Organs of the immune system

The immune system can be distinguished by function as the: Primary Lymphoid organs  --> Thymus & Bone marrow Secondary lymphoid organs –>  Lymph nodes, Spleen & Various Mucosal Lymphoid Tissues (MALT), such as Gut-Associated Lymphoid Tissue (GALT). Tertiary lymphoid organs –> Cutaneous-associated lymphoid tissues Primary Lymphoid organs: Only after a lymphocyte has matured a primary lymphoid organ is the cell immune competent. In mammals, T-cells mature in the "Thymus" and B-cells mature in the "Bone marrow” (in Bursa of Fabricus in birds). There are TWO cells in the Primary Lymphoid Organs – Thymus and Bone Marrow. Secondary lymphoid organs Lymph nodes and the spleen are the most highly organized of the secondary lymphoid organs. Less-organized lymphoid tissue, collectively called "Mucosal-Associated lymphoid Tissue"(MALT), is found in various body sites. MALT includes -> Peyer's patches (in the small intestine), the tonsils, &


The thymus is a flat, bi-lobed organ situated above the heart. Immature T-cells are simply referred as "Thymocyte", these are densely packed outer compartment (or) cortex of the thymus lobule. Some of the thymic epithelial cells in the outer cortex, called "nurse cells", have long membrane extensions that surround as many as so thymocytes, forming large multicellular complexes The average weight of the thymus is 70 grams in infants; its age dependent involution levels an organ with an average weight of only 3 grams in the elderly. Explanations about Thymus:The thymus covered by a fibrous capsule. It is formed of two lobes. Each lobe of the thymus is organized into lobule which is separated from one another by septacalled trabeculae. With in each lobule, the cells are arranged into an outer cortex and an inner medulla. The cortex is tightly packed with proliferating immature lymphocytes while the medulla contains more mature cells. As the cells mature, they move from th…

Bone marrow

In birds, a lymphoid organ called the "Bursa of fabricius", which is a gut-associated lymphoid tissue, is the primary site of B-cell maturation. In mammals, such as primates and rodents, there is no Bursa, but Bone marrow serve as the "Bursal equivalent", where B-cell maturation occurs. In rabbits, uses gut-associated tissues such as the Appendix as primary lymphoid tissue for important steps in the proliferation and diversification of B-cells.


It is largest secondary lymphoid organ. The spleen is a large, ovoid secondary lymphoid organ, situated at high in the left abdominal cavity. Spleen specializes in filtering blood and trapping blood-borne antigens. It can respond to systemic infections. The spleen is surrounded by a capsule that extends a number of projections into the interior to form a compartmentalized structure. They are of two types: a) Red pulp       b)White pulp a)Red pulp:Populated by macrophages & numerous red blood cells (erythrocytes). It is the site where old and defective RBC are destroyed and removed. b)White pulp:Populated mainly by T-Lymphocytes.

Mucosal - Associated Lymphoid Tissue (MALT):

It is non-encapsulated clusters of lymphoid tissue with immune function is the "mucosal immune system". It is common around the membrane lining the respiratory, digestive and urogenital tracts (gateways to the body for involving organisms) The MALT of the gut is called "Gut-Associated lymphoid tissues"(GALT). It includes payer's patches and focal accumulations of lymphocytes in the lamina propria. The MALT filters out antigens that enter in air and food (or) come from microorganisms growing in the intestines. The antigen activated B-Lymphocytes of the bodies first line defense against infection by microorganisms. These B-cells also make Ig.E as a main response to helminthes. The mucosal layer of the alimentary canal, respiratory and urinogential tracts is provided with dispersed groups of lymphoid tissues known as MALT. These tissues are usually without a capsule (uncapsulated). In human beings the payers patches, the tonsils and appendix are good examples of lym…

Peyer’s Patches

Peyer’s patches are secondary lympoid tissues. They are MALT (Mucosa Associated Lymphoid Tissue). They are collection of lymphoid nodules packed together to form oblong elevations of the mucus membrane of the small intestine. They consist of a mixture of T and B lymphocytes where the T-cells predominate. The cells secrete Ig.A , gets transported across the mucosa into the intestinal lumen where it neutralizes viruses, bacteria and toxins and blocks the entry of these antigens into circulation by acting as an antiseptic paint.

Lymph node

Lymph nodes are encapsulated bean-shaped structures containing a reticular network placed with lymphocytes, macrophages & dendritic cells. The overall architecture of a lymph node supports an ideal micro environment for lymphocytes to effectively encounter and respond to trapped antigens. The lymph node can be divided into three roughly concentric regions based on morphology: Cortex:The Cortex contains lymphocytes (mostly B-cells), macrophages & follicular dendritic cells arranged in primary follicles. This cortex region some times referred to as "Thymus-independent area. Medulla:It is innermost layer of lymph node Para-cortex:Beneath the cortex, which is populated largely by T-Lymphocytes and also contains interdigitating dendritic cells thought to have migrated from tissues to the node. These interdigitating dendritic cells express high levels of class n MHC molecules, which are necessary for presenting antigen to TH-cells. These paracortex regions sometimes referred to as…