Lymphoid Organs

Primary (central) organs:

The primary lymphoid organs include the bone marrow and thymus.

Bone Marrow is where B maturation occurs.

Thymus is where T-cell maturation occurs. T cell progenitors formed during hematopoiesis enter the thymus gland and mature there to become antigen committed, immunocompetent T cells. In the course of thymocyte maturation the antigenic diversity of the T cell receptor is generated by a series of random gene rearrangements. After expressing antigen binding receptors, these cells are subjected to a selection process so that only T cells recognize antigenic peptides in the context of self-MHC molecules which are released form the thymus.

The thymus is divided into an outer cortex and inner medulla. The outer cortex contains many immature thymocytes which undergo rapid proliferation coupled with a large rate of cell death. The inner medulla is thought to contain thymocytes that are more mature.

A decline in immune functions that accompanies aging, leading to an increase in infections results primarily form changes in the T cell components of the immune system. The thymus reaches its maximal size at puberty and then atrophies, with a significant decrease in both cortical and medullary cells and an increase in the total fat content of the organ. Whereas the average weight of the thymus is 70 g in infants it is only 3 g in the elderly.

Secondary organs

The secondary lymphoid organs include the lymph nodes and spleen and mucosal associated lymphoid tissue. As blood circulates under pressure, the fluid component of the blood (plasma) seeps through the thin wall of the capillaries into the surrounding tissue. Much of this fluid, called interstitial fluid, return to the blood through capillary membranes. The rest, called lymph, flows into a network of tiny open lymphatic capillaries and then into a series of progressively larger collecting lymphatic vessels. The largest lymphatic vessel, the thoracic duct empties into the left subclavian vein near the heart.

A naive lymphocyte is not able to mount an immune response until it has been activated to become an effector cell. Activation of a naive cell occurs in specialized microenvironments within secondary lymphoid tissue (e.g., peripheral lymph nodes, Peyer's patches, tonsils, and spleen). Naive cells circulate indiscriminately to secondary lymphoid tissue by recognizing adhesion molecules on HEVs. High-endothelial venules (HEVs) which are special regions (such as those often found on postcapillary venules in the lymph node discussed below) which have specialized cells with a plump, high shape which express a variety of cell-adhesion molecules liked CAMs of the selectin family, the mucin-like family and the immunoglobulin superfamily (ICAMs). Within these microenvironments, dendritic cells capture antigen and present it to the naive lymphocyte, resulting in its activation.

Lymph Nodes: Various lymphoid tissues are located along the vessels of the lymphatic system. For example, antigen are carried into lymph nodes by the lymph, it is trapped, processed and presented together with class II MHC molecules by dendritic cells resulting in T_H cell activation. A lymph node is very efficient at trapping antigen carried into it by the afferent lymphatics.

Lymph nodes are found in clusters throughout the body and are composed of an outside cortex (which contains B cells located within follicles and germinal centers) and inner medula (which contains antibody-producing plasma cells). lymphocytes enters via the afferent lymphatic channels and exits via the efferent lymphatic vessel. Blood borne lymphocytes also migrate into the node by passing between specialized endothelia cells that line postcapillary venules of the node.

Germinal center (GC) is the microenvironment that allows the generation of B cell memory. There B cells proliferate and undergo somatic mutation, isotype switching, affinity selection, and differentiation into memory B cells or plasma blasts. The GC also contains T cells, follicular DCs, and GC DCs.

Lumph nodes contain mostly T cells.

Spleen: The spleen  is a large secondary lymphoid organ situated high in the left abdominal cavity. Unlike lymph nodes which are specialized to trap localized antigen from regional tissue spaces, the spleen is adapted to filtering blood and trapping blood born antigens.

The spleen contains two main compartments; an outer red pulp and inner white pup. These compartments are separated by a diffuse marginal zone. The white pulp surrounds the branches of the splenic artery, forming a periarteriolar lymphoid sheath (PALS), populated mainly by T lymphocytes. Blood borne antigens and lymphocytes enter the spleen through the splenic artery which empties into the marginal zone. In the marginal zone, antigen is trapped by interdigitating dendritic cells which carry it to the PALS.

The spleen contains a mixture of T and B cells and 10% macrophage and DCs.

Mucosal associated lymphoid tissue line the digestive, respiratory and urogenital system. The eptithelial cells of mucous membranes play an important role in promoting the immune response by delivering small samples of foreign antigen from the lumina of the respiratory, digestive, and urogenital tracts to the underlying mucosal associated lymphoid tissue. This antigen transport is carried out by specialized cells, called M Cells.