The document summarizes the key secondary lymphoid organs and tissues - lymph nodes, spleen, mucosal-associated lymphoid tissue (MALT), and their roles in the immune system. Lymph nodes filter lymph and initiate immune responses. The spleen filters blood and mounts immune responses. MALT includes structures like the tonsils, Peyer's patches, and appendix that sample antigens in mucosa. Together these secondary lymphoid organs help the immune system recognize and respond to pathogens.

1. SECONDARY
LYMPHOID ORGANS
Supervisor: Dr. Jaishanker pillai H P M.Sc.,M.Phil.,B.Ed.,Ph.d.,FBSS.
YHU microbiology faculty
Name : Ovya pugalenthi aruna
Roll no: 16
Date: 9/11/2020
Subject: microbiology
Year: 3rd year general medicine

2. CONTENT
Introduction
Lymph nodes
Spleen
Mucosal Associated-lymphoid tissue
Summary
Conclusion
Bibliography

3. INTRODUCTION
The secondary lymphoid organs are the sites
where the lymphocytes localise, recognise foreign
antigen and mount immune response against it.
Secondary lymphoid organs include lymph nodes,
spleen, tonsils, appendix, and clumps of
lymphoid tissue in small intestine called payer’s
patches.

4. LYMPH NODES
Bean-shaped
Generally only 10 mm by 2.5 cm in size
Nodes are encapsulated structures positioned along lymphatic
vessels to filter lymph and facilitate antibody production.
Its the major site where APC’s present antigen to activate T
cells and where T cells helps B cells undergo immunoglobulin
class switching.
Appears in human foetus in the third month.

5. STRUCTURE OF LYMPH NODE
Made up of 3 regions
1.Outer cortex = B cells, macrophages,
follicular dendritic cells.
2.Middle paracortex = T cells, macrophages,
dendritic cell.
3.Inner medulla = both B tells and T cells,
plasma cells, macrophages.

6. Antigen exposure
Primary follicle becomes
secondary follicle
Contains germinal center
Follicular dendritic cells
Activate B cells to differentiate
into plasma cells and memory
cells
Medullary cords = lymphocytes
arranged along strands of
connective tissue.
Medullary sinuses = medullary cords
separating large sinuses that
contain plasma cells
Site where T cells responses to lymph-
born antigens are initiated.

7. Circulation in lymph node
Afferent lymphatics
Subcapsular sinus
Trabecular sinus
Medullary sinus
Efferent lymphatics
Blood enters through an
artery at the hilum
Arterioles branch from
hilar artery to feed into
capillary beds
Capillary beds are drained
by high endothelial venules
HEVs drain into hilar vein
L
Y
M
P
H
A
T
I
C
B
L
O
O
D

8. MEDICAL APPLICATION
Neoplastic proliferation of
lymphocytes, producing a
malignant lymphoma, may
occur diffusely but is often
located in one or more lymph
nodes. Such growth can
completely obliterate the
normal architecture of the node
and convert it to an enlarged,
encapsulated structure filled
with lymphocytes, a condition
called lymphadenopathy.

9. Metastatic cancer cells detached from a primary
tumor can enter lymphatics and are carried to nearby
lymph nodes, especially the sentinel lymph node
that is the first one downstream of the region with the
tumor. Cells from well- established tumors are often
immunosuppressive themselves and may continue
growth as a secondary tumor within lymph nodes.
During cancer surgery lymph nodes in the lymphatics
draining the tumor area are examined by pathologists
for the presence of cancer cells. The presence of such
metastatic cells in lymph nodes is a key determinant in
most staging systems for various types of cancer and
an important prognostic indicator.

10. SPLEEN
Filters the blood
Only lymphoid organ which is not supplied with
lymphatic vessels.
Located high in the left upper quadrant of the abdomen.
Surrounded by a capsule that penetrates into the tissue as
septa called tubecula.
Divided into Red Pulp and White Pulp

11. Monitoring antigens in blood
Proliferation of lymphocytes
Production of antibodies
Immune Functions Of the Spleen
Hematopoietic functions of the spleen
Formation of blood cells in fetal life
Removal and destruction of RBCs & platelets.
Retrieval of iron from RBC hemoglobin

12. WHITE AND RED PULP
White pulp, only 20% of the spleen, is secondary
lymphoid tissue associated with small central
arterioles that are also enclosed by periarteriolar
lymphoid sheaths (PALS) of T cells.
Red pulp, which filters blood, removes defective
erythrocytes, and recycles haemoglobin iron,
consists of splenic cords with macrophages and
blood cells of all kinds and splenic sinusoids.

13. Longitudinal section of white pulp (W) in
a PALS surrounding a central arteriole.
Surrounding the PALS (periarteriolar
lymphoid sheaths) is much red pulp(R).
A
A large nodule with a germinal centre
forms in the PALS and the central arteriole
is displaced to the nodule’s periphery.
Small vascular sinuses can be seen at the
margin between white (W) and red (R)
pulp.
B

14. The splenic red pulp is composed entirely
of sinusoids (S) and splenic cords (C),
both of which contain blood cells of all
types.
The cords, often called cords of
Billroth, are reticular tissue rich in
macrophages and lymphocytes.
The sinusoids (S) are lined by endothelial
cells with large nuclei bulging into the
sinusoidal lumens.
The unusual endothelial cells are called
stave cells and have special properties
that allow separation of healthy from
effete red blood cells in the splenic cords
(C).

15. SPLENIC CIRCULATION
Closed and open circulation

16. Medical application
Enlargement of the spleen,
splenomegaly, can occur from a variety
of causes, including lymphoma or other
malignant growth, infections such as
mononucleosis, or sickle cell disease and
other types of anaemia.
The splenic capsule is relatively thin, and
an enlarged spleen is susceptible to
traumatic rupture, a potentially life-
threatening occurrence due to loss of
blood into the abdominal cavity.
Such rupture may require prompt surgical
removal of the spleen, splenectomy, after
which most functions of the organ are
carried out by other lymphoid organs, with
erythrocyte removal occurring in the liver
and bone marrow.

17. TEST YOUR
KNOWLEDGE 1

18. 1. Which of the following is not a immune
function of the spleen?
A. Monitoring antigens in blood
B. Production of antibodies
C. Removal and destruction of
RBCs & platelets.
D. Proliferation of lymphocytes
Ans: c

19. TEST YOUR
KNOWLEDGE 2

20. 1. Recycling of iron and heme, the major complex
containing iron, occurs most actively in which lymphoid
organ(s)/tissue(s)?
A. Lymph nodes
B. Peyer’s patches
C. Tonsils
D. Spleen
E. Lymphatic vessels
Ans : d

21. TEST YOUR
KNOWLEDGE 3

22. 1. A 12-year-old African-American girl presents with anemia and a large
percentage of her peripheral erythrocytes appear sickle-shaped.
Genetic testing reveals homozygosity for sickle cell disease. In which
of the following sites will the abnormal RBCs be removed from the
circulation?
A. Thymic cortex
B. Medullary sinuses of lymph nodes
C. Periarteriolar lymphoid sheathes of splenic
white pulp
D. Thymic medulla
E. Splenic cords (of Billroth)
Ans : e

23. MUCOSAL-ASSOCIATED LYMPHOID TISSUE
GALT includes the tonsils, adenoids, diffuse
lymphoid areas along the gut, and specialised
regions in the intestine called payer’s patches.
Less well-organised MALT also occur in the
respiratory system and is called bronchial-
associated typhoid tissue (BALT).

24. MALT can be subdivided into:
1. GALT (gut-associated lymphoid tissue, such as the Peyer's
patches in the lining of the small intestines, as well as the
adenoids, tonsils, and appendix)
2. BALT (bronchial-associated lymphoid tissue in the bronchi)
3. SALT (skin-associated lymphoid tissue beneath the
epidermis)
4. NALT (nose-associated lymphoid tissue)
5. LALT (larynx-associated lymphoid tissue)
6. CALT (conjunctiva-associated lymphoid tissue in the eye)

25. The major function of SALT is to confine
microbial invaders to the area immediately the
epidermis and to prevent them from gaining
access to the bloodstream.
Types of SALT cells:
1. Langerhans cell : a dendritic cell that
phagocytosis microorganisms that
penetrate the the skin. Once it
internalise a microorganism, it migrates
from epidermis to nearby lymph nodes,
where it presents antigen to activate
nearby lymphocytes, including a
specific immune response to that
antigen
2. Interaepidermal lymphocyte: a
specialised T cell having potent
cytolytic and immunoregulatory
response to antigen.
Skin associated lymphoid tissue

26. TONSIL (NALT)
Palatine tonsils are located in the posterior lateral walls
of the oral cavity and are partly encapsulated.(paired)
Lingual tonsils are situated along the surface of the
posterior third of the tongue.(paired)
Pharyngeal tonsil is a single medial mass situated in
the posterior wall of the nasopharynx. Hypertrophied
regions of pharyngeal tonsils resulting from chronic
inflammation are called adenoids. (paired)
Inflammation of the tonsils, tonsillitis, is more
common in children than adults. Chronic
inflammation of the pharyngeal lymphoid tissue and
tonsils of children often produces hyperplasia and
enlargement of the tonsils to form “adenoids,”
which can obstruct the eustachian tube and lead to
middle ear infections.
Medical application

27. WALDEYER’S
RING
The three main kind of
tonsils constitute an
anatomical structure called
WALDEYER’S RING.

28. TEST YOUR
KNOWLEDGE 4

29. 1. Which structure is partly encapsulated?
A. Appendix
B. Lymph node
C. Palatine tonsil
D. Peyer’s patch
E. Thymic (Hassal’s) corpuscle
Ans : c

30. PEYER PATCHES
The epithelium directly over a Peyer
patch lymphoid nodule has unique cells
called M (microfold) cells with short
apical folds but no brush border.
The basal surface of M cells forms a
large intracellular pocket that harbours
a transient population of T and B
lymphocytes (L) and dendritic cells (D)
which move through the openings in
the basement membrane (BM).
Darker cytoplasm of adjacent
enterocytes (E) with brush borders (B).

31. Extend from laminapropria to submucosa.
Nodules of payers patches consist of 30- 40
lymphoid follicles.
B cell follicles with germinal centre’s are
predominantly seen in payers patches.
Germinal centres are surrounded by region
surrounding T cells activity.

32. APPENDIX
Its lamina propria and
submucosa filled with
lymphocytes and lymphoid
follicles (L). The small
lumen contains a sample of
the microbial flora of the
intestine, along with
undigested material.

33. SUMMARY
Each lymph node filters lymph and provides a site for B-cell activation and differentiation to antibody
secreting plasma cells. A lymph node has three functional but not physically separate compartments:
an outer cortex, a underlying paracortex, and an inner medulla adjacent to the hilum and efferent
lymphatic.
The secondary lymphoid organs include highly organised lymphoid organs such as lymph nodes and
the spleen, as well as less organised accumulations of lymphoid organs scattered strategically
throughout the body.
Lymphatic vessels are responsible for flow of lymph within the lymphoid system and are a part of the
body's fluid recirculation system. The lymph flows through regional lymph nodes and eventually
enters the circulatory system at the heart to maintain the fluid volume of the circulation.
The spleen is a large lymphoid organ without a cortex/medulla structure; instead, it has two
intermingled but functionally different regions: white pulp and red pulp.
MALT is found in the mucosa of most tracts but is concentrated in the palatine, lingual and
pharyngeal tonsils, Peyer patches, and the appendix.

34. CONCLUSION
I would like to conclude by mentioning the major
features and functions of secondary lymphoid organs
and tissues. The lymph nodes are encapsulate structure
that filter the lymph and then produce an immune
response against trapped microbes. Spleen filters the
blood and is the only lymphoid organ which is not
supplied with the lymphatic vessel. And antigen
sampling occurs and immune responses are initiated in
MALT.

35. Online source –
https://www.histology.leeds.ac.uk/lymphoid/lymph_spleen.php
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2766168/
https://bio.libretexts.org/Bookshelves/Microbiology/
Book%3A_Microbiology_(Kaiser)/Unit_6%3A_Adaptive_Immunity/
12%3A_Introduction_to_Adaptive_Immunity/12.4%3A_The_Lymphoid_System
Offline source-
JUNQUEIRA’S BASIC HISTOLOGY text and atlas FOURTEENTH
EDITION (2016) ,Antony L. mescher, Pg no: 281-292.(published by Mc
GRAW.HILL education)
PRESCOTT’SMICROBIOLOGY ,TENTHEDITION (2017),Willy/
Sherwood/Woolverton, Pg no: 724-726.(published by Mc GRAW.HILL
education)
Bibliography