The document summarizes the major lymphoid organs of the immune system. It describes the primary lymphoid organs, the thymus and bone marrow, where lymphocyte maturation occurs. The secondary lymphoid organs, lymph nodes, spleen, MALT and GALT, trap antigens and allow interactions between lymphocytes and antigens. The thymus specifically mediates T cell maturation and selection, while lymph nodes contain B cell follicles and T cell zones to initiate adaptive immune responses to lymph-borne pathogens and antigens.

1. Organs of the
Immune System
Amandeep Singh
Assistant Professor
Department of Biotechnology
GSSDGS Khalsa College Patiala

2. Types of Lymphoid Organs
1. Primary lymphoid organs
2. Secondary lymphoid organs
Primary lymphoid organs
A. Thymus
B. Bone marrow
Where maturation of lymphocytes
takes place.
Secondary lymphoid organs
A. Lymph node
B. Spleen
C. MALT
D. 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 primary lymphoid organs.
• To make immunocompetent cells (capable of mounting
an immune response)

4. Thymus- Structure
• Site of T-cell maturation.
• Flat, bi-lobed organ situated above the heart, behind the sternum.
• Each lobe is surrounded by a capsule & is divided into lobules, which
are separated from each other by strands of connective tissues called
trabeculae.
• Each lobule is organized into 2 compartments:
1. Cortex: Densely packed with immature T cells or thymocytes
2. Medulla: Lightly packed with thymocytes

5. Thymus- Structure
• Both Cortex and medulla of thymus are criss-crossed by a
3-D stromal cell network composed of epithelial cells,
dendritic cells and macrophages which lead to the maturation
of thymocytes.
• Some thymic epithelial cells in the outer cortex called nurse
cells, have long membrane extensions that surround as many
as 50 thymocytes, forming large multicellular complex.

6. Thymus- Function
Production of T Cell Receptor (TCR) on surface of T-cells
TCR
Those which
recognize Ag-
MHC complex
Those which
recognize Self Ag-
MHC complex
Those which do
not recognize Ag-
MHC complex
DieSurvive Die
95% of thymocytes die by apoptosis in thymus without even reaching maturity.
Foreign Ag
Foreign
Ag
Self body Ag
Clonal Selection Clonal Deletion Clonal Deletion

7. Deficiency of Thymus development
DiGeorge Syndrome Nude Mice
Humans Mice
Thymus fails to develop
• Aging decline thymus function
Avg. wt. = 70 gm in infants
3 gm in elders

8. Cross Sectional Diagram of a portion
of thymus

9. Secondary Lymphoid Organs
1. Lymph node
2. Spleen
3. MALT (Mucosal Associated Lymphoid Tissue)
4. GALT (Gut Associated Lymphoid Tissue)

10. Lymphatic System
Tissue
Lymph
Blood at
high
pressure Blood Vessel
Lymph
Antigen

11. Lymph node
• Encapsulated bean shaped structures containing a reticular
network packed with lymphocytes, macrophages, dendritic cells.
• Lymph node is divided into 3 regions:
1. Cortex
2. Paracortex
3. Medulla

12. Cortex Paracortex Medulla
B-cells, macrophages,
follicular dendritic cells.
T-cells, Interdigitating
dendritic cells.
Antibody secreting
plasma cells.
These cells lead to the
production of primary
follicles.
Have MHC-II on the surface
and present Ag to Helper-T
cells.
Primary follicle + Antigen =
Secondary follicles
Thymus independent zone. Thymus dependent zone.

13. Structure of
Lymph
node
Structure of a lymph node. (a) The three layers of
a lymph node support distinct microenvironments.
(b) The left side depicts the arrangement of
reticulum and lymphocytes within the various
regions of a lymph node. Macrophages and
dendritic cells, which trap antigen, are present in
the cortex and paracortex. TH cells are
concentrated in the paracortex; B cells are located
primarily in the cortex, within follicles and
germinal centers. The medulla is populated largely
by antibody-producing plasma cells. Lymphocytes
circulating in the lymph are carried into the node
by afferent lymphatic vessels; they either enter the
reticular matrix of the node or pass through it and
leave by the efferent lymphatic vessel. The right
side of (b) depicts the lymphatic artery and vein
and the postcapillary venules. Lymphocytes in the
circulation can pass into the node from
The postcapillary venules by a process called
extravasation (inset).

14. Spleen
• Graveyard of RBCs
• Destroy blood born antigens
• Antigens are supplied by blood to the spleen
Structure:
Surrounded by a capsule that extend projections (trabeculae) into the interior
to form a compartmentalized structures.
2 compartments:
1. Red Pulp
2. White Pulp
Both are separated by marginal zone in between.

15. Structure of Spleen
Red Pulp
• Network of sinusoids populated by macrophages and RBCs
• Site where old and defective RBCs are destroyed and removed.
• Macrophages contain engulfed RBCs or iron pigments from degraded Hb.
White Pulp
• It surrounds the branch of splenic artery, forming a Peri-Arteriolar Lymphoid
Sheath (PALS).
• Populated mainly by T-lymphocytes
• Primary lymphoid follicles (B-cells) are attached to PALS.
Marginal Zone
• Peripheral to PALS
• Lymphocytes and macrophages

16. Mechanism of action of Spleen
• Blood born antigens and lymphocytes enter the spleen through splenic
artery.
• This empty into marginal zone.
• In marginal zone, Ag trapped by dendritic cells, carry it to PALS.
• In PALS, Initial activation of B and T cells occur.
• Which move to Primary follicle in marginal zone to give rise to
Secondary follicle.

17. Spleen

18. MALT (Mucosal Associated
Lymphoid Tissue)
• Mucus lining of respiratory, digestive and genito-
urinary tract are protected by group of lymphoid
tissues called MALT.
Example:
• MALT of Intestine, which consist of M-cells, IEL,
Lamina Propia, Peyer’s Patches in Submucosa.

19. MALT of Intestine
IEL
Majority of these ells are T-cells which express TCR (αδ TCR) on the surface
and suitable for antigens in the GIT.
Lamina Propia Lymphocytes
Contain loose cluster of cells resembling follicles.
Contain large number of B-cells, activated Helper T cells and macrophages.
Submucosal Peyer’s Patches
30-40 lymphoid follicles present in submucosa below lamina propia.
Follicles undergo proliferation following antigen encounter into secondary
follicles with germinal centers.

20. MALT of Intestine
M Cells
• Flat surfaced epithelial cells (without microvilli)
• Active pinocytic cells
• Capable of endocytosing antigen from lumen.
• Basolateral deep pocket , that contain B cells, T cells and Macrophages
• Site bearing M-cell is called Inductive site.
• M-cells are utilized by Salmonella typhi, Vibrio cholera and Polio virus to infect
lumen.

21. MALT of Intestine
Peyer’s Patch Intestinal Mucosa

22. MALT of Intestine
M-Cell at an Inductive site

23. CALT (Cutaneous Associated
Lymphoid Tissue)
The epidermal cells called keratinocytes
secret cytokines, express MHC-II
molecules and function as antigen
presenting cells.
Dendritic cells also present called as
Langerhan's cells which internalize
antigens and then migrate to lymph node
as veiled cells to activate Helper- T cells.
Dermis contain scattered B and T
lymphocytes and macrophage to further
strengthen the immune system.