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Flashcards in Lymphoid Organs Deck (46)
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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)

1. produced in germinal centers

2. forced to periphery of nodules

3. enter sinuses

4. leave node by way of efferent lymphatics

5. enter blood circulatory system by way of thoracic duct

6. most return to nodes by HEV

7. 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

1. play a role in lymphocyte recirculation (contribute to specificty)

2. decrease in number with age (matches immunologic function)

3. may be involved in metastasis of lymphoid malignancies (lymphomas)


reasons why recirculation of lymphocytes is critical to efficient immune surveillance

1. permits lymphocytes to encounter Ags

2. facilitates communication with other leukocyte types

3. 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

1. filtration of lymph

2. production/selection of B lymphocytes

3. immune response to lymph-borne antigens


thymus general characteristics (where, embryology, etc.)

1. situated in superior mediastinum

2. dual embryological origin

-epithelial lining of 3rd/4th brachial (pharyngeal) pouches + surrounding mesenchyme

3. develops early, and fully developed at birth (max size at 1 yo)

4. involutes with age, parenchyma replaced by fat and CT


fetal thymus structure

1. thin capsule of dense CT

2. 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

1. thymocytes - thymic lymphocytes predominate in cortex (proliferate, select, apoptose)

-populate T dependent regions of other lymphoid organs

2. 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)

1. secretion - dense secretory granules, and thymosin polypeptides that promote differentiation of T lymphocytes

-may also make thymopoietin with trophic action on lymphoid system

2. form supporting framework (cytoreticulum) of organ (has desmosomes and tonofilaments)

3. form Hassall's corpuscles - unique to thymic medulla

4. 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)