organs of the immune system

Primary lymphoid tissues

sites of leukocyte development

• thymus and bone marrow are primary/central lymphoid organs

• where maturation of lymphocytes (T, B, NK, NKT cells) takes place

secondary Lymphoid tissues

sites of lymphocyte activation

• lymph nodes, spleen, mucosal associated lymphoid tissue (MALT), gut-associated lymphoid tissue (GALT)

• trap antigens so that they interact with mature lymphocytes and for antigen dependent activation of T and B cells

tertiary lymphoid tissues 

sites of lymphoid activity

• normally contain fewer lymphoid cells than secondary lymphoid organs, but can import lymphoid cells during and inflammatory response 

• eg cutaneous-associated lymphoid tissue (CALT)

bone marrow

Primary lymphoid tissue for producing leukocytes

extracellular matrix:

• attachment of stem cells and hematopoietic progenitor cells via adhesion molecules

stromal cells (endothelial, perivascular, nerves, macrophages, and osteoblasts)(anything that supports the development of immune cells):

• physical support (scaffold) for the growth of hematopoietic cells

• provide nutrients, produce hematopoietic growth factors, and express adhesion molecules that induce differentiation 

thymus

• primary

• site of T and NKT cell maturation 

• bi-lobed encapsulated organ with lobules separated by connective tissue strands 

• outside (cortex) : densly packed with immature, proliferating thymocytes 

• inside (medulla): sparsly populated with mature thymocytes

• cortical epithelial cell, medullary epithelial cells dendritic cells, and macrophages help with thymocyte development and maturation

lymph nodes

• secondary 

• provides a site for interaction between lymphocytes and antigen presenting cells 

• site for phagocytosis of organisms that enter the lymph to prevent them from entering the blood

parts of the lymph node

• cortex: primary follicles (naive B cells, follicular dendritic calls, macrophages), and secondary follicles (activated B cells in germinal centres)

• Paracortex: T cells, interdigitating dendric cells

• medull: phagocytic macrophages, antibody secreting plasma cells, some activated/memory B and T cells moving into the lymph

• reticular network (stroma): made of an extracellular matrix, reticular fibers, and fibroblastic reticular cells. provides structural support for lymphocyte compartments 

spleen

• secondary

• where immune responses are mounted against antigens in the blood, and where old red blood cells are phagocytosed and recycled 

• surrounded by a capsule

• white pulp: generation of T cell responses and B cell responses (antibodies) against blood borne antigens

• marginal zone: interdigitating dendritic cells trap blood borne antigens and transport them into the white pulp

• red pulp: defective red blood cells and blood borne pathogens get phagocytosed by macrophages

organization of white pulp in spleen

• formed by lymphocytes surrounding the arterioles running through the spleen 

• arterioles surrounded by periarteriolar lymphoid sheath and contains T cells

• follicles contain B cells

• germinal centre full of activated proliferating B cells. surrounded by a b cell corona and the marginal zone is rich in macrophages and marginal B cells

• lymphocytes and antigens enter perifolicular zone first, then move in through the white pulp

lymphoid structures at mucosal surfaces

• mucosal surfaces protected by malt 

• GALT includes tonsils, adenoids, appendix, peyers patches (specialized structures in small intestine)

• peyers patches: antigens collected from GI tract by specialized epithelial cells called microfold or m cells. peyers patches also include B cell follicles with germinal centres separated by t cell dependant areas, and the subepithelial dome rich in dendritic cells

• similar but less organized lymphoid tissue also exist at other mucosa