The presentation refreshes the ideas about lymphatic system and its functions

1. Aswathi K S
1st MSc BCM
Central University of Kerala

2.  A network of vessels
that collect fluid that has
escaped into the tissues
from capillaries of the
circulatory system.

3. The four parts of lymphatic system are
1. LYMPH
2. LYMPH VESSEL
3. LYMPHOID TISSUE/ LYMPHOID
ORGAN
4. LYMPHOCYTES and PHAGOCYTES

4.  A clear fluid composed mainly of water,
electrolytes and some plasma proteins
 Transported in lymphatic pathway from
lymphatic vessels to collecting ducts and end
disposed into venous blood
 When blood circulates in high pressure, the
fluid (plasma) portion seeps through thin
capillary walls into surrounding tissue.
 This interstitial fluid bathes the cells and is
returned to blood through walls of venules.
 The remainder enters a network of thin walled
tubes called lymphatic vessels and now is
called lymph

5.  Reabsorbed by lymphatic capillaries in all body
tissues where excessive tissue fluids occur.
 Because of lack of pumping organ, lymph
movement is largely dependent on skeletal
muscle activity.
 Movement normally constant and smooth,
except when obstruction (small clot, tumor)
occurs which tend to back up the lymph, and
result in edema (fluid accumulation in tissues)

6.  Lymphatic capillary run
parallel to blood
capillaries in all body
tissues and allow
diffusion of fluid from
intestitial spaces into
lymphatic pathway.
 Capillaries also
responsible for
absorbing short chain
fatty acids in the small
intestine using

7.  Lymphatic capillaries collecting ducts
Lymphatic vessels Lymphatic trunks
Lymphatic ducts.

8.  Structurally identical to veins- vessel wall
composed of 3 thin layers of tissue, and contain
valves to prevent backflow.
 They form specialized lymphatic organs called
lymph nodes which store macrophages and
lymphocytes to eliminate foreign substance in the
lymph.
 Larger connecting vessels converge to form
Lymphatic trunks. Lymphatic trunks deliver lymph
to two lymphatic ducts which eventually drain all
lymph fluid back to the blood : THORACIC duct
returns lymph from left side of the body to left
subclavian vein, and RIGHT LYMPHATIC duct
returns lymph from right part of the body to right
subclavian vein

9. A number of morphologically and functionally
diverse organs and tissues contribute to the
development of immune responses. These organs
can be distinguished by function as :
1. PRIMARY LYMPHOID ORGAN
i. THYMUS
ii. BONE MARROW
2. SECONDARY LYMPHOID ORGAN
i. LYMPH NODE
ii. SPLEEN
iii. MALT, CALT, GALT, BALT…

10.  Sites where immature lymphocytes generated by
haematopoesis mature and become commited to
specific antigen
 Only after lymphocytes mature within primary
lymphoid organs are the cells
immunocompetent.
 T cells arise in bone marrow and mature in
thymus.
 In many mammals, B cells originate in bone
marrow
 Primary lymphoid organs are THYMUS and

11.  Flat, bilobed organ
situated above the heart.
 Each lobe surrounded
by a capsule and divided
into lobules, which are
separated from each
other by strans of connective tissue called trabeculae.
 Each lobule organized into 2 compartments: the outer
compartment cortex, and the inner compartment
medulla.

12.  Cortex is densly packed with immature T cells and
medulla is sparsely packed with thymocytes.
 Both medulla and cortex criss-crossed by a 3D stromal
cell network of epithelial cells, dentritic cells and
macrophages.
 Many of the stromal cells interact physically with

13.  The function of thymus is to generate and select a
repertoire of T cells that will protect the body from
infection.
 As thymocytes develop, an enormous diversity of T
cell receptors is generated by gene rearrangement,
which produces some T cells with receptors capable
of recognizing antigen-MHC complexes.
 Thymus induces death of cells that cannot recognize
antigen-MHC complexes and those that react with
self antigens.
 More than 95% of thymocytes die by apoptosis in
thymus before maturity.
 Experiments conducted on thymectomized mice
showed dramatic decrease in circulating T
lymphocytes and absense of cell mediated immunity.
 Thymic function is known to decline with age. Thymus
reaches maximal size at puberty and atrophies, with

14.  Average weight of thymus in human infants is
30g, and age dependent involution leaves the
organ with a weight of 3g in elderly.
 By the age of 35, thymic T cell generation falls to
20% of in infants and by 65, fallen to only 2%.
 In one experiment, thymectomized mouse were
implanted with thymus from either one day old or
33 month old mice. Mice receiving newborn
thymus graft showed increased immune response
than mice receiving 33 month old thymus.

15.  Complex tissue which is
the site of
haematopoesis and fat
deposition.
 Haematopoetic cells
generated in bone
marrow move through
walls of blood vessel
and enter blood stream,
which carries them out
of the marrow and
distributes these cells to
the rest of the body.
 In humans and mice, it
is the site of B cell

16.  Immature B cells arise from lymphoid progenitor cells,
proliferate and differentiate in bone marrow. Stromal
cells in bone marrow interact directly with B cells and
secrete various cytokines required for development.
 Bone marrow B cells are source of 90% IgG and IgA
in the plasma.
 In birds B cell maturation occurs in bursa of Fabricius
– a lymphoid organ associated with the gut, is the site
of B cell maturation.
 In cattle and sheep, primary lymphoid tissue hosting
B cell maturation, proliferation and diversification in
early gestation is fetal spleen.
 In later gestation, this function is assumes by a patch
of tissue in intestinal wall called ileal Payers patch
which contain large amount of B cell and T cell.
 The rabbit uses gut associated tissue, especially the
appendix

17.  Lymph nodes and spleen are the most highly
organized secondary lymphoid organs.
 In addition to lymphoid follicles, they have
additional distinct regions of T-cell and B –cell
activity, and are surrounded by a fibrous capsule.
 Less organized lymphoid tissue, collectively called
Mucosa Associated Lymphoid Tissue (MALT) is
found in various body sites.
 MALT includes Payers patches (in small intestine),
the tonsils, and the appendix as well as numerous
lymphoid follicles within lamina propria of intestinal
lining and in mucous membranes lining the upper
airways , bronchi, and genitourinary tract.

18.  Sites where immune responses
are mounted to antigens in
lymph.
 Encapsulated bean shaped
structures containing a reticular
network packed with
lymphocytes, macrophages and
dendritic cells.
 1st structure to encounter antigen
that enter the tissue spaces.
 As lymph percolates through,
antigen is trapped by cellular
network of phagocytic cells and
dendritic cells.
 Ideal environment for lymphocyte

19.  Lymph node can be
divided into 3 centric
regions: the Cortex, the
Paracortex and the
Medulla.
 The outer most cortex
contains
lymphocytes(mostly B),
macrophages and
follicular dendritic cells
arranged in primary
follicles.
 On antigenic challenge,
primary follicle enlarges
to secondary
follicle with germinal
centre.
 Children with B cell

20.  Beneath cortex, paracortex: largely populated by T
lymphocytes and dendritic cells that migrated from
tissues to the node.
 These dendritic cells express high levels of class II
MHC molecules – necessary for antigen
presentation to TH cells.
 Medulla is more sparsely populated with lymphoid
lineage cells, and many are plasma cells actively
secreting antibodies.
 An antigen is carried to node by lymph, it is trapped,
processed and presented with MHC molecule by the
dendritic cells in paracortex, resulting in activation of
TH cells.
 Initial activation of B cell occur in paracortex.
Activated TH cells and B cells form a small foci, with

21.  Within 4-7 days few B and TH cells move to primary
follicle of cortex and interact with dendritic cells to
form secondary follicle with germinal center.
 Some plasma cells in germinal center move to
medullary areas and many migrate to bone marrow.
 The afferent lymph vessel pierce node and empty
lymph to sub capsular sinuses, where the lymph
percolate slowly and allow phagocytic cells and
dendritic cells to trap antigen.
 The lymph leaving the node is rich in antibody, which
is secreted by medullary plasma cells and has 50x
higher concentration of lymphocytes.
 Higher concentration of lymphocyte is due to higher
proliferation at lymph node.
 Blood bourne lymphocyte migrate to nodes through
walls of post capillary venules lined by epithelial cells ,
called High Endothelial Venules (HEV)

22.  Situated high in the left abdominal
cavity, an ovoid organ.
 Filter blood and trap blood borne antigen: thus
respond to systemic infections.
 Not supplied by lymphatic vessels. Instead blood
borne antigen is carried to spleen by splenic artery.
 Surrounded by a capsule from which a number of
projections (trabeculae) extend into interior to form
compartments.
 Compartments are of two types: the RED pulp and
the WHITE pulp, separated by a diffuse marginal
zone.
 Red pulp consist of network of sinusoids populated
by macrophages, RBC and lymphocytes. It is the site

23.  The white pulp surrounds the branches of splenic
artery, forming Periarteriolar Lymphoid Sheath(PALS)
populated by T lymphocytes.
 primary lymphoid follicles are attached to PALS and
are rich in B cells, some of them contain germinal
centers.

24.  Blood borne antigen and lymphocytes enter spleen
through splenic artery, and empties into marginal
zone.
 In marginal zone, antigen is trapped by dendritic cell
and carried to PALS.
 Initial activation of B and T cells takes place in T cell
rich PALS.
 Dendritc cells capture antigen and present it to TH
cells by class II MHC molecules. Activated TH cells
activate B cells.
 B cells n TH cells migrate to primary follicles in
marginal zone and on antigenic challenge, they
develop into charecteristic secondary follicles with
germinal centers.
 In germinal centers rapidly dividing B cells and

25.  The vulnerable membrane surfaces of respiratory,
digestive and urogenital tracts are defended by
group of organized lymphoid tissue called MALT.
 Secondary lymphoid tissue associated with
respiratory epithelium is Bronchus Associated
Lymphoid Tissue (BALT)
 That of digestive tract is called Gut Associated
Lymphoid Tissue (GALT)
 MALT ranges from loose, barely organized clusters
of lymphoid cells in lamina propria of intestinal villi
to well organized structures such as Payers
patches in intestinal lining.
 MALT also includes tonsils and appendix.

26.  MALT contains large population of plasma cells,
greater than concentration in spleen, lymph node
and bone marrow combined.

27.  Outer mucosal epithelial layer contains intraepithelial
lymphocytes (IEL)
 Lamina propria contains large number of B cells,
plasma cells, TH cells and macrophages in loose
clusters.
 Antigen transport from lumina to underlying mucosa
associated lymphoid tissue carried out by M cells.

28.  M cells have deep invaginations and antigens in the
intestinal lumen are endocytosed into vesicles that
are transported from luminal membrane to
underlying pocket membrane.
 Vesicle fuse with pocket membrane delivering
antigen to lymphocytes and antigen presenting cells.
 Antigen hence transported result in activation of B
cells that secrete IgA, which combat many type of
infection at mucosal sites.

29.  Skin is the largest organ of the body and plays an
important role in defense mechanism as anatomical
barrier.
 Epidermal layer of cells composed of specialized
keratinocytes, which produce cytokines that induce
local inflammatory reaction.
 Scattered in epidermis are langerhans cells, which
undergo maturation and migrate to lymph nodes
where they act as potent activators of naïve TH cells.
 Epidermis also contains intra epidermal lymphocytes,
which are mostly T lymphocytes,
 The dermal layer of skin contains scattered T cells

30.  T CELLS
 B CELLS
 NATURAL KILLER CELLS
 MONOCYTES AND MACROPHAGES
 MAST CELLS
 DENDRITIC CELLS
 GRANULOCYTIC CELLS: Neutrophil.
Eosinophil, Basophil
 FOLLICULAR DENDRITIC CELLS

31.  Immunity
 fluids from all capillary beds are filtered
 immune cells stand ready to respond to foreign
cells or chemicals encountered
 Lipid absorption
 Lacteals in small intestine absorb dietary lipids
 Fluid recovery
 absorbs plasma proteins and fluid (2 to 4 L/day)
from tissues and returns it to the bloodstream
 interference with lymphatic drainage leads to
severe edema