Lymphoid Organs Flashcards
what are the primary and secondary lymphoid organs?
Primary: bone marrow, thymus (production of lymphocytes)
Secondary: lymph nodes, spleen, tonsils, Peyer’s patches (microenvironment for interactions of lymphocytes and Ags)
all important components of immune system, where responses are largely initiated and generated
extralymphoid tissues/organs of immune system
- aggregates of lymphoid tissue in non-lymphoid organs: GALT (digestive system), respiratory tract, urinary tract, reproductive tract, SALT (skin)
- lymphocytes of blood/lymph
- wandering lymphocytes
lymphocyte reticulation
from blood to lymphoid organs back to blood
- may have to go through lymph nodes after the initial lymphoid organ, leaving thru thoracic duct back to circulation
- facilitates immune surveillance
- ensures rapid response to Ag
- connects the three components of immune system (primary, secondary, extralymphoid)
reticular connective tissue and two common types
contains cells and fibers (type III collagen)
- forms sponge-like meshwork that supports lymphocytes
1. lymphoid tissue - free cells are largely lymphocytes
2. myeloid tissue - free cells are developing erythrocytes and granular leukocytes (bone marrow)
reticular (dendritic) cells
in reticular lymphoid tissue
- large cells of mesenchymal origin that maintain ECM
- have numerous cytoplasmic processes that wrap around reticular fibers
- acts as macrophages, ferritin storage, and Ag processing and presentation
- have trophic role in blood cell formation
reticular fibers
component of lymphoid reticular tissue
-seen in H&E preps, PAS+, reduced in silver salts, , made of type III collagen
types of lymphoid tissue
loose: open meshwork of cells and fibers, with numerous fixed (reticular) cells
dense: denser meshwork of cells and fibers, with numerous free (lymphocyte) cells
nodular: spherical aggregates of mostly B lymphocytes (lymphatic nodule)
- lacks CT capsule, but compact and spherical
- both primary and secondary nodules
primary nodular tissue
unstimulated (not under antigenic challenge)
-tightly packed small lymphocytes
secondary lymphatic nodular tissue
under antigenic stimulation with germinal centers
- dark zones: B lymphocytes (proliferation via clonal expansion)
- light zone: non-dividing B lymphocytes (selection, apoptosis, differentiation), T helper cells
- mantle zone: young plasma cells and memory B cells (temporary storage)
- appear during primary antigenic response, involute in about 4 weeks
also have framework of follicular dendritic cells and reticular fibers
lymph node general characteristics
kidney-shaped w/ consex (afferent lymphatic vessels) and concave sides (hilus, efferent lymphatic vessels)
- capsule of dense irregular CT
- CT trabeculae invaginate and provide structural support, subdivide node into incomplete compartments
cortex of lymph node
outer, more densely stained region
- loose lymphoid tissue (subcapsular and peritrabecular sinuses)
- lymphatic nodules (may have germinal centers, but contain mostly B lymphocytes)
medulla of lymph node components
dense lymphoid tissue (medullar cords)
-mostly B lymphocytes and plasma cells
loose lymphoid tissue (medullar sinuses)
- numerous reticular cells
- communicate with peritrabecular sinuses and efferent lymphatics
- have permeable walls that permit free passage of wandering cells
flow of lymph through a lymph node
for immune and filtration functions
- afferent lymphatics (enter from multiple sites on convex surface; unique to lymph nodes)
- subscapular sinus
- peritrabecular (intermediate) sinuses
- medullar sinuses
- efferent lymphatics (leave node at hilus)
both entry/exit have valves to ensure one-way lymph flow
filtration of lymph
occurs in sinuses
lymph moves slowly
reticular cells are phagocytes
-failure will faciilitate spread of infection and/or metastases
lymphocyte circulation (not the same as lymph flow)
- produced in germinal centers
- forced to periphery of nodules
- enter sinuses
- leave node by way of efferent lymphatics
- enter blood circulatory system by way of thoracic duct
- most return to nodes by HEV
- recirculation is critical to efficient immune surveillance
high endothelial venules
HEV - how 90% of lymphocytes return to circulation (other 10% by afferent lymphatic vessels)
- lined by tall endothelial cells w/ vascular addressins on their surface
- small lymphocytes have “homing receptors”
- other cell adhesion molecules (selectins, integrins, CHO, clever-1, Ig superfamily) involved in diapedesis
- seen in tonsils, lymph nodes and Peyer’s patches, but not spleen/thymus
medical relevance of HEVs
- play a role in lymphocyte recirculation (contribute to specificty)
- decrease in number with age (matches immunologic function)
- may be involved in metastasis of lymphoid malignancies (lymphomas)
reasons why recirculation of lymphocytes is critical to efficient immune surveillance
- permits lymphocytes to encounter Ags
- facilitates communication with other leukocyte types
- allows targeting of lymphocytes
paracortical zone of lymph nodes function
also deep/tertiary/inner cortex
- between cortex and medulla, where most HEVs are
- under thymic influence, so mostly T lymphocytes
- made of deep cortex units
–semi-rounded structures contiguous with cortex and bulging into medulla
–centered on opening of afferent lymphatic, and sometimes used to form complexes
paracortical zone’s deep cortex structure (central and peripheral portions)
- central portion with few reticular fibers, high concentration of small lymphocytes, and site of cellular storage and proliferation
- dense framework of reticular fibers, with fewer lymphocytes
–the site of HEVs and rapid migration of lymphocytes
functions of lymph nodes
- filtration of lymph
- production/selection of B lymphocytes
- immune response to lymph-borne antigens
thymus general characteristics (where, embryology, etc.)
- situated in superior mediastinum
- dual embryological origin
- epithelial lining of 3rd/4th brachial (pharyngeal) pouches + surrounding mesenchyme - develops early, and fully developed at birth (max size at 1 yo)
- involutes with age, parenchyma replaced by fat and CT
fetal thymus structure
- thin capsule of dense CT
- CT septae (trabeculae) subdivide into 2 main lobes and numerous lobules (have cortex and medulla) with continuity
thymus cortex
peripheral, more darkly staining zone
- dense lymphoid tissue
- lymphocytes predominate
- site of thymocyte proliferation, apoptosis, and selection
thymus medulla
central, more lightly staining zone
- loose lymphoid tissue
- rich in epithelial-reticular cells
- site of thymocyte maturation
principle cell types w/in the thymus
- thymocytes - thymic lymphocytes predominate in cortex (proliferate, select, apoptose)
- populate T dependent regions of other lymphoid organs - epithelial reticular cells - only from thymus, not phagocytic, don’t manufacture reticular cells, and not APCs
- originate in endodermal lining of 3rd/4th branchial pouches (desmosomes and tonofilaments)
functions of epithelial reticular cells
(in thymus)
- secretion - dense secretory granules, and thymosin polypeptides that promote differentiation of T lymphocytes
- may also make thymopoietin with trophic action on lymphoid system - form supporting framework (cytoreticulum) of organ (has desmosomes and tonofilaments)
- form Hassall’s corpuscles - unique to thymic medulla
- contribute to blood-thymus barrier - protects cortical lymphocytes from circulating Ags, providing immunologically priveileged site for differentiating thymocytes
Hassall’s corpuscles
unique to thymic medulla
- concentrically arranged epithelial reticular cells that frequently keratinize or calcify
- decrease in number and increase in size with age
- probably degenerated structures, that may remove apoptotic thymocytes or generate regulatory T cells?
mesenchymal reticular cells (dendritic cells)
only a small fraction of thymic stroma
- identical to reticular cells of lymph nodes
- phagocytic for T-cell selection debris, so appear black
types of cells in thymus cortex VS thymus medulla
C: mostly thymocytes (production, selection, apoptosis)
M: mostly epitheial reticular cells (selected thymocytes begin maturation into T lymphocytes, and presence of Hassall’s copuscles)
functions of the thymus
- development of T diverse lymphocytes that respond to foreign Ag (thymosin)
- trophic role in development of other lymphoid organs (thymopoietin)
general characteristics of spleen
largest single accumulation of lymphoid tissue
- complex filter for blood interposed in bloodstream
- immunodefense
- dense, irregular CT capsule with branching network of trabeculae from capsule (provides support and contains blood vessels, small amount of smooth muscle)
splenic pulp components
red pulp (80% of splenic parenchyma)
white pulp (20% of splenic parenchyma), stains more purple from lymphocytes
splenic red pulp
- contains large quantity of blood
- made up of splenic sinuses (discontinuous sinusoid) and splenic cords
splenic sinuses of splenic red pulp
- long, irregular vascular channels
- elongate endothelial cells with spaces between them
- discontinuous basement membrane
- macrophages are on the outside surface of basement membrane (w/in cords)
- allow free exchange between cords and sinuses
splenic cords of splenic red pulp
- plates of cellular tissue lying between sinuses
- site of destruction of RBC
- site of storage of blood
splenic white pulp
- composed of lymphoid tissue
- periarterial lymphatic sheath (PALS) - surrounds major arterial branches (central arteries) as T-dependent area
- lymphatic nodules - within PALS have mostly B lymphocytes, and may or may not have germinal centers
marginal zone of spleen
- found at junction of red and white pulp
- most arterial blood runs thru marginal zone
- contains lymphocytes, dendritic cells, and macrpohages
4. site of initiation of immune responses to blood-borne Ags
- blood enters red pulp from here
blood flow thru spleen
- splenic artery (enters at hilus)
- trabecular arteries
- central arteries (arterioles in periarterial lymphatic sheaths and nodules to supply lymphatic tissue)
- penicillar arteries (straight branches into red pulp)
- capillaries (open/closed theories of splenic circulation)
- splenic sinuses
- red pulp veins
- trabecular veins
- splenic veins (leaves spleen at hilus)
open/closed theories of splenic circulation
Closed: blood flows directly from capillaries to sinuses (faster)
Open: blood flows from capillaries into cords into sinuses (slower, but the one most blood follows)
splenic sinuses
wide vascular channels with splaces between endothelial cells
- discontinuous basement membrane (gaps) make it highly porous
- macrophages are outside basement membranes (w/in cords)
- free exchange between sinuses and cords
functions of spleen
- production of lymphocytes (in germinal centers of 2ary lymphatic nodules)
- immune response to blood-borne Ags (dentritic cells in marginal zone trap Ags, process and present to lymphocytes, which migrate to nodules (B) or PALS (T) to differentiate)
- destruction of old RBCs (by macropahges)
- storage of blood, esp. cells and platelets (spongy nature of red pulp is important)
- reservoir of monocytes can be rapidly released to regulate inflammation
tonsils generalized characteristics and functions
- aggregates of lymphoid tissue that lies beneath an epithelium in upper pharynx (aperture thru which oral cavity communicates with pharynx)
- consist of lymphatic nodules (with germinal centers) embedded in dense lymphoid tissue
- reach maximal development in childhood
Functions: detect and respond to pathogens in oral cavity, and produce lymphocytes
pharyngeal tonsils (adenoids)
located in roof of nasopharynx
- covered by pseudostratified ciliated columnar (respiratory) epithelium
- characterized by presence of numerous surface folds (pleats) rather than deep crypts
- surrounded by thin capusle of dense, irregular CT
palatine tonsils
- located in lateral wall of oropharynx
- covered by stratified squamous epithelium often infiltrated by lymphocytes
- epithelium invaginates to form numerous deep crypts
- crypts contain desquamated epitheial cells, lymphocytes, and bacteria (detrius)
- surrounded by thick capsule of dense, irregular CT
lingual tonsils
- located at base of tongue
- smaller and more numerous than other tonsils
- covered by stratified squamous epithelium
- each lingual tonsil has a single large crypt, usually filled by detrius
- often associated with mucous glands and skeletal muscle of tongue
