1. HISTOLOGY OF
LYMPHOID ORGANS

2. INTRODUCTION
 Immune systems consists of : 4
 Lymphoid organs
 Heterogeneous group of motile cell types
 2 components of immune systems : 3
 Innate Immune System  non spesific (complement,
macrophages & Neutrophils, Natural Killer cells/NK
cells)  nonclonal defense mechanism
 Adaptive Immune System  specific (T Lymphocytes, B
lymphocytes,Antigen Presenting Cells/APCs)

3.  Adaptive immune system : 4
 Humoral immune response : B lymphocytes
produce antibodies  phagocytosis & digestion
of bacteria by macrophages & neutrophils
leukocytes
 A cell-mediated immune response :T lymphocytes
bind to surface of parasites or virus-infected cells
 lyse them by secreting a membrane-disrupting
protein & a Hydrolytic Enzyme

4. Lymphoid organs : 1, 3
• Primary/central organs :Thymus
& Bone Marrow  responsible for
development & maturation of
lymphocytes
• Secondary/Pheripheral organs :
Lymph nodes, Spleen,Tonsils,
solitary nodules, Peyer’s Patches
of ileum, Appendix

5. Origin of Immune System
Cells
 Pluripotent hematopoetic stem cell in bone
marrow growth factors stimulation 
proliferation and maturation of the cells 
formed elements of the blood
 Stem cell  myeloid progenitor cell & lymphoid
progenitor cells
 Lymphoid progenitor cells  B lymphocytes &T
lymphocytes

6. Growth factors on hematopoietic
system
 G-CSF, GM-CSF, M-CSF, IL- 1, IL-3, IL-4, IL-6, EPO,
TPO etc.
 Cytokines are a unique family of growth factors 
messenger molecules that can communicate
signals from one cell type to another
 Secreted primarily from leukocytes but also produced by
various cells of the body  interleukin (IL)
 IL instruct the receiving cells to proliferate, differentiate,
secrete additional cytokines, migrate or die
 IL Stimulate both the humoral and cellular immune
responses, as well as the activation of phagocytic cells
 The list of identified interleukins grows continuously

7. Origin of the main types of
lymphocytes. B lymphocytes and
natural killer lymphocytes are
formed in the bone marrow and
leave the bone marrow already
mature, to seed the secondary
lymphoid organs and transit
through the blood, epithelia, and
connective tissues. Immature
CD4– and CD8–T lymphocyte
precursors are transported by the
blood circulation from the bone
marrow to the thymus, where
they complete their maturation
and leave as either CD4+ or CD8+
cells.

8. CLONAL SELECTION OF
LYMPHOCYTES
 In bone marrow &Thymus  primary lymphoid
organ
 Single type receptor on Lymphocytes can
recognize all possible antigens  but self
tolerance
 Lymphocytes with receptors not self tolerance
are eliminated by apoptosis  clonal deletion

9. B Lymphocytes Maturation
 Bone marrow  Lymphoid stem cell  Pro B cell
 Pre B Cell  Immature/naive B cells (Ig M)
mature B Cells (Ig M + Ig D)  blood stream &
circulate  secondary lymphoid organ
 Proliferation and maturation of B-cell responses are
mediated by cytokines

10. T Cells Maturation
 Fetal liver/bone marrow  PreT Cell 
migrate toThymus  Stage 1:T cells with CD
4- & CD8- (double negative)  Stage 2:T Cells
with CD 4+ & CD 8+ (Double positive)  Stage
3 : matureT Cell with CD4+ or CD 8+ (single
positive)

11. Lymphoid
organs
T lymphocytes
%
B Lymphocytes
%
Thymus 100 0
Bone marrow 10 90
Spleen 45 55
Lymph nodes 60 40
Blood 80 20
Approximate percentage of lymphocytes in
lymphoid organs1

12. THYMUS
 In superior mediastinum 1, 2
 2 lobes Thin capsules  septa 
subdivide into incomplete lobules 1, 3
 Each lobule consist of cortex &
medulla:1,3
A.Cortex :
• Darker than medulla  due to
large number of T lymphocytes
• Also contain macrophages &
Epithelial Reticular Cells
• 95-98% of developingT cells die
by apoptosis in cortex 
phagocytosed by macrophages

13. B. Medulla : 1, 3
– Stain lighter than cortex  less
T cells population & large
number of epithelial reticular
cells
– 3 types of epithelial reticular
cells in medulla :
o Type IV cells
o Type V cells
o Type VI cells  Hassl’s
Body / Thymic Corpuscle
(found only in medulla,
cornified, even calcified,
unknown function)

14. Thymus vascular supply1, 3
 Blood-thymus barrier  formed by continuous
cappillaries in cortex with thick basal lamina,
invested by sheath of type I epithelial reticular cells
 preventing contact of developingT Cells to
blood-borne macromolecules
 Self macromolecules crossed barrier  to select &
eliminateT cells react with self antigens  clonal
selection & clonal deletion
 No barrier in medulla
 T cells leave medulla via veins drainning the
thymus

15. Hormones in thymus 1, 3
 Epithelial reticular cells produce :
 Thymosin
 Thymopoietin
 Thymulin
 Thymic humoral factor
 Facilitate T cell proliferation & expression of surface
markers
 Other hormones influenceT cells maturation :
 Corticosteroids  decreaseT cells number in cortex
 Thyroxin  stimulates epithelial reticular cells to increase
thymulin production
 Somatotropin  promotesT cells development in thymus
cortex

16. THYMUS INVOLUTION4
 Start after puberty
 Parenchym replaced adipose
tissue and connective tissue
 Decrease weights : 40 g at
puberty, 10-15 g late in life
 After involution, thymus still
has its function as a maturation
place forT cells

17. LYMPH NODE
 Kidney shape, encapsulated
(capsul of Conn.Tissue
Trabeculae)
 Location : neck, axilla, scrotum,
blood vessels in thorax, etc 1, 2
 Have Afferent lymph vessel &
Efferent lymph vessel 1
 Hilum : concave depresion
which arteries & nerve enter,
veins & lymphatic vessels
leave1,2
 Parenchym composed ofT cells,
B cells, APCs & macrophages3

18.  On average, naive lymphocyte spend less than
½ hour in circulation before homing to another
lymphoid organ
 2 main ports of entry into Lymph Node :
 By High EndothelialVenule (HEV)
 Specialized type of post capillary venule, lined by
cuboid or high endothelial cells
 Found only in secondary lymphoid organs except
spleen
 Main site of B &T lymphocytes entry from blood  by
diapedesis
 By afferent lymph vessel  Site of some memory
cells, free antigens & or antigen-loaded APC

19. BLOOD & LYMPH CIRCULATION OF LYMPH NODE

20. CORTEX 1, 2
o Outer Cortex
 Lymphoid nodules
 B cells  imunocytes
 Germinal center/secondary
nodules  only in response
of antigenic challenge
 Reticular cells & fiber
o Inner Cortex/Paracortical Area 
T cells activated & proliferated
o Subcapsular Sinus &
intermediate/Peritrabekular
Sinus

21. Section of a lymph node showing the cortex
and the medulla and their primary
components. B: (1) Capsule; (2) lymphoid
nodule with germinative center; (3) subcapsular
sinus; (4) intermediate sinus; (5) medullary
cords; (6) medullary sinus; (7) trabecula. H&E
stain. Low magnification. (Courtesy of PA
Abrahamsohn.)
Section of a portion of the outer cortex of a
lymph node showing the capsule,
subcapsular sinuses, diffuse lymphoid
tissue, and lymphatic nodules. H&E stain.
Medium magnification. (Courtesy of PA
Abrahamsohn.)

22. MEDULLA : 1, 2, 3
Medullary Cords :
 B cells, plasma cells,
macrophages
 Reticular cells & fiber
 More irregular
trabeculae than in
cortex
Medullary Sinus 
continue with subcapsular
sinus & intermediate sinus
end up in efferent
lymph vessels

23. SPLEEN
 Largest lymphoid organ in body 3
 Hilum
 Capsul  trabeculae
 Consist of : 1, 2, 3
A. White Pulp :
 Formed by :
 Lymphoid nodules  B cells
 Peri Arterial Lymphatic
Sheath/PALS  formed by
T cells surroundingA.
Centralis
 Lymphoid nodules  germinal
centre due to antigenic
challenge

24. B. Marginal zone 3
 Separate white pulp to red pulp
 Composed of plasma cells,T cells, B cells,
macrophages,APCs
 Marginal sinuses
 Contain an abundance of blood antigens 
plays major role in immunologic activities of
spleen

25. C. Red Pulp :1, 3
• Consist of :
 Splenic Cords / Billroth’s
Cords  macrophages,T
cells, B cells, plasma cells,
blood cells
 Splenic Sinusoids :
 Endothelial cells
fusiform, elongated
 Discontinuous basal
lamina

26. Blood Circulation of Spleen

27. TONSILS
 Incompletely encapsulated aggregates of
lymphoid nodules 1
 Based on location : palatine, pharyngeal, lingual
tonsils1
 Produce lymphocytes1

28. PALATINE TONSILS
 A pair, in pars oralis
pharynx1
 Consist of : 1, 2, 3
 Stratified squamous
Epithelium
 A band of lymphoid nodule
with germinal center
 Crypts :
 Invagination of epithelium
 10-20 crypts/tonsil
 Contain food debris, dead
leucocytes, desquamated of
epithelial cells,bacteria etc
 Capsule  partially at the
base
The palatine tonsil consists of diffuse lymphocytes and
lymphoid nodules disposed under a stratified squamous
epithelium. One of the crypts of the tonsil is shown; the
crypts often contain dead epithelial and inflammatory cells.
B: (1) Crypt; (2) stratified squamous epithelium; (3)
lymphoid nodules; (4) diffuse lymphoid tissue; (5)
germinative center; (6) capsule; (7) mucous glands.
Hematoxylin and eosin (H&E) stain. Low magnification.
(Courtesy of PAAbrahamsohn.)

29. PHARYNGEAL TONSILS
 Single in posterior nasopharynx1, 2
 Consist of :1, 2, 3
 Pseudostratified ciliated columar epithelium
 Lymphoid nodules
 No crypts, only shallow longitudinal infolding called
pleats
 Thinner capsule thanT. Palatina

30. LINGUAL TONSILS
 Smaller & more numerous than other tonsils
 At base of tongue
 Consist of :1, 2, 3
 Stratified Squamous Epithelium
 Lymphoid nodules  germinal center
 Each lingual tonsils has a single crypts

31. MUCOSA-ASSOCIATED LYMPHOID TISSUE /
MALT3
Section of lung showing a collection of
lymphocytes in the connective tissue of
the bronchiolar mucosa, an example of
mucosa-associated lymphoid tissue
(MALT). Pararosaniline—toluidine blue
(PT) stain. Low magnification.
 Non capsulated
 Lymphoid nodules in
mucosa or submucosa of
GI tract, respiratory tract,
urinary tract.
 Gut-Associated lymphoid
tissue (GALT)  peyer’s
patches (B Cells surround
byT cells & APCs)
 Bronchus-associated
lymphoid tissue (BALT) 
similar to peyer’s patches

32. REFERENCES :
1. Basic Histology Text & Atlas , 10th ed. , L. Carlos
Junquira MD, Jose Carneiro MD, Robert O. Kelley PhD,
Lange Medical Books, Mc Graw-Hill , 2003. Pp 265 –
290.
2. Essentials of Human Histology, 2nd Edition, William J.
Krausse PhD, Little Brown & Company (Inc), 1996. Pp
197-228
3. Color Textbook of Histologi, 2nd edition, Gartner LP, Hiatt
JL, WB Saunders Company, Philadelphia, Pennsylvania,
2001. Pp 273-299
4. Consise Histology, 2nd edition, Don W Fawcett, Ronald P
Jensh, Arnold publisher, London, 2002. Pp 148-161