The Cleanup Crew
The lymph system uses a number of different organs to perform its functions. Primary lymphoid organs are those where lymphocytes are formed and mature. Secondary lymphoid organs are those organs that act as filters in the lymph system.
Primary Lymphoid Organs
Not all WBCs reach their final mature (differentiated) state in the marrow. Agranular lymphocytes are split between two maturation locations.
Bone Marrow
While all lymphocytes begin their lives in the bone marrow, only the B lymphocytes mature there. As naïve (immature) B cells, they are not able to recognize antigens with their receptors and are incapable of mounting an immune response. The bone marrow is compartmentalized in such a way that these immature cells are unable to enter the systemic circulation until they pass inspection, at which time they are mature and are released into circulation.
If a cell produces a B cell receptor that binds too tightly or too weakly to the antigen, then the B cell is destroyed. The most common method is apoptosis (programmed cell death or cellular suicide). B cells must also be able to distinguish between host cells and tissues and foreign pathogens before they are let out into circulation.
Name that cell
The B in “B cells” represents the maturation in the bone marrow, while the T in “T cells” is because these cells travel to the thymus to complete their maturation process.
Thymus
Located in the mid thorax (chest) in a location most often referred to as the mediastinum, the thymus is an amorphous, bilobed organ that sits superior to the aortic arch and extends upward toward the neck. Connective tissue covers the entire organ, making it an encapsulated lymphoid organ. The thymus is divided internally and is isolated immunologically from the body by cells called reticular cells. These cells provide structural support for lymphocytes and secrete hormones that stimulate their production.
Beneath the capsule of connective tissue is the cortex and below that a deeper middle region called the medulla. Different types of reticular cells are specific to these locations and isolate each region of the thymus from another. For example, some cells line the capsule and isolate the thymus from the body, while others create a boundary between the cortex and medulla. Other cells cover the capillaries and blood vessels to create a blood-thymus barrier and prevent rogue T lymphocytes from escaping into the body before they are screened.
Naïve T cells undergo a screening process similar to what occurs with B cells. In the thymus they are first screened for their antigen-binding abilities. In the medulla they are screened for their ability to distinguish between host cells and tissues and foreign pathogens. If they pass, they leave the thymus.
Once mature, lymphocytes migrate out into the body and accumulate in various locations, such as in organs of the digestive and respiratory tract, as well as stand-alone lymphoid organs. Here, they are able to quickly and effectively encounter pathogens and mount an immune response.
Secondary Lymphoid Organs
Secondary lymphoid organs are those organs that serve as filters in the lymphatic system.
Lymphoid Nodules
Often confused with lymph nodes, nodules are simply aggregates of lymphocytes. Largely composed of B lymphocytes, nodules also consist of antigen-presenting cells and reticular cells for structure and anchorage. If the nodule is a solid dark color, it is referred to as a primary nodule (follicle). This indicates that the cells of this nodule have yet to be challenged with antigen. After the antigenic challenge, the center of the nodule (germinal center) becomes lighter as the lymphocytes proliferate and become antibody-generating plasma cells. These lighter-centered nodules are called secondary nodules. T lymphocytes also reside in the nodules but to a lesser degree.
Lymph Nodes
These encapsulated, bean-shaped lymphoid organs interrupt the path of the larger lymphatic vessels. Lymphatic vessels join at the convex surface of the node. They empty lymph into the cortex of the lymph node, which is subdivided into compartments by connective tissue extensions of the capsule called trabeculae. It is within this region that the round lymphoid nodules are found.
T lymphocytes are found in the deeper or subcortical region of the node. Lymph flows through the cortex and down into the medulla (deeper middle portion) of the node, where spaces or sinuses allow the lymph to drain from the node and into lymphatic vessels.
Spleen
While the spleen does have lymphoid characteristics, it also functions in the cleaning, destruction, and removal of dead RBCs. As the splenic artery enters the spleen, progressively smaller-diameter vessels branch off and run through the bulk of the spleen. Central arteries are the smaller tributaries that are surrounded by lymphocytes, which form a periarterial lymphatic sheath (PALS). This PALS is further encompassed by a lymphoid nodule, much like that found in the lymph node. Both the PALS and the nodules make up the white pulp of the spleen.
Vessels continue to branch and radiate through the nodules until they form splenic sinuses in the area between the white pulp. This is the location of the red pulp. From these sinuses, plasma freely escapes and flows throughout the spleen, including the white pulp where antigen-presenting cells capture and display antigens to the local lymphocytes.
Tonsils
There are three sets of tonsils in the back of the mouth, all of which contribute to the lymph system’s cleaning function. The three sets are:
· the adenoids
· the lingual tonsils
· the palatine tonsils
Generally when people refer to “the tonsils” they mean the palatine tonsils. These organs can become inflamed because of tonsillitis and are often surgically removed. Filled with lymphoid nodules and positioned at the boundary between the oral and pharyngeal cavities, these organs are in the perfect location to detect any pathogens trying to gain entry into the body through the oral cavity.
Also protecting the oral cavity, but to a lesser degree, are the lingual tonsils. These are located on the lateral borders of the tongue and are much smaller in mass than that of the palatine tonsils. These are filled with lymphoid nodules.
The final set of tonsils is the pharyngeal tonsils, better known as the adenoids. These are positioned higher in the pharynx at the boundary between the oral and pharyngeal cavities, and provide protection from any pathogen seeking to gain entrance to the body.
Peyer’s Patches
While lymphocytes are scattered throughout the body and temporarily accumulate in areas of infection, in the final portion of the small intestine, permanent lymphoid nodules called Peyer’s patches are present. They monitor bacteria growth in the intestines and prevent infection there.
Diffuse Lymphoid Tissue
Although not permanent nodules, resident lymphocytes are found in the underlying layers of both the gastrointestinal and the respiratory tracts. Classified as diffuse lymphatic tissue, or mucosa-associated lymphoid tissue (MALT), in the gastrointestinal tract it is known as the gut-associated lymphoid tissue (GALT) and in the respiratory tract it is called bronchus-associated lymphoid tissue (BALT).