A brief overview of the biology of the thymus, T cell development and the immunological and pathological aspects of thymus function. The spleen is described in a nutshell. Suitable for teaching undergraduates and reviewing for postgraduates.

2. Designed to gather,
destroy infectious
microorganisms
Figure 20.10

4. Anterior view of chest showing location and size of adult thymus

5. •
•
•
The thymus is a primary lymphoid organ where T
lymphocytes develop and undergo maturation.
It consists of two lateral lobes, situated partly in
the thorax above the heart, partly in the neck.
Each lobe is surrounded by a capsule and is
divided into lobules, which are separated from
each other by strands of areolar connective tissue
called trabeculae.

6. 1
4
3
2
Haematoxylin and eosin
1 - lobules
2 - interlobular
connective tissue
3- cortex
4 - medulla

7. Each follicle is organized into two compartments:
•
•
•
The cortex, is densely packed with immature T
cells, called thymocytes .
The medulla where there are fewer lymphoid cells
(thymocytes).
It also contains nest-like
corpuscles of Hassall.
bodies,
concentric

8. •
Stromal-cell network composed of epithelial cells,
dendritic cells, and macrophages.
•
They make up the framework of the organ and
contribute to the growth and maturation of
thymocytes.
•
Many of these stromal cells (e.g. Nurse Cells,
cTEC, mTEC) interact physically with the
developing thymocytes.

9. •
The thymus continues to grow between birth and
puberty and then begins to atrophy; this thymic
involution is directed by the high levels of circulating
sex hormones.
Age
Mass
birth
about 15 grams;
puberty
about 35 grams
twenty-five years
25 grams
sixty years
less than 15 grams
seventy years
as low as 5 grams

10. •
•
•
Development and maturation of T lymphocytes.
Education of developing thymocytes by positive and
negative selection.
o Generation of MHC restriction (positive selection).
o Induction of central tolerance (negative selection).
Generation of diversity of TCR and immunological
repertoire.

14. Color Atlas of Immunology

15. Journey
Through
the
Thymus
Nat. Imm.
Rev.

17. •
•
The thymus induces the death of those T cells that
cannot recognize antigen-MHC complexes and those
that react with self-antigen– MHC and pose a danger
of causing autoimmune disease.
More than 95% of all thymocytes die by apoptosis in
the thymus without ever reaching maturity.

18. •
•
•
Thymectomized neonate mice show a dramatic
decrease in circulating lymphocytes of the T-cell
lineage and an absence of cell-mediated immunity.
DiGeorge Syndrome characterized by congenital
thymic aplasia, or congenital deficiency of a thymus.
There is an absence of circulating T cells and of cellmediated immunity and an increase in susceptibility
to infectious diseases.

19. •
•
•
•
•
Severe combined immunodeficiency syndromes
(SCID).
Autoimmune
Polyendocrinopathy-CandidiasisEctodermal Dystrophy (APECED) caused by
mutations in the Autoimmune Regulator (AIRE)
gene.
Myasthenia gravis.
Thymomas and Lymphomas.
HIV/AIDS.

20. •
•
Surgical removal or involution of the thymus does
not result in T cell immunodeficiency !!!!!!!!!!!!!!???
Sufficient T cells are generated during fetal life that
are long-lived and can perform homeostatic
proliferation throughout the lifetime of the patient.
•
Secondary thymi.
•
Extra-thymic development of T-cells.

21. The Belly of The Beast
A living Hell for intruders

22. •The
spleen plays a major role in mounting immune
responses to antigens in the blood stream.
•It
is a large, ovoid secondary lymphoid organ situated
high in the left abdominal cavity.
•While
lymph nodes are specialized for trapping
antigen from local tissues, the spleen specializes in
filtering blood and trapping blood-borne antigens;
thus, it can respond to systemic infections.

24. The red pulp (RP) is composed of open sinusoids
containing blood.
The white pulp (WP) contains lymphocytes. (White?)

26. •
•
•
red pulp: consists of a network of sinusoids
populated by macrophages and numerous red blood
cells (erythrocytes) and few lymphocytes;
it is the site where old and defective red blood cells
are destroyed and removed.
white pulp: surrounds the branches of the splenic
artery, forming a periarteriolar lymphoid sheath
(PALS) populated mainly by T lymphocytes.