Organs of the Immune System
Organs of the Immune System
- Organs of the Immune system are consisting of Primary lymphoid organs and secondary lymphoid organs.
- The bone marrow and thymus are the primary lymphoid organs where B and T lymphocytes mature, respectively.
- Secondary organs provide a site for foreign antigen contact (. The spleen, lymph nodes, and various mucosal-associated lymphoid tissues (MALT) are examples of secondary lymphoid organs .
- The primary and secondary organs are classified based on their roles in adaptive and innate immunity.
Figure 1: Organs of the Immune System
Primary Lymphoid Organs
Primary Lymphoid organs are:
- Bone Marrow
1. Bone Marrow
- Bone marrow is one of the largest tissues in the body, filling the core of all long flat bones.
- It is the primary source of hematopoietic stem cells, which develop into erythrocytes, granulocytes, monocytes, platelets, and lymphocytes.
- Each of these lines has its own set of precursors that stem from the pleuripotent stem cell.
- Some lymphocyte precursors remain in the bone marrow to mature and become NK and B cells. B cells got their name because they were discovered to mature in birds in an organ called the bursa of Fabricius, which is similar to the appendix in humans.
- After searching for such an organ in humans, it was discovered that B-cell maturation occurs within the bone marrow itself. As a result, the naming of these cells was appropriate. Other lymphocyte precursors migrate to the thymus, where they mature into T cells.
- Immature T cells appear in the fetus as early as 8 weeks of gestation period. Thus, lymphocyte differentiation appears to occur very early in fetal development and is required for the infant to acquire immune competence by the time he or she is born.
- T cells acquire their distinguishing features in the thymus, a small, flat, bilobed organ located in the thorax, or chest cavity, just below the thyroid gland and overlying the heart.
- The thymus in humans reaches a weight of 30 to 40 g by puberty and then gradually shrinks.
- The thymus was thought to produce enough virgin T lymphocytes early in life to seed the entire immune system.
- Each lobe of the thymus is divided into smaller lobules filled with epithelial cells that play an important role in differentiation.
- T cells mature over a 3-week period as they pass through the thymic cortex to the medulla.
- As T cells mature, different surface antigens are expressed. In this way, a repertoire of T cells is created to protect the body from foreign invaders. Mature T lymphocytes are then released from the medulla.
Secondary Lymphoid Organs
The following are secondary lymphoid organs:
- Lymph nodes
- Mucosal-associated lymphoid tissues
1. Lymph nodes
- Lymph nodes serve as central collection points for lymph fluid from neighboring tissues.
- Lymph fluid is a blood filtrate formed by the passage of water and low-molecular-weight solutes through blood vessel walls and into the interstitial spaces between cells. Some of this interstitial fluid returns to the bloodstream via venules, but a portion flows through the tissues and is eventually collected in thin-walled vessels known as lymphatic vessels.
- Lymph nodes are found along lymphatic ducts and are particularly abundant near joints and where the arms and legs connect to the body.
- Filtration of interstitial fluid from around cells in tissues is an important function of these organs because it allows lymphocytes to interact with foreign antigens from the tissues.
- While the spleen protects us from foreign antigens in the blood, the lymph nodes provide an ideal environment for contact with foreign antigens that have penetrated the tissues.
- Lymph fluid flows slowly through spaces called sinuses, which are lined with macrophages, creating an ideal environment for phagocytosis.
- The node tissue is divided into three sections: the outer cortex, the paracortex, and the inner medulla.
- Afferent lymphatic vessels allow lymphocytes and any foreign antigens to enter nodes. Numerous lymphocytes enter the nodes from the bloodstream via specialized venules known as high endothelial venules found in the paracortical areas of the node tissues.
- The cortex, the outermost layer, contains macrophages and B cell aggregations in primary follicles similar to those found in the spleen. These are mature, resting B cells that haven’t been exposed to antigens yet.
- Follicular dendritic cells, which are specialized cells, are also found here. These cells have a large number of antibody receptors and aid in the capture of antigens for presentation to T and B cells.
- Secondary follicles are made up of proliferating B cells that have been stimulated by antigen.
- The interior of a secondary follicle is known as the germinal center because it is here that B cells undergo transformation. When exposed to an antigen, plasma cells that actively secrete antibodies and memory cells that are on the verge of forming plasma cells are formed. As a result, the lymph nodes provide an ideal environment for B-cell memory formation.
- T lymphocytes are primarily found in the paracortex, which is located between the follicles and the medulla. T lymphocytes are found near APCs known as interdigitating cells.
- The medulla is less densely populated than the cortex, but it does contain T cells (in addition to B cells), macrophages, and a large number of plasma cells.
- Particulate antigens are removed from the fluid as it travels from cortex to medulla across the node.
- Fluid and lymphocytes leave the body via efferent lymph vessels. These vessels combine to form a larger duct, which eventually connects to the thoracic duct and the venous system. As a result, lymphocytes can continuously circulate between lymph nodes and the peripheral blood.
- Lymphocytes can respond to a specific antigen proliferate in the lymph node.
- Lymph nodes enlarge due to the accumulation of lymphocytes and other cells, a condition known as lymphadenopathy.
2. mucosal-associated lymphoid tissues
- MALT stands for mucosal-associated lymphoid tissues .
- MALT can be found in the digestive, respiratory, and urogenital tracts.
- Tonsils, appendix, and Peyer’s patches, a specialized type of MALT located in the lower ileum of the intestinal tract, are a few examples.
- Because these mucosal surfaces are important entry points for foreign antigens, they are home to a large number of macrophages and lymphocytes.
- The skin is the body’s largest organ, and the epidermis contains a number of intraepidermal lymphocytes.
- The majority of these are T cells, which are uniquely positioned to combat antigens that enter the body through the skin. Monocytes, macrophages, and dendritic cells can also be found here.
- The spleen, the largest secondary lymphoid organ, is approximately 12 cm long and weighs 150 g in adults.
- It is surrounded by a thin connective tissue capsule and is located in the upper-left quadrant of the abdomen, just below the diaphragm.
- The organ functions as a large discriminating filter, removing old and damaged cells as well as foreign antigens from the blood.
There are two types of splenic tissue:
- red pulp
- and white pulp
- The red pulp accounts for more than half of the total volume and serves to destroy old red blood cells (RBCs).
- Blood flows from the arterioles into the red pulp and out the splenic vein.
- The white pulp contains the lymphoid tissue, which is arranged around arterioles in a periarteriolar lymphoid sheath and accounts for approximately 20% of the total weight of the spleen. This sheath is mostly made up of T cells.
- Primary follicles are attached to the sheath and contain B cells that have not yet been stimulated by antigens.
- A marginal zone containing dendritic cells that trap antigens surrounds the PALS.
- Lymphocytes enter and exit this area via the many capillary branches that connect to the arterioles.
- The spleen receives approximately 350 mL of blood per minute, allowing lymphocytes and macrophages to constantly scan for infectious agents or other foreign matter.
- All of these secondary organs serve as potential sites of contact with foreign antigens, increasing the likelihood of an immune response.
- T and B cells are segregated and perform specialized functions within each of these secondary organs.
- B cells are responsible for humoral immunity or antibody formation and differentiate into memory cells and plasma cells.
- T cells contribute to cell-mediated immunity by producing sensitized lymphocytes that secrete cytokines.
- Cell-mediated immunity and humoral immunity are both components of the adaptive immune response.