- organs where maturation and education of lymphocytes occurs

- Bone marrow (B cells) and thymus (T cells)

Primary Lymphoid tissue

- 'peripheral' tissue

- site of antigen-driven proliferation and differentiation

(lymph nodes, spleen, MALT, lymphoid nodules)

Secondary Lymphoid Organs

- scattered lymphocytes and plasma cells

- common beneath wet epithelia

- only efferent lymph vessels present

Diffuse lymphatic tissue

- rounded aggregation of lymphocytes and plasma cells

- contains a primary and secondary nodule

Lymph nodules

- lacks a germinal center (central pale zone)

- rounded aggregation of lymphocytes and plasma cells

Primary lymph nodule

- has a pale zone (germinal center) due to large lymphocytes (lymphoblasts) present

- also contains 'follicular dendritic cells'

- the rim (mantle and marginal zones) is darker because of small lymphocytes crowding

- found in secondary lymphoid organs and inflammatory or antigenic stimulated areas

Secondary lymph nodule

Lymph node

- afferent lymph vessels empty into these sinuses immediately beneath the node capsule

- Cancer cells are first found in this region of the lymph node

Subcapsular sinus

- blood supply enters the lymph node here

- capillaries then surround the nodules in the cortex

Hilum

- site of lymphocyte re-circulation

- T-lymphocytes leave the blood circulation and enter lymphatic circulation

- cuboidal shaped endothelium

HEVs (high endothelial venules)

- region of lymph node that contains T-lymphocyte dependent zone

- inner or deep cortex

Paracortex

- allow lymphocytes to gain access to node parenchyma

Selective surface adhesion molecules

- the subcapsular sinuses empty into these sinuses in the cortex

Trabecular sinuses

- these sinuses are found in the medulla of the lymph node and funnel lymph fluid into the efferent lymph vessels in the hilum

Medullary sinuses

- Filters lymph

- site of lymphocyte proliferation & differentiation

- usually in series along lymph channels

Lymph nodes

- B lymphocytes are mainly present here

Cortex (germinal center)

- T-lymphocytes are mainly present here

paracortex (inner cortex)

- B-lymphocytes and plasma cells present here

Medullary cords

- specialized fibroblasts

- produce type III collagen forming the lymph node stroma

- produce cytokines that attract lymphocytes & dendritic cells

Reticular cells

- bone marrow-derived antigen-presenting cells

- present in the paracortex (deep cortex)

- involved in presenting antigen to T-lymphocytes

Dendritic cells (IDCs)

- reside in the follicles

- have long cytoplasmic processes to which antigen-antibody complexes adhere

- NOT antigen presenting cells

- participate in B-lymphocyte proliferation

Follicular dendritic cells (FDCs)

- found in most regions of the lymph node, prominent in germinal centers

- contain phagocytosed material

- function as antigen-presenting cells

Macrophages

- made of paired lobes subdivided into lobules, thin capsule, incomplete septa

- developed from 3rd/4th pharyngeal pouch endoderm

-regresses with age

- produces T cells and humoral factors

Thymus

- has densely packed lymphocytes undergoing selection, differentiation & maturation

- several populations of thymic epithelial cells

- macrophages are busily eating apoptotic lymphocytes

Thymic cortex

- has antigen-presenting dendritic cells

- populations of thymic epithelial cells

- Hassall's corpuscles

- Mature T-cells

Thymic medulla

- participate in formation of blood-thymus barrier

- formation of a loose meshwork ('reticular') by adjoining cytoplasmic processes

- participate in education of T-lymphocytes

- produce of a variety of cytokines and hormones

- form Hassall's corpuscles

thymic epithelial cells

- express MHC I & II

- subscapular epithelial cells (participate in the 'blood-thymus' barrier)

- nurse cells/inner cortical epithelial cells (form a loose meshwork by adjoining cytoplasmic processes)

Cortical epithelial cells

- express MHC I & II

- extensive cell processes for a network (reticulum)

medullary epithelial cells

- made up of flattened, concentrically-oriented keratinizing epithelial cells

- do not express MHC II

Hassall's corpuscles

- endothelium of capillary with tight junctions

- basal lamina of capillary

- thin connective tissue compartment (macrophages)

- basal lamina next to a continuous layer of cells

- thymic epithelial cells with tight junctions

Blood-thymus barrier

- fat replaces most of the thymic tissue at this period of development

puberty

- stem cells from marrow develop into immunologically competent T cells in thymus

- most die

- some migrate to T lymphocyte zones of secondary lymphoid sites

T cell production line

- affect the maturation of lymphocytes in other lymphatic organs

- thymosin

Humoral factors

- largest single lymph organ in the body

- filters blood (defense and immune mechanisms)

- produces lymphocytes, with the B-cells producing antibodies

- destroys worn out blood cells

- hematopoiesis in the embryo

- blood storage (enlarged --> may harbor too many platelets)

Spleen

Spleen

- stained mostly dark blue due to large # of lymphocytes

- central (follicular) arteries are present

- surrounded by a periarterial lymphocytic sheath (PALS)

- nodules are present at intervals along the central arteries where B lymphocytes prevail

White Pulp

- 75% of splenic volume

- consists of splenic (Billroth) cords and venous sinusoids

- erythrocytes are present

Red pulp

- tissue between the sinusoids and contains macrophages and erythrocytes

splenic (Billroth) cords

- lined by elongated endothelial cells

- enclosed by circular reticular fibers

Venous sinusoids

- trabecular arteries in fibrous trabeculae

- central arteries in white pulp

- penicillar (straight) arteries

- open circulation

- venous sinusoids

splenic blood supply

- branches of the central arteries

- near ends these vessels are ensheathed by a layer of macrophages, reticular and lymphatic cells

- ends called arterial capillaries

Penicillar (straight) arteries

- associated with 'wet' epithelia

- tonsils, peyer's patches, vermiform appendix

- found in digestive tract (GALT) and lower respiratory tract (BALT)

Mucosal Associated Lymphoid Tissue (MALT)

- covered with a mucosa

- consist of aggregates of nodules with efferent lymph vessels only

- MALT organs and a part of Waldeyer's ring around superior portion of the pharynx

Tonsils

- largest tonsil

- covered with stratified squamous non-keratinized epithelium

- has deep crypts

Palatine tonsil

- buried in the posterior tongue

- tongue muscle surrounds it (3 planes)

- shallow crypts

Lingual tonsil

- also called adenoid

- pseudostratified columnar epithelium (and focally squamous mucosa) covers it

- in the posterior nasopharynx

Pharyngeal tonsil

- consists of aggregates of nodules in the wall of the ileum

- covered by flattened M cells

- endocytose macromolecules and transport them to underlying lymphocytes

Peyer's Patches

- part of the lower GI tract

- can inflame and need to be removed (itis --> ectomy)

Vermiform appendix