1. ORGANS OF THE
IMMUNE SYSTEM
PRESENTED BY PRANZLY

2. TYPES
PRIMARY LYMPHOID
ORGANS
• Bone marrow
• Thymus
where maturation of
lymphocytes takes place
SECONDARY LYMPHOID
ORGANS
• Spleen
• Lymph nodes
• Mucosal associated
lymphoid tissues (MALT)
• gut-associated lymphoid
tissue (GALT)
which trap antigen and
provide sites for mature
lymphocytes to interact with
that antigen

3. PRIMARY LYMPHOID ORGANS
• Immature lymphocytes generated in hematopoiesis
mature and become committed to a particular
antigenic specificity within the primary lymphoid
organs
• Only after a lymphocytes has matured within a
primary lymphoid organ is the cell
immunocompetent (capable of mounting an
immune response).
• T cells arise in the thymus, and in many mammals—
humans
• B cells originate in bone marrow

4. BONE MARROW
• supports self-renewal and differentiation of hematopoietic stem cells
(HSCs) into mature blood cells.
• bone marrow is the site of B-cell origin and development
• the long bones (femur, humerus), hip bones (ileum), and sternum tend to be
the most active
• contains several cell types that coordinate HSC development, including
1. OSTEOBLASTS, versatile cells that both generate bone and control the
differentiation sites of hematopoiesis of HSCs,
2. ENDOTHELIAL CELLS that line the blood vessels and also regulate HSC
differentiation,
3. RETICULAR CELLS that send processes connecting cells to bone and
blood vessels, and, unexpectedly,
4. SYMPATHETIC NEURONS, which can control the release of
hematopoietic cells from the bone marrow

5. microenvironments within
the bone marrow
ENDOSTEAL NICHE
the area directly
surrounding the bone and
in contact with bone-
producing osteoblasts) .
appears to be occupied by
quiescent HSCs in close
association with
osteoblasts that regulate
stem cell proliferation
VASCULAR NICHE
the area directly
surrounding the blood
vessels and in contact with
endothelial cells
•appears to be occupied by
HSCs that have been
mobilized to leave the
endosteal niche to either
differentiate or circulate

6. Thymus
• the site of T-cell development and maturation.
• It is a flat, bilobed organ situated above the heart.
• Each lobe is surrounded by a capsule and is divided
into lobules, which are separated from each other
by strands of connective tissue called trabeculae.
Lobules consists of two compartments
CORTEX
the outer compartment, is densely
packed with immature T cells, called
thymocytes
MEDULLA
the inner compartment, is sparsely
populated with thymocytes

7. composed of epithelial cells, dendritic cells, and macrophages, which make up
the framework of the organ and contribute to the growth and maturation of
thymocytes.
in the outer cortex, called nurse cells, have long membrane extensions that
surround as many as 50 thymocytes, forming large multicellular complexes
FUNCTION- to generate and select a repertoire of T cells that will protect the
body from infection.

8. SECONDARY LYMPHOID ORGANS
• Lymph nodes and the spleen are the most highly
organized of the secondary lymphoid organs and are
compartmentalized from the rest of the body by a
fibrous capsule.
• lymphoid tissue is organized into structures called
lymphoid follicles,
• Until it is activated by antigen, a lymphoid follicle—
called a primary follicle—comprises a network of
follicular dendritic cells and small resting B cells.
• After an antigenic challenge, a primary follicle
becomes a larger secondary follicle—a ring of
concentrically packed B lymphocytes surrounding a
center (the germinal center)

9. SPLEEN
• It is a large, ovoid secondary lymphoid organ
situated high in the left abdominal cavity.
• the spleen specializes in filtering blood and
trapping blood-borne antigens; thus, it can respond
to systemic infections.
• Functions- in iron metabolism, thrombocyte
storage, haematopoiesis)
• compartments
1. the red pulp and white pulp, which are separated
by a specialized region called the marginal zone

11. • THE SPLENIC RED PULP
consists of a network of
sinusoids populated by
red blood cells,
macrophages, and some
lymphocytes. It is the
site where old and
defective red blood
cells are destroyed and
removed
• THE SPLENIC WHITE
PULP surrounds the
branches of the splenic
artery, and consists of
the periarteriolar
lymphoid sheath (PALS)
populated by T
lymphocytes as well as
B-cell follicles.
The marginal zone, which borders the white pulp, is
populated by unique and specialized macrophages and B
cells, which are the first line of defense against certain
blood-borne pathogen

12. LYMPH NODES
divided into three roughly
concentric regions
THE CORTEX
The outermost layer,,
contains lymphocytes
(mostly B cells), macro-
phages, and follicular
dendritic cells arranged in
primary follicles
THE PARACORTEX
which is populated
largely by T
lymphocytes and also
contains
interdigitating
dendritic cells
THE MEDULLA
The innermost layer,
is more sparsely
populated with
lymphoid-lineage cells;
of those present, many
are plasma cells
actively secreting
antibody molecules

14. WHEN A FOREIGN ANTIGEN GAINS ENTRANCE TO THE TISSUES, IT IS PICKED UP BY THE LYMPHATIC SYSTEM
(WHICH DRAINS ALL THE TISSUES OF THE BODY) AND IS CARRIED TO VARIOUS ORGANIZED LYMPHOID TISSUES
SUCH AS LYMPH NODES
ANTIGEN-PRESENTING CELLS THAT ENGULF AND PROCESS THE ANTIGEN ALSO CAN GAIN ACCESS TO LYMPH
AS LYMPH PASSES FROM THE TISSUES TO LYMPHATIC VESSELS, IT BECOMES PROGRESSIVELY
ENRICHED IN SPECIFIC LEUKOCYTES, INCLUDING LYMPHOCYTES, DENDRITIC CELLS, AND
MACROPHAGES.
WHERE THE LYMPHOCYTES CAN INTERACT WITH THE TRAPPED ANTIGEN AND UNDERGO ACTIVATION
ALL IMMUNE CELLS THAT TRAFFIC THROUGH TISSUES, BLOOD, AND LYMPH NODES ARE GUIDED
BY SMALL MOLECULES KNOWN AS CHEMOKINES. THESE PROTEINS ARE SECRETED BY STROMAL
CELLS, ANTIGENPRESENTING CELLS, LYMPHOCYTES, AND GRANULOCYTES, AND FORM
GRADIENTS THAT ACT AS ATTRACTANTS AND GUIDES FOR OTHER IMMUNE CELLS

15. LYMPHOID TISSUE
1. Mucosa-associated lymphoid tissue (MALT).
2. Bronchus-associated lymphoid tissue (BALT)
3. Nasal-associated lymphoid tissue (NALT),
4. Gut-associated lymphoid tissue (GALT)
5. Cutaneous-Associated Lymphoid Tissue

16. MUCOSA-ASSOCIATED LYMPHOID TISSUE
(MALT)
• MALT INCLUDES
1. TONSILS
2. PEYER’S PATCHES (IN THE SMALL INTESTINE),
3. THE APPENDIX,
4. AS WELL AS NUMEROUS LYMPHOID FOLLICLES
WITHIN THE LAMINA PROPRIA OF THE INTESTINES
AND IN THE MUCOUS MEMBRANES LINING THE
UPPER AIRWAYS, BRONCHI, AND GENITOURINARY
TRACT

17. TONSILS
• THE TONSILS ARE FOUND IN THREE LOCATIONS:
1. LINGUAL AT THE BASE OF THE TONGUE;
2. PALATINE AT THE SIDES OF THE BACK OF THE
MOUTH;
3. AND PHARYNGEAL (ADENOIDS) IN THE ROOF OF
THE NASOPHARYNX
ALL THREE TONSIL GROUPS ARE NODULAR
STRUCTURES CONSISTING OF A MESHWORK OF
RETICULAR CELLS AND FIBERS INTERSPERSED
WITH LYMPHOCYTES, MACROPHAGES,
GRANULOCYTES, AND MAST CELLS.

18. PEYER’S PATCHES
• Peyer’s patches, nodules of 30 to 40 lymphoid
follicles, extend into the muscle layers that are
just below the lamina propria.

19. GUT-ASSOCIATED LYMPHOID
TISSUE (GALT)
• The outer mucosal epithelial layer contains intraepithelial
lymphocytes (IELs), many of which are T cells.
• The lamina propria, which lies under the epithelial layer, contains
large numbers of B cells, plasma cells, activated T cells, and
macrophages in loose clusters
• In the digestive tract, specialized M cells transport antigen across
the epithelium, they are flattened epithelial cells lacking the
microvilli that characterize the rest of the mucosal epithelium.
• M cells have a deep invagination, or pocket, in the basolateral
plasma membrane; this pocket is filled with a cluster of B cells, T
cells, and macrophages

20. M cells are located in so-called
inductive sites—small regions of
a mucous membrane that lie
over organized lymphoid
follicles.
Antigens transported across the
mucous membrane by M cells
can activate B cells within these
lymphoid follicles.
The activated B cells
differentiate into plasma cells,
which leave the follicles and
secrete the IgA class of
antibodies.

21. Cutaneous-Associated Lymphoid
Tissue (CALT)
• The epidermal (outer) layer of
the skin is composed largely of
specialized epithelial cells
called keratinocytes.
• These cells secrete a number
of cytokines that may function
to induce a local inflammatory
reaction.
• These cells express high levels
of class II MHC molecules and
function as potent activators of
naive TH cells
• The epidermis also contains so-
called intraepidermal
lymphocytes.