Primary: organs where the maturation and education of lymphocytes occurs, e.g. Bone marrow (B cells) and thymus (T cells)

Secondary: lympoid organs (and tissues) everywhere else; they are 'peripheral', are the sites of antigen-driven proliferation and differentiation, e.g. lymph node, spleen, and diffuse lymph tissues such as (lymph nodules, MALT, etc.)

Lymph nodule/lymphoid follicle: collection of lymphocytes and plasma cells; found anywhere there is
antigenic stimulation

Lymph node: anatomical structure, ball-shaped organ of the immune system, distributed widely throughout the body and linked by lymphatic vessels

Lymphoid node/follicle (as opp. to lymph node)

Rounded concentrations of lymphoid tissue that has no capsule

Primary nodule: lacks germinal center (central pale zone)

Secondary nodule: has germinal center, pale because lymphocytes (lymphoblasts) are present

Germinal center (pale area): (1) morphologic indication of antigenic stimulation

(2) Where B-cell clonal expansion occurs

Lymph Node

ball-shaped organ of the immune system; garrisons of B, T, and other immune cells. filters lymph; site of lymphocyte proliferation and differentiation; usually in series along lymph channels

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HEVs (High endothelial venules) - postcapillary venules in the paracortex, that have a high, cuboidal endothelium;

• site of lymphocyte re-circulation, where T-lymphocytes leave the blood circulation and enter the lymphatic circulation.(blood-> lymph)
  
• "paracortical or T-lymphocyte" dependent zone; aka "inner" or "deep" cortex
• recirculation process
  • selective surface adhesion molecules on the endothelial cells of the HEVs allow lymphocytes to gain access to node parenchyma, then they exit the efferent lymph vessels

Reticular cells- Fibroblasts making type III collagen

Macrophages- Phagocytes & antigen-presenting cells

Follicular Dendritic Cells- Present in germinal centers associated with B cells; have long processes with adherent Ag-Ab complexes; lack MHC II & thus are not APCs

(Interdigitating) Dendritic cells- Bone-marrow derived antigen-presenting cells, associated with T-cells

Thymus -

paired lobes, subdivided into lobules, and thin capsule is present, with incomplete septa

Embryonic deriv: 3rd pharyngeal pouch and part of 4th

Cortex: (1) densely packed lymphocytes undergoing selection, differentiation, & maturation

(2) several populations of thymic epithelial cells

(3) macrophages (busily phagocytosing apoptotic lymphocytes)

Medulla: (1) antigen-presenting dendritic cells

(2) more populations of thymic epithelial cells

(3) hassall's corpuscles

(4) mature T-cells, undergoing more selection, not as densely packed as in the cortex

Thymus -

paired lobes, subdivided into lobules, and thin capsule is present, with incomplete septa

Functions: T-cell production and produce humoral factors (thymosin) which affect the maturation of lympocytes in other lymphatic organs

Embryonic deriv: 3rd pharyngeal pouch and part of 4th

Cortex: (1) densely packed lymphocytes undergoing selection, differentiation, & maturation

(2) several populations of thymic epithelial cells

(3) macrophages (busily phagocytosing apoptotic lymphocytes)

Medulla: (1) antigen-presenting dendritic cells

(2) more populations of thymic epithelial cells

(3) hassall's corpuscles

(4) mature T-cells, undergoing more selection, not as densely packed as in the cortex

Thymic epithelial cells: have desmosomes and tonofibrils (keratin intermediate filaments), but not reticular cells in the sense of modified fibroblasts

Functions: (1) formation of blood-thymus barrier; (2) formation of a loose meshwork (reticular) by adjoining cytoplasmic processes (not type III collagen, however); (3) partipation in education of T-lymphocytes; (4) production of variety of cytokines and hormones; (5) Formatoin of Hassall's corpuscles, function unknown

include

Thymocytes (Lymphocytes undergoing maturation as T-cells)
Macrophages
Dendritic cells (Bone marrow-derived APCs)
Thymic epithelial cells:
*participate in thymocyte development
*participate in “blood-thymus” barrier
*produce thymic hormones & cytokines
- Cortical epithelial cells

Subcapsular epithelial cells: Express MHC I & II; have tight-junctions, participate in blood-thymus barrier;
Inner cortical epithelial cells (nurse cells): Express MHC I & II; form an interconnected network and partially surround developing thymocytes, providing a specialized microenvironment

- Medullary epithelial cells

Medullary epithelial cells: Express MHC I & II; extensive cell processes for a network (reticulum);
Hassall’s corpuscle cells: made up of flattened, concentrically-oriented kertainizing epithelial cells (forming round balls of keratinizing cells); Lack MHC II; are keratinized

Blood thymus barrier:

(1) endothelium of capillary with tight junctions

(2) basal lamina of capillary

(3) thin connective tissue compartment (macrophages)

(4) basal lamina next to a continuous layer of thymic epithelial (epithelial reticular) cells

(5) thymic epithelial cells with tight junctions

children: Thymus is most fully developed in .

Puberty:  involution occurs, and fat replaces most of the thymic tissue;

adulthood: the thymus is almost all fat, with scattered hassall's corpuscles and a little lymphatic tissue left to show that it is a thymus

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Spleen

Functions: filters blood; produces lymphocytes, w/ B-cells producing Abs; Destroys worn-out cells; hematopoiesis in the embryo; blood storage - only a minor function in healthy humans (enlarged spleens may sequester clinically significant volumes of platelets)

White pulp - fresh state appearance, staining dark blue in H&E due to lymphocytes; central arteries (follicular arteries) are present; they are surrounded by a periarterial lymphocytic sheath (PALS). T-lymphocytes are most abundant in the PALS. Nodules are present at intervals along the course of the central arteries and here B-lymphocytes prevail.

Red pulp (75% of volume) - consists of splenic cords (tissue between the sinusoids and contains many macrophages and erythrocytes) and venous sinusoids; elongated endothelial cells line the sinusoids and are enclosed by circular reticular fibers (the barrel analogy with staves surrounded by hoops).

Blood supply  - (1) Trabecular arteries - larger arteries in fibrous trabecule

(2) central arteries in white pulp - the PALS and lymph nodules surround these

(3) Penicillar (straight arteries) - These are branches of the central arteries; Near their ends these vessels (really arterioles) are ensheated by a layer of macrophages, reticular cells, and lymphatic cells. At their ends the penicillar arteries are called arterial capillaries.

Open circulation - arterial capillaries emptying into splenic cord spaces; blood passing between endothelial cells of sinusoids to get back to circulation (as opp. to closed circulation)

Venous sinusoids - elongated endothelial cells; from them, the blood courses to pulp (connecting) venules and veins which then enter the trabecular veins

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Palatine tonsils

- - Largest of the tonsils
- - Covered by stratified squamous mucosa
- - Deep crypts

All tonsils are...

(1) covered with mucosa and consist of aggregates of nodules with efferent lyph vessels only. The epithelium is often infiltrated with lymphocytes

(2) MALT organs and are part of a collection of MALT tissues collectively called Waldeyer's ring, because they are in a vaguely ring-like arrangement around the superior portion of the pharynx

Pharyngeal tonsils (adenoid)

- - Small, shallow crypts
- - Pseudostratified ciliated columnar
(respiratory epithelium) –squamous metaplasia is common

All tonsils are...

(1) covered with mucosa and consist of aggregates of nodules with efferent lyph vessels only. The epithelium is often infiltrated with lymphocytes

(2) MALT organs and are part of a collection of MALT tissues collectively called Waldeyer's ring, because they are in a vaguely ring-like arrangement around the superior portion of the pharynx

Lingual Tonsils

- - Small, shallow crypts
- - Covered by stratified squamous mucosa with underlying tongue muscle

All tonsils are...

(1) covered with mucosa and consist of aggregates of nodules with efferent lyph vessels only. The epithelium is often infiltrated with lymphocytes

(2) MALT organs and are part of a collection of MALT tissues collectively called Waldeyer's ring, because they are in a vaguely ring-like arrangement around the superior portion of the pharynx

Payers Pathces

MALT tissue consisting of aggregates of nodules in the wall of the hileum; epithelial covering is not simple columnar of the intestine, but flattened M cells (for microfold) which endocytose macromolecules and transport them to the underlying lymphocytes