Immune organs: Primary and Secondary Lymphoid organs

Primary lymphoid organs à maturation of lymphocytes takes places.

Thymus

It is the site for T cell development and maturation.

Bilobed organ situated above the heart. Each lobe is surrounded by a capsule and divided into lobules, which are separated from each other by strands of connective tissue called trabeculae. Each lobule is organized into two compartments: cortex which is densely packed with immature T cells called Thymocytes whereas the inner compartment or medulla is sparsely populated with thymocytes.

Both the compartments consist of epithelial cells, dendritic cells, and macrophages which make up the framework of the organ and contribute to the growth and maturation of thymocytes.

Function of thymus: to generate and select a repertoire of T cells that will protect the body from infection. As thymocytes develop, enormous diversity of T cell receptors is generated by gene rearrangement. 

It produces some T cells with receptors capable of recognizing antigen MHC complexes. But only small portion of cells survive and rest will undergo apoptosis. 

Bone marrow

It is a complex tissue that is the site of haematopoiesis and a fat depot. Hematopoietic cells are generated here move through the walls of blood vessels and enter the bloodstream, which carries them out of the marrow and distributes these various cell types to the rest of the body.

It is site of B cell origin and development. Immature B cells proliferate and differentiated within the Bone marrow and stromal cells within the bone marrow interact directly with the B cells and secrete various cytokines that are required for development. 

Secondary lymphoid organs à site for mature lymphocytes to interact with antigen.

Lymphatic system

lymphoid follicles: primary follicle à network of follicular dendritic cells and small resting B cells. After antigenic challenge, a primary follicle becomes secondary follicle à a ring of concentrically packed B lymphocytes surrounding a center (the germinal center) in which one finds a focus of proliferating B lymphocytes and an area that contains nondividing B cells and some helper T cells interspersed with macrophages and follicular dendritic cells.

Lymph nodes and spleen à surrounded by fibrous capsules.

MALT à less organized lymphoid tissue found in various body sites.

Lymph node:

Encapsulated bean shaped structures containing a reticular network packed with lymphocytes, macrophages and dendritic cells. They are organised in such a way to encounter antigens that enter the tissue spaces.

Cortex à outermost layer contains B cells mostly, macrophages and follicular dendritic cells arranged in primary follicles. After antigenic challenge, the primary follicles enlarge into secondary follicles, each containing germinal center.

Paracortex à it is beneath cortex which is populated largely by T cells and also contains dendritic cells that migrated from tissue to the node.

Medulla à more sparsely populated with lymphoid lineage cells, and of those present, many are plasma cells actively secreting antibody molecules.

Lymph nodes traps the antigens from local tissues.

Spleen:

Present in the high left abdominal cavity. It is large ovoid, plays a major role in mounting immune responses to antigens in the blood stream.  Spleen filters the blood and traps the blood borne antigens. So it responds to systemic infections. It is not supplied by lymphatic vessels, instead antigens and cells are carried into spleen through splenic artery.  It is surrounded by capsules from which a number of projections extend into the interior to form a compartmentalized structure.

Two compartments:

Red pulp: network of sinusoids populated by macrophages, numerous red blood cells, and few lymphocytes. Defective and old RBCs are destroyed and removed.

White pulp: it surrounds a branches of splenic artery forming a periarteriolar lymphoid sheath (PALS) populated mainly by T cells. Primary lymphoid follicles are attached to PALS.

Marginal zone contains lymphocytes and macrophages. Antigens are trapped in marginal zone by dendritic cells, which then carry it to the PALS. And also lymphocytes enter through the same route as of the antigen.

MALT (mucosa associated lymphoid tissue)

The vulnerable membrane surface of digestive, respiratory and urogenital systems are defended by a group of organized lymphoid tissues called MALT.

Respiratory epithelium à bronchus associated lymphoid tissue (BALT)

Epithelium of the digestive tract à Gut- associated lymphoid tissue (GALT)

MALT includes tonsils, and appendix.

M cells carry the antigen from lumina of the tracts to the underlaying MALT. They are flattened epithelial cells lacking the microvilli that characterize the rest of the mucous epithelium. They have deep invagination, or pocket in the basolateral, plasma membrane which is filled with a cluster of B cells, T cells and Macrophages.