biological filters; only that spleen filters blood whereas lymph node filters lymph.

The functional anatomy of spleen promotes interaction between foreign antigens and their specific lymphocytes. Spleen has two compartments: red pulp and white pulp.

Red pulp contains macrophages and dendritic cells; dendritic cells interdigitate and form a meshwork that physically traps microbes that circulate in blood. Moreover, macrophages and dendritic cells are antigen-presenting cells; they ingest microbes (present in blood) and present antigens derived from the microbes to lymphocytes, which are specific for the displayed foreign antigen. Actually, dendritic cells and macrophages present blood-borne antigens to specific lymphocytes (which populate the spleen), initiating an immune response.

White pulp is the layer that contains B and T-lymphocytes. These lymphocytes are normally at rest; however, they become activated once they recognize a foreign antigen displayed by APCs of the red pulp. Most foreign antigens stimulate both antibody formation (humoral response) and formation of T-cell receptors (cellular response).

An individual can actually survive without a spleen, but he/she will suffer infections more often.

Lymph node: Lymph nodes are small bean-shaped organs scattered along the entire length of lymphatic vessels. Thousands of lymph nodes occur all over the body; and they most often occur in clusters, which drain lymph from a particular organ. Moreover, lymph nodes are most prevalent around the neck, armpit, groin, and knees.

The body system recognizes pathogens and foreign antigens, which enter through the periphery (skin and mucus membranes) and deliver them to the lymph and the lymphatic system, by way of antigen presenting cells.

An important responsibility of lymphatic system is to convey lymph (containing lymphocytes and other white blood cells) to and from lymph nodes. Lymph, particularly conveys antigen-presenting cells to lymph nodes.

Lymph nodes act as both filters that filter lymph, as well as stores that house significant amounts of lymphocytes. B-lymphocytes directly bind (using their B-cell receptors) and incapacitate bacterial cells and intact and whole antigens that may circulate in lymph. (T-lymphocytes are unable to bind intact antigens directly; they only bind processed antigen fragments displayed by antigen presenting cells).

Lymph nodes are both essential and critical in your body’s immune responses; actually, a good number of your immune responses begin in the lymph nodes. When you are infected, pathogen appears in lymph node and makes the lymph nodes to produce more infection-fighting immune cells. This increase in activity may cause your lymph nodes to swell and feel soft, tender and painful. You most likely feel these lymph nodes swellings around the groin, armpit and underarm, and neck.

Can you think of other lymphoid organs and their roles in immune response? Then send us a comment, question or suggestion.

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