Lymphoid organs such as the bone marrow, thymus, lymph nodes, and spleen are involved in conducting immune responses. The bone marrow and thymus are primary lymphoid organs that produce naïve lymphocytes. Secondary lymphoid organs such as lymph nodes and the spleen facilitate interactions between antigens and lymphocytes. Tertiary lymphoid organs are sites of infection that immune cells must survey and regulate the influx of effector cells to. The document then discusses the cells and processes involved in the primary and secondary lymphoid organs.

1. Dr Alok Tripathi
Department of Biotechnology,
Saaii College of Medical Science & Technology, Kanpur
aquaimmuno@yahoo.co.in
09451662231

2. LYMPHOID ORGANS
Definition: organs participate in
conducting immune operations are
highly localized microenvironments.
are structural basis of
immune response
3 functional
environments
PRIMARY
lymphoid organs
– mature but
naïve lymphocyte
production
SECONDARY –
mechanism of co-
localizing
antigens and
lymphocytes
TERTIARY – sites of infection
must be able to survey for it
and also regulate the influx of
effector cells

4. PRIMARY LYMPHOID TISSUE
– bone marrow and thymus
antigen recognition
development
positive/negative selection
signaling apparatus in response
to antigenic stimulation
capacity to home to proper
microenvironment

5. BONE MARROW – for B Cells
contains no lymphatic channels but is vascularized
hematopoetic stem cells are nestled next to osteoblasts and receive
survival signals from them
• very inefficient process (frameshift
recombination)
• pro-thymocyte also is made from
hematopoetic stem cell
• stromal cells: support
development/maturation of lymphocytes in
marrow
• secrete cytokines (IL-7)
antigen independent maturation
of B-cells
• cell-cell contact w/ stromal cells also directs
lymphopoesis
C-KIT and stem cell factor
interact on B-cell surface for
lymphopoesis

7. THYMUS – many epithelial-derived cells
CORTEX:
CAPSULE = lining;
invaginates to form lobes
lined w/ early T-cell
progenitors
Cortical Epithelial Cells –
dendridic cells, stimulate T-
cell maturation
Macrophages –
clearance of
unproductive
rearrangements
Interdigitating Cells –
boundary between cortex and
medulla
also called “thymic dendridic
cells”
bone-marrow derived
MHCI/MHCII presentation

8. The thymus is enclosed by a
thin connective tissue capsule
from which numerous septa
extend into the thymus
subdividing the two lobes into
numerous lobules (about 0.5 -
2 mm in diameter).
Blood vessels enter and leave
the thymus via the connective
tissue septa.
Each lobulus is divided into a
darker peripheral zone, the
cortex, and a lighter, central
zone, the medulla.
Medullary tissue is
continuous from lobule to
lobule throughout each lobe.

9. Thymus: Cortex+ Medulla

10. Cells in Thymus
Reticular cells
Reticular cells sheathe
the cortical capillaries
• are quite abundant.
• cytoplasm eosinophilic, with large,
ovoid and light nuclei may contain 1
or 2 nucleoli.
• cells branched, and their slender
processes are connected with the
processes of other reticular cells to
form a cellular reticulum
• This cellular network (reticular fibres
are scant in the thymus) provides
support for other cells of the thymus.
• form an epitheloid layer which
delimits the cortical tissue from
the connective tissue
• and secrete substances
important for thymic function.
• → create and maintain the
microenvironment necessary
for the development of T-
lymphocytes in the cortex.

12. Macrophages occur in both
cortex and medulla. They are
difficult to distinguish from the
reticular cells in H&E stained
sections.
Lymphocytes are present in
both cortex and medulla,
• more numerous (denser) in the cortex.
• sizes are variable (5 - 15 µm) in the
cortex but generally small in the
medulla.
• The vast majority of them will be
developing T-lymphocytes.
• They are also called thymic lymphocytes
or thymocytes.

13. Function of the Thymus
necessary for the development of the recirculating pool of small,
long-lived lymphocytes, the T-lymphocytes.
T cells mainly responsible for the cell-mediated immune response.
Stem cells invade the cortical regions of the thymus, where they
divide to form lymphocytes.
Only a small fraction (10-30%) of the cells generated in the cortex
leave the thymus.
They migrate via the medulla into the blood stream to populate
the T-lymphocyte areas of other lymphoid tissues and organs.
Cells which do not express the necessary receptors to recognize AP to them or which react
incorrectly towards "self-antigens" die and are removed by cortical macrophages.
Since the function of the thymus is to produce lymphocytes for the
other lymphoid tissues it is a primary lymphoid organ.

14. SECONDARY LYMPHOID TISSUES – lymph
nodes (true and musocal), spleen
mechanisms for APC or antigen to access necessary
microenvironment from site of infection
• - specialized vascularization (lymphatics and high endothelial venules)
to recruit lymphocytes and send them back out (effect lymphatics)
• - distinct regions for B (follicles) and T cell priming (between follicles)

15. FOLLICLES
primary follicles – B cells have not
yet encountered antigen
secondary follicles – rapid growth
of B cells after antigenic exposure
germinal center and marginal
zone
Lymphocyte compartmentalization
is a directed process
chemokine secretion by HEV to
recruit lymphocytes out of blood
activated effector cells no longer
express chemokine receptors

16. FOLLICULAR
dendridic cells
- not bone
marrow
derived; NOT
APC’s
make chemokines
(BLC) and organize
follicles
HEV and Stromal cells secrete
SLC/ELC  attracts T cells
Epithelial
Dendridic
Cells (APC’s)
also respond
to SLC
ensures
Dendridic
cells and T-
cells
COLOCALIZE
HEL/OVA
experiment
demonstrated
that T cells
move to
follicular
areas upon
antigen
exposure,
causing B cell
proliferation

17. NAÏVE  EFFECTOR TRANSITION:
occurs in lymph nodes
T cells: decreased activation
threshold (no costimulation)
• effector function activity (helper or
killer)
• homing capacity (directed to
tertiary, not secondary tissues)
B cells: memory and
effector (plasma) cells
• affinity maturation
• class switching  antibody
secretion

18. OVERVIEW
no antigen  B and T
cells come in from high
endothelial venule migrate
to follicle/dendridic cell
So they LEAVE via the
efferent lympathics
antigen present 
B cell enters and remains in
follicle (binds soluble
antigen) antigen on DC
causes CD4 TH2
differentiation
CD4 proliferates and
releases cytokines
(IL4/5/6)
CD4 follows SLC and
moves to follicular
margin for T/B
interaction
•CD40L----CD40
•TCR/CD4 -----MHC: peptide
•CD28 ----B7 (stim)
•CTLA4 --- B7 (inhib)
migration occurs in the
absence of CCR7
receptors
**localization is
very
important**

19. B cell proliferation and class
switching
CD40L is not
constitutively expressed
by all T cells, but CD40 is
by B cells
• requires that TCR crosslinking
first takes place
CD40 binds CD40L 
• survival advantage (upregulates
anti-apopotitic proteins)
• proliferation
• upregulation of costimulatory
molecules for T cells

20. GERMINAL CENTER PROLIFERATION
clonal expansion
occurs in presence
of
antigen/CD40L
DARK ZONE
(centroblasts)
during this point
that hypermutation
is taking place
centroblasts move
to light zone
 now are
CENTROCYTES
interact with
follicular dendridic
cells 
differentiate into
memory or plasma
cells
if they bind more
antigen than
dendridic cells

21. Phagocyte:
Macrophage (MФ) & Neutrophils (PMN)
A phagocyte is a cell that ingests
(and destroys) foreign matter, such
as microorganisms or debris via a
process known as phagocytosis,
• in which these cells ingest and kill offending
cells by cellular digestion.
These phagocytes are extremely
useful as an initial immune system
response to tissue damage.

22. Blood Cells
Granulocytes
Cells with various types of granules
Agranulocytes
Cells without granules
Eosinophils
Stain with acidic dyes
Lymphocytes
Basophils
Satin with basic dyes
Monocytes
Neutrophils
Stain with neutral dyes

23. MФ Macrophages adapted specially for sustained
battles against foreign agents.
In addition, they help to
clean up and remove
damaged tissues.
Immature macrophages which are
circulating in the bloodstream are
called monocytes.
These macrophages cannot react
immediately, but once they have
developed, they are often referred to as
'killing machines' they act by
phagocytizing and destroying anything
that isn't recognized as belonging to the
body.

24. A number
of cell types
are closely
related to
MФ -
Dendritic cells
(including
Langerhans
cells)
Microglia Kupffer cells
Osteoclasts

25. Neutrophil
• express receptors for
immunoglobulin and
complement and are
involved in the acute
inflammatory response.
Morphology
• Large mononuclear cells with granular cytoplasm
• Smaller cells with multi-lobed nucleus and neutral cytoplasmic
granules
Location
• Often resident in tissues
(remove routine cell debris)
• Blood – requires recruitment to
site of infection

26. Killing
ability
After killing
Antigen
presentation
• Require activation by
bacterial molecules ±IFNg
• Activated during
recruitment, then able to kill
internalised bacteria
automatically
• Migrate to local lymph
nodes
• Die at site by apoptosis
(then taken up by
macrophages)
• Can present antigen (Class
II upregulated by IFNg)
• Cannot present antigen
(don't normally express
Class II)
Neutrophil

27. Eosinophil granulocyte
Eosinophil granulocytes, commonly referred to as eosinophils (or less
commonly as acidophils), are white blood cells that are responsible for
combating infection by parasites in the body.
Transparent in vivo, these cells appear brick-red when stained with eosin
using the Romanowsky method (and are thus, 'eosin (or acid)-loving' cells,
hence the name).
The red color is visible as small granules within the cellular cytoplasm.
These granules contain histamine and proteins such as eosinophil peroxidase,
RNase, DNases, lipase, plasminogen, and Major Basic Protein that are toxic
to both parasites and the host's tissues.
Eosinophils make up about 2.3% of the all white blood cells, and are about
10-12 micrometres in size.
A key mediator in eosinophil activation is interleukin 5.

28. Functions
of
eosinophils
• Eosinophils play a role in fighting
viral infections which is evident
from the abundance of RNAses
they contain within their granules.
• Eosinophils also play a role in the
allergic response, and in fibrin
removal in inflammation.
• Eosinophils are considered the
main effector cells in asthma
pathogenesis and are associated
with disease severity.
• Eosinophils fight helminth (worm)
colonization.

29. Basophil
granulocyte
 least common of the granulocytes, representing about 1% of
circulating leukocytes.
 They contain large cytoplasmic granules which obscure the
cell nucleus under the microscope.
 when unstained, the nucleus is visible and it usually has 2
lobes.
 A cell in tissues, the mast cell, has many similar
characteristics.
 For example, both cell types store histamine, a chemical that
is secreted by the cells when stimulated in certain ways
(histamine causes some of the symptoms of an allergic
reaction).
 Like all circulating granulocytes, basophils can be recruited
out of the blood into a tissue when needed.

30. Function
Basophils tend to appear in specific kinds of inflammatory
reactions, particularly those that cause allergic symptoms.
While the exact purpose of basophils has never been proven,
they appear often in tissues where parasites are found.
They can be found in unusually high numbers at sites of
exoparasite infection, e.g., ticks.
also appear in tissues where allergic reactions are occurring
and probably contribute to the severity of these reactions.
Basophils have protein receptors on their cell surface that
bind IgE antibody very tightly.
It is the bound IgE antibody that confers a selective
response of these cells to environmental substances, for
example, pollen proteins.

31. Secretions
When activated, basophils secrete histamine, several proteoglycans, lipid
mediators like leukotrienes, and several cytokines.
Histamine and proteoglycans are pre-stored in the cell's granules while the
other secreted substances are newly generated.
Each of these substances contributes to inflammation.
Recent evidence suggests that basophils are an important source of the
cytokine, IL-4, perhaps more important than T cells.
Interleukin-4 is considered one of the critical cytokines in the development of
allergies and the production of IgE antibody by the immune system.
other substances that can activate basophils to secrete which suggests that
these cells have other roles in inflammation.

32. Lymphocyte
A lymphocyte is a
type of white blood
cell involved in the
human body's
immune system.
There are two broad
categories of
lymphocytes, namely
T cells and B cells.
Lymphocytes play an
important and
integral part of the
body's defenses.
T cells are chiefly responsible for cell-
mediated immunity whereas B cells are
primarily responsible for humoral
immunity (relating to antibodies).
T cells are named such
because these lymphocytes
mature in the thymus; B
cells, named for the bursa of
Fabricius in which they
mature in bird species, are
thought to mature in the
bone marrow in humans.
In the presence of an
antigen, B cells can
become much more
metabolically active and
differentiate into plasma
cells, which secrete large
quantities of antibodies.

33. Microscopically, in a Wright's stained peripheral blood smear, a normal
lymphocyte has a large, dark-staining nucleus with little to no basophilic
cytoplasm.
In normal situations, the coarse, dense nucleus of a lymphocyte is
approximately the size of a red blood cell (about 7 micrometres in diameter).
Some lymphocytes show a clear perinuclear zone (or halo) around the nucleus
or could exhibit a small clear zone to one side of the nucleus.
It is impossible to distinguish between T cells and B cells in a peripheral blood
smear.
Normally, flow cytometry testing is used for specific lymphocyte population
counts.
When one must specifically determine the percentage of lymphocytes that
produce a particular secretion (say, a specific antibody or cytokine), the
ELISPOT or secretion assay techniques can be used instead.

34. The HIV hijacks and destroys T cells (specifically, CD4+
lymphocytes).
Without this key defense, the body is susceptible to
opportunistic diseases that otherwise would not kill healthy
people.
A lymphocyte count is part of a peripheral complete blood
cell count and is expressed as percentage of lymphocytes to
total WBC counted.
An increase in lymphocytes is usually a sign of a viral
infection
A general increase in the number of lymphocytes is known
as lymphocytosis whereas a decrease is lymphocytopenia.

35. Monocyte
A monocyte is a
leukocyte, part of the
human body's immune
system that protect
against blood-borne
pathogens and move
quickly to sites of
infection in the tissues.
It is one of the five
major types of white
blood cell, based on the
appearance of white
blood cells in stained
smears as viewed under
a light microscope.
Microscopic
anatomy
• Monocytes are 13 to 25 μm
in diameter. On a Wright's
stained peripheral blood
smear they appear larger
than red blood cells and
have a bluish-grey
cytoplasm with a large
cytoplasm to nucleus ratio.
Monocytes are typically
identified by flow
cytometry by surface
expression of the protein
CD14, a receptor for
bacterial endotoxin that
gives rise to septicaemia.

36. Physiology
Monocytes are
produced by
the bone
marrow from
haematopoietic
stem cell
precursors,
circulate in the
blood stream
for about one
to three days
and then
typically move
into tissues
throughout the
body. In the
tissues
monocytes
mature into
different types
of
macrophages
at different
anatomical
locations.
Monocytes are
responsible for
phagocytosis
Monocytes can
perform
phagocytosis
using
intermediary
(opsonising)
proteins such
as antibodies
or
complement.
Monocytes are
also capable of
killing infected
host cells via
antibody,
termed
antibody-
mediated
cellular
cytotoxicity.
Vacuolization
may be present
in a cell that
has recently
phagocytized
foreign matter.
Monocytes
which migrate
from the blood
stream to other
tissues are
called
macrophages.
Macrophages
are responsible
for protecting
tissues from
foreign
substances but
are also the
predominant
cells involved
in
atherosclerosis.

37.  Diagnostic use
 A monocyte count is part of a complete blood count
and is expressed either as a ratio of monocytes to the
total number of white blood cells counted, or by
absolute numbers. Both may be useful. Monocytosis
is the state of excess monocytes in the peripheral
blood. It may be indicative of various disease states.