Lymphoid Organs Flashcards
Distribution of lymphoid organs
- bone marrow
- thymus
- lymph nodes
- spleen
- tonsils
- Peyer’s patches
- important components of the immune system
- immune responses are largely initiated and generated within the lymphoid organs
central components of the immune system
-primary lymphoid organs-bone marrow and thymus
peripheral lymphoid organs
- secondary
- lymph nodes
- spleen
- tonsils
- Peyer’s patches
extralymphoid tissues and organs
- GI
- Resp
- Urinary
- Repro
- skin
- blood
- lymph
- wandering lymphocytes
lymphocyte recirculation
- blood-lymphoid organs-blood
- permits continuous immune surveillance and ensures rapid response to antigens
- connects the three components of the immune system
small/medium lymphocytes
- 30% of leukocytes
- B cells
- T cells
- NK cells
reticular cells
- make reticular fibers
- form a spongelike meshwork that supports the lymphocytes
- mesenchymal origin
- large cells
- numerous cytoplasmic processes
- processes wrap around reticular fibers
types of reticular tissue in hematopoietic organs
- lymphoid tissue-free cells are largely lymphocytes
- myeloid tissue-free cells are developing erythrocytes and granular leukocytes (bone marrow)
functions of mesenchymal reticular cells
- synthesis and maintenance of the ECM
- phagocytosis
- trophic role in blood cell formation
- storage place of ferritin
- antigen processing and presentation-dendritic in both sense of the word
loose lymphoid tissue
-reticular cells
dense lymphoid tissue
-lymphocytes
nodular lymphoid tissue
- compact
- spherical
- lack a CT capsule
- consist mostly of B cells
- primary and secondary with germinal center
- not limited to lymphoid organs
secondary lymphatic nodules
- germinal center with numerous activated B cells
- antigenic stimulation
- surrounded by mantle
germinal center
- B cell:
- proliferation
- selection
- apoptosis
- differentiation
- storage
- appear during primary antigenic response and involute in about 4 weeks
lymph nodes
- in line filters of lymph system
- capsule
- trabeculae
- hilum
- medulla
- cortex
- capsule
- subcapsular sinus (made of loose lymphoid tissue)
components of the cortex
- loose lymphoid tissue:subcapsular and peritrabecular sinuses
- lymphatic nodules-primary and/or secondary contain mostly B cells
components of the medulla
- dense lymphoid tissue:medullary cords populated largely by B cells and plasma cells
- loose lymphoid tissue: medullary sinuses (numerous reticular cells)
- structures of the sinuses facilitates the filtering function of the nodes
flow through a lymph node
- afferent vessels with valves pointing in
- subcapsular sinus
- trabecular sinus
- medullary sinus
- efferent vessels valves pointing the other way
High endothelial venule
- fat cells
- lymphocytes leave the efferent vessel and come back through the arteries to HEVs
- homing addressins on HEVs with corresponding receptors on lymphocytes
- helps cells perform diapedesis to get out of BV
- found in tonsils, Peyer’s patches, but not in spleen or thymus
key molecules in lymphocyte homing
- selectins
- integrins
- carbs (sugars and mucins)
- immunoglobulin superfamily members (ICAM VCAM)
medical relevance of HEVs
- play a role in lymphocyte recirculation-contribute to specificity
- decrease in number with age (immunologic function decreases with age)
- implicated in metastasis of lymphoid malignancies
deep cortex
- aka paracortex or tertiary cortex
- located between the cortex and the medulla
- location of HEVs
- most lymphocytes are T
functions of lymph nodes
- filtration of lymph
- production and selection of B lymphocytes
- immune response to lymph born antigens
fetal thymus
- dual embryological origin- epithelial lining of 3rd and 4th brachial pouches and surrounding mesenchyme
- develops early
- maximum size within first year of life
- capsule
- trabeculae
- lobular
- cortex-dense lymphoid tissue
- medulla-loose lymphoid tissue
adult thymus
- atrophy begins by the age of 2
- much of the parenchyma is replaces by CT
principal cell types within the thymus
- thymocytes-predominate in cortex-proliferate, selection, apoptosis, some travel to other organs
- epithelial reticular cells-derived from endoderm of 3rd and 4th brachial pouches
- desmosomes and tonofilaments (mesenchymal reticular cells have no desmosome)
epithelial reticular cell
- not normally phagocytic
- do not make reticular fibers
- not antigen presenting cells
functions of epithelial reticular cells
- secretion-provide supporting framework (cytoreticulum)-thymosins, thymopoietin
- form Hassall’s corpuscles
- contribute to blood thymus barrier
thymosins
- family of polypeptides that promote T cell differentiation
- may also produce a factor that has a trophic role in the lymphoid system
Hassall’s corpuscles
- in medulla of thymus
- concentric epithelial reticular cells
- frequently keritanize of calcify
- decrease in number but increase in size with age
- may remove apoptotic thymocytes??
- may play a role in generation of regulatory T cells?????
blood thymus barrier
- epithelial reticular cells help form a barrier to the movement of macromolecules from the blood into the thymic cortex
- provides and immunologically privileged site for the differentiating thymocytes
principal cell types in the thymus
- thymocytes
- epithelial reticular cells
- mesenchymal reticular cells-black in thymus because they are phagocytes
cortex of thymus
- mostly thymocytes
- production
- selection
- apoptosis
medulla of thymus
- mostly epithelial reticular cells
- selected thymocytes begin maturation into T cells
- presence of Hassall’s corpuscles
functions of the thymus
- development of a diverse population of T cells that can respond to foreign antigens (thymosin)
- trophic role in the development of other lymphoid organs (thymopoietin)
spleen
- largest single accumulation of lymphoid tissue
- filtration of blood
- immunodefense
- white pulp (20%), capsule, red pulp (80%)
- trabeculum
red pulp
- splenic cord
- splenic sinus (discontinuous sinusoid)
- has central arteries
PALS
-periarterial lymphatic sheath
white pulp
-PALS and lymphatic nodules
marginal zone
- between white and red pulp
- most arterial blood runs through MZ
- contains macrophages, T and B cells, dendritic cells
- site of initiation of immune resonses
- blood then enters the red pulp
blood flow through the spleen
- splenic artery
- trabecular arteries
- central arteries
- penicillar arteries
- capillaries
- sinuses
- red pulp veins
- trabecular veins
- splenic vein
closed circulation
-blood flows directly from caps to sinuses
open circulation
- blood flows from caps into cords before it enters sinuses
- most blood flows this way
endothelium of splenic sinuses
- elongated cells
- spaces between them
- gaps in the BM
- highly porous
functions of the spleen
- production of lymphocytes
- immune response to blood borne antigens
- destruction of old erythrocytes (by macrophages)
- storage of blood (esp in cells and platelets)
- a reservoir of monocytes that can be rapidly released to regulate inflammation
palatine tonsil
- aggregates of lymphoid tissue lying beneath the epithelium in the upper pharynx
- pharyngeal, palatine, lingual
- epithelium, lymphoid tissue, crypt, capsule nodules
functions of tonsils
- detection and response to pathogens in the oral cavity
- production of lymphocytes
