This document summarizes the key lymphoid organs and tissues involved in the immune system. It describes the bone marrow and thymus as the primary lymphoid organs where immune cells mature and differentiate. It then discusses secondary lymphoid tissues like lymph nodes, spleen, tonsils, and mucosa-associated lymphoid tissue that provide sites for immune cell interaction and response initiation upon antigen exposure. Specific cell types, structures, and functions are outlined for each lymphoid organ discussed.

1. Lecture # 9

2. Cells, tissues and organs
that monitor body surfaces
and internal fluid
compartments;
 react to (+) potentially
harmful antigenic substances
(infectious microorganisms,
viral entities, toxins, foreign
cells and tissues, & normaL
cells that transformed into
cancer cells)
recognized as ("non-self“).

3. - Organs in which immune
cells undergo maturation,
and/or differentiation, and
proliferation.

4.  Bone marrow and Thymus
 Sites of immune cell maturation and
differentiation.
 Tissues in which lymphocytes are generated
and differentiate into mature naïve B cells
and T cells.
 Cells here do not come into contact with an
antigen.

5. Primary site of
hematopoiesis.
Immune cells arise from
progenitors in the bone
marrow.
Site for B cell maturation.

7. Bone Marrow
 Rearrangement of genes that encode the B
cell receptor that will recognize foreign
antigen, but not foreign molecules.
 B cell receptor – membrane
immunoglobulins
 Screening process – self-reactive B cells
are eliminated
 Naïve, mature B cells (functional): released
in the bone marrow.
 seed the peripheral lymphoid tissues
 circulate in immunosurveillance

9. Thymus
 Flat, bilobed organ
situated above the heart
and below the thyroid
gland.
 Encapsulated organ

10. It increases in size until
it reaches its peak
development during
adolescence.
Becomes smaller with
age.
A
B

13. Pre-T cells from the bone
marrow migrate to the
cortex to undergo
maturation.
The thymus is where T
cells are "educated" to
distinguish self from
nonself.

14.  Construction of T cell receptor (TCR) - gene
rearrangement (a random process).
 Thymic Selection: a screening process
 only the T cells with TCRs that can recognize
antigens (TCRs beneficial to the host) will survive
and mature.
 Mature T cells leave the thymic medulla,
enter the blood stream.
 Seed the secondary lymphoid tissues
 circulate in immunosurveillance.

15. Table 1. Approximate Percentage of B and T Lymphocytes in
Lymphoid Organs (Junquiera et al., 2005)
Lymphoid
Organ
T Lymphocytes, (%) B Lymphocytes, (%)
Thymus 100 0
Bone marrow 10 90
Spleen 45 55
Blood 75 35

16. Lymph nodes, spleen,
tonsils, Peyer’s
patches, MALT, and
cutaneous
immune system
Exposure to antigens
initiates immune
responses in the
secondary lymphoid
tissues.

17.  Secondary lymphoid tissues provide a place
where lymphocytes can talk to each other,
and to other cells.
 They provide an environment for antigen
focusing, where lymphocytes can 'study' an
antigen, and sharpen up the immune
response by clonal expansion and affinity
maturation.
 They provide a home for lymphocytes, where
they can be available when they're needed.

18. LYMPH NODES
Small encapsulated
structures located at
the junction of the main
lymphatic tracts.
Serve as central
collecting points for
lymph fluid from
adjacent tissues; mainly
functions for filtration.
BONE
MARROW
APPENDIX
THYMUS
BRONCHUS
ASSOCIATED
LYMPHOID
TISSUE
TONSIL
Axillary node
Intercostal node
SPLEEN
PEYER’S PATCHES
Lumbar node
Iliac node
Inguinal node
Cervical node

19. LYMPH NODES
Filtering function - to
trap antigens and cells
containing antigen that
flow into them via
afferent lymphatics.
To provide a site for
clonal expansion of
lymphoid cells recruited
from the millions of
cells that enter and
leave via various routes.

20. Lymph fluid
 fluids and low-molecular-weight solutes
drained from the tissues (by passing out of
blood vessel walls and into the interstitial
spaces between cells).
 flowing through thin-walled lymphatic
vessels.

21. Afferent lymphatic vessel: the
entrance of lymph fluid which contains
the antigens and cells
Efferent lymphatic vessel: where
drained lymph fluid along with
lymphocytes exit; connected with
the thoracic duct and venous
system.
Subcapsular sinus – lined with macrophages where
antigen processing takes place.
Node: cortex, paracortex and medulla

23.  Cortex – mostly B cells
 Paracortex – mostly T cells; APCs; HEV (high
endothelial venules)
 Interfollicular region – T cells; APCs
 Medulla – less densely populated area but
contains some T cells, B cells and macrophages.

24.  Primary follicles – small rounded masses of
cells which are inactive due to absence of
antigenic stimulation. Contains mature,
resting B cells
 B cells that are not yet stimulated by an antigen
 Secondary follicles – larger masses or follicles
containing germinal centers generated
during an encounter with antigens carried by
the lymph.
 B cells are stimulated / activated by antigens

25. • a mass of activated B
cells
• site where B cells
proliferate and
differentiate into plasma
cells.
Plasma cell-release
antibodies
Lymphocyte
B cell
Plasma cell

26. SS – subcapsular space
T – trabecula

29.  the largest secondary lymphoid organ
 located in the upper left quadrant of the
abdomen just below the diaphragm
 a large discriminating filter
 filters out old and damaged cells and foreign
antigens from the blood

31.  makes up more than one-half of the total
volume;
 its function is the destruction of old red
blood cells – by splenic macrophages
 blood flows from the arterioles into the
red pulp and exits through the splenic
vein
 red matrix; composed of sinusoids and
splenic cords of cells (cords of Billroth);
vascular areas

33.  contains the lymphoid tissue
 arranged around arterioles in a periarterial
lymphatic sheath (PALS)
 with lymphoid follicles attached; PALS and
lymphoid follicles are surrounded by a
marginal zone.

35.  Periarterial lymphatic sheath (PALS)
• Contains primarily T cells; also macrophages,
plasma cells and granulocytes.
• Lymphocytes enter and leave this area by
means of the many capillary branches that
connect to the arterioles
 White marginal zone
• contains dendritic cells and macrophages; CD+4
T cells and B cells

36.  MALT (mucosa-associated lymphoid tissue)
1. GALT – gut-associated lymphoid tissue
 Peyer’s patches
2. BALT – bronchus-associated
lymphoid tissue
 Tonsils
Exhibit primary and secondary
lymphoid nodules

37.  consist of diffusely distributed lymphoid cells
and follicles that underlie all regions coated
with mucosa.
 has similar immune tissue components as the
lymph nodes and spleen.
 the main difference of MALT: immune tissue
components are not encapsulated; scattered
diffusely.

39. - involved in defense against pathogens that may be colonizing the gut.
Esophagus

40.  Payer’s patches: represents a specialized
type of MALT; form larger aggregates of
lymphoid nodules (colon, appendix, ileum)

41. Notice the germinal center where B-cells proliferate. These are a
major source of antibody production.

42.  small masses of macrophages and lymphoid
tissue found in the mucous membrane lining
of the oral and pharyngeal cavities
 Function: to respond to pathogens entering
the respiratory and
alimentary tracts
Palatine
tonsil

43. GC- Germinal center

44. The pharyngeal tonsil is distinguished from the palatine by the
presence of pseudostratified columnar epithelium (arrows).

45.  Cutaneous-associated lymphoid tissue
 Immune cells present in the epidermis and
dermis of the skin
 Activated Keratinocytes – produce a number
molecules that play an important role in host
defenses
 Langerhan’s cells – APC’s in the skin
 T cells – uniquely positioned to combat any
antigens that enter through the skin