Organs of the Immune system

Immune system consists of two organs primary and secondary lymphoid organs.

1-Primary (Central) lymphoid organs.
2-Secondary (Peripheral) lymphoid organs.
1-Thymus Gland.
2-Bone Marrow.
They consist of ….
Spleen.
Lymph nodes.
Mucosa- associated lymphoid tissue.
The sites of immune cell birth and the locations where they mature are considered primary lymphatic organs.

Locations in the body where immune cells become activated, reside, or carry out their functions are called secondary lymphatic organs

1. Organs of the Immune system
Hawler Medical University
College of Health Sciences
Medical Microbiology Dept.
Dr. Amer Ali Khaleel
(Ph.D. Medical Immunology)
Lecture 3
2
nd
stage

2. Organs of the Immune System
•Immune system consists of two organs primary and secondary
lymphoid organs.
1-Primary (Central) lymphoid organs.
2-Secondary (Peripheral) lymphoid organs.

3. •The primary lymphoid organs are those organs where T-Lymphocytes
and B-Lymphocytes mature and acquire their antigen specific receptor.
•Primary lymphoid organs consist of the following organs:
1-Thymus Gland.
2-Bone Marrow.
1-Primary (Central) lymphoid organs

4. 1-Thymus Gland:
•Thymusisaprimarylymphoidorganfoundinallvertebrates.
•Inmammals,thymusisabio-lobedorganfoundinthemiddleofthe
chestjustabovetheheart.
•Alllymphocytesoriginatewithinthebonemarrow;Tcellsaresentat
anearlyagetothethymusforadvancededucationtodifferentiate
betweenselfandnon-selfantigens.
•ThymusisthesiteofT-lymphocyte(Tcell)maturation.

5. 1-Thymus Gland:
•Duringitsmaturationinthethymus,theT-cellcomestoexpressa
uniqueantigen-bindingreceptoronitsmembranecalledT-Cell
receptor(TCR).
•Thesecellsdevelopspecificityandarereleasedintothecirculationas
matureTcells,subsequentlymigrateandsettleinothersecondary
lymphoidorgans.

6. Figure: Anatomy of Thymus
This figure just for preview

7. Thymus Education:
•PriorenteringthethymusT-cellprecursorislackantigenreceptors(TCR=TCell
Receptor),CD4andCD8moleculesontheirsurface(doublenegative
thymocytes).
•DuringpassagethroughthecortexofthethymustheydifferentiatetoTcellthat
expressTCR,CD4+andCD8+(doublepositivethymocyte).Positiveselection
•Inthemedulla,thesecellsdifferentiateintoexpresseitherCD4+ifcontactsacell
bearingclassIIMHCproteinsorCD8+ifcontactacellsbearingclassIMHC
proteins(singlepositivethymocyte).Negativeselection

8. •The majority (98%) of these T cells will die and only
(2%) will survive as mature cells.
•Negative & Positive selections are two processes
eliminateimmature T cells lacking proper antigen
receptor specificities.
•When T cell express the CD4 receptor become helper
T cell while those express the CD8 receptor become
cytotoxic T cell.
Thymus Education:

10. •Single positive functional T cells (mature) which then leave the thymus through
post-capillary venules to enter the peripheral blood circulation through which
they are transported to the secondary lymphoid organs in which T cells encounter
and respond to foreign antigens.
Figure : T-cell precursors migrate from the bone marrow to the thymus to mature.
This figure just for preview
Thymus Education:

11. 2-Bone marrow:
•Blymphocytes are formed (originate) within the Bone marrow.
•B lymphocyte (often simply calledB cells).
•B cells are home schooled and remain within the bone marrow.
•It is a lymphocyte cell and derived its name from the site of maturation.
•After maturation, when they leave bone marrow, they express antigen binding
receptor known a B-cell receptor (BCR), At this stage this B-cell is known as naïve
B-cell, means do not previously encountered with antigen, these B cells then migrate
to secondary lymphatic organs, such as the spleen or lymph nodes, where they wait to
encounter foreign antigen.

12. 2-Bone marrow:
•Binding of the antigen to BCR cause the
B-cell to divide rapidly and produced
two types of B-cell (1) Memory B cell
(2) Plasma cell or effector B-cell.
•Some of the activated B-cells do not
differentiate into plasma cells but rather
remain dormant as memory B-cells until
activated once again by a new quantity
of the same antigen.

13. Figure : B cells develop in bone marrow and then migrate to
secondary lymphoid tissues.
This figure just for preview

14. T cells undergo both positive and negative
selection whereas B cells undergo only
negative selection.

15. 2-Secondary (Peripheral) lymphoid organs
•One way to think of the secondary lymphatic organs is that
they are where the of immunity takes place.
•After maturation, B-cell and T-cell migrate via blood
vascular system to the secondary lymphoid organs.

16. 2-Secondary (Peripheral) lymphoid organs
•The secondary lymphoid organs are specially evolved to be sites within
the body where naïve immune cells have the maximum possibility of
meeting and responding to antigen.
•They consist of ….
1.Spleen.
2.Lymph nodes.
3.Mucosa-associated lymphoid tissue.

17. The lymph nodes, spleen, and various lymphoid tissues
contain large numbers of T and B cells and are stationed
throughout the body, ready to encounter antigen and become
activated.

18. 1-The Spleen:
•The spleen is the largest of the secondary lymphoid organs in the adult.
•It is located on the left side of the body just above the kidney.
•It is enclosed by a capsule and divided by trabecula into communicable
compartments.
•It is composed of white pulp, rich in lymphoid cells and located around small
arterioles in follicular area.
•Red pulpcontains many sinuses as well as large quantities of erythrocytes,
macrophage and some lymphocytes.
•Approximately (50%) of spleen cells are B cell and (30-40%) T cell.
•The spleen is also referred to as thegraveyard of RBCs.

19. It plays the dual role of
removing aged or injured
red blood cells in the red
pulp and producing
antibodies in the white
pulp.

20. Figure: Anatomy of the spleen.
This picture just for preview

21. 2-Lymph Node:
•They are small, kidney bean-shaped structures that are located throughout the body
along the lymphatic routes designed to filter antigens incoming from the tissue spaces.
•They contain specialized compartments where immune cells congregate, and where
they can encounter antigens.
•The lymph nodes are composed of medullaand cortex which surrounded by a capsule
of connective tissue.
•The cortical region contains primary lymphoid follicles, after antigenic stimulation
enlarge to form secondary follicles with germinal center rich with B cells.
•The paracortical area contains T cells and dendritic cells. Antigen presenting occurs
in this area that result in T cell activation
•The medullary area is rich with plasma cells.

22. Lymph nodes have two
primary functions:
Filter out potentially harmful
particles from the lymph
&
act with immune
surveillance provided by
macrophages and
lymphocytes.

23. This picture just for preview
Figure: Anatomy of the Lymph Node.

24. Spleen is somewhat similar to a lymph node
except that it serves as a filter for blood instead
of lymph.

25. 25
Whenapartofthebodyisinfected,thenearbylymphnodes
canbecomeswollen.Forexample,ifapersonhasathroat
infection,thelymphnodesintheneckmayswell.

26. 3-Mucosa-Associated Lymphoid Tissue (MALT):
•MALT is located within the lining of the major tracts (digestive,
respiratory and urogenital) system have a combined surface area and
the major sites of entry of most pathogens.
•These vulnerable membrane surface are defended by a group of
organized lymphoid tissues known collectively as mucosal associated
lymphoid tissue (MALT).
Examples:
•Gut-associated lymphoid tissue (GALT),
•Bronchus-associated lymphoid tissue (BALT)
•Conjunctiva-associated lymphoid tissue (CALT).
•Skin -associated lymphoid tissue (SALT).

27. Lymphatic System:
•A subsystem of thecirculatory
systemin thevertebratebody that
consists of a complex network
ofvessels,tissues, andorgans.
•The lymphatic system, for most
people, is associated with the
immune system to such a degree
that the two systems are virtually
indistinguishable.
This picture just for preview

28. This picture just for preview
The lymphatic system is
commonly divided into the
primary lymphoid organs,
which are the sites of B and T
cell maturation, and the
secondary lymphoid organs, in
which further differentiation
oflymphocytesoccurs.
What are the main organs of
the lymphatic system?

29. Thelymphaticsystemhasthreeprimaryfunctions:
1.The lymphatic system helps maintain fluid balancein the body by
collecting excess fluid and particulate matter from tissues and
depositing them in the bloodstream.
2.It also helps defend the body against infection by supplying disease-
fighting cellscalled lymphocytes. which help rid the body of disease-
causing microorganism.
Functions of the Lymphatic System

30. 3.Theorgansandtissuesofthelymphaticsystemarethemajorsitesof
production,differentiation,andproliferationoftwotypesof
lymphocytes—theTlymphocytesandBlymphocytes.
Functions of the Lymphatic System

31. AsAPCs(Macrophage,DC&B-cells)ingestanddigesttheforeign
particlesanddebrisfromtheinitialinflammatorylesion,withinhours
theybegintotransportthatmaterialtotheclosestdrainingsecondary
lymphoidorgan.
Dendriticcellsarethemostefficientattheprocessofantigentrapping
andtransport,andastheyarriveinthelymphnodes(enteringthrough
lymphaticsfromthetissuefluids)orthespleen(ifcarriedviatheblood),
theirsurfacesarecoveredwithMHCclassIIandpeptidecomplexes.
So,thepresentthiscomplextoadaptiveimmunecells.
Transportation of Antigen to Secondary Lymphoid Organs:

32. This picture just for preview

33. This picture just for preview

34. Recap:
•The sites of immune cell birth and the locations where they mature
are considered primary lymphatic organs.
•Locations in the body where immune cells become activated, reside,
or carry out their functions are called secondary lymphatic organs.

35. Any
Questions
??!!!
Next Lecture
Host Defense Mechanism