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Immune system
Immune System
Organs
Lymphoid Organs
•
A number of morphologically and functionally diverse organs and tissue
contribute to the development of immune responses.
•
These organs can be distinguished by function as primary and
secondary organs.
•
The thymus and bone marrow are the primary (or central) lymphoid
organs, where maturation of lymphocytes takes place.
•
The secondary (or peripheral) lymphoid organs include lymph nodes, the
spleen, the various mucosa-associated lymphoid tissues (MALT) such
as gut-associated lymphoid tissue (GALT) and bronchus associated
lymphoid tissue (BALT). These organs provide sites for mature
lymphocytes to interact with antigen.
Bone Marrow
Bone Marrow
• Bone marrow is a complex tissue that is site of hematopoisis and fat
deposit.
• In fact, with the passage of time, fat eventually fills 50% or more of the
marrow compartment of bone.
• Hematopoietic cells generated in bone marrow move through the walls
of blood vessels and enter the blood stream, which carries them out of
the marrow and distributes these various types to the rest of the body.
Thymus
Thymus
•
These thymocytes then move into the medulla, where they undergo
further differentiation and selection and finally migrate via circulation
to the secondary lymphoid organs/ tissues where they are able to
respond to microbial antigens.
•
Most (95%) of the thymocytes generated each day in the thymus die
by apoptosis with less than 5% surviving.
•
Molecules important to T cell function such as CD4, CD8 and T cell
receptor develop at different stages during the differentiation process.
•
The main functions of the thymus as a primary lymphoid organ are:
1.
2.
To produce sufficient numbers (millions) of different T cells each
expressing unique T cell receptors such that, within this group, there
are at least some cells potentially specific for huge number of
microbial antigens in our environment (generation of diversity).
To select for survival those T cells which bind weakly to self MHC
molecules (positive selection), but then to eliminate those which bind
too strongly to these same self MHC molecules (negative selection) so
that the chance for an autoimmune response is minimized.
Different Kinds of Dendritic Cells &
Their Origins
Dendritic cells



Found mainly in
lymphoid tissue
Function as antigen
presenting cells
(APC)
Most potent
stimulator of T-cell
response
Different Kinds of Dendritic Cells &
Their Origins
Human Lymphoid System
Structure of a Lymph Node
Circulation of lymphocytes
Structure of the Spleen
The spleen is the largest lymphoid organ. It is specialized for trapping blood-borne
antigens.
The white pulp forms a sleeve, the periarteriolar lymphoid sheath (PALS), around the
arterioles; this sheath contains numerous T cells. Closely associated with PALS is the
marginal zone, an area rich in B cells
Sites of
effector function
Primary (Central):
Bone marrow
Thymus
Secondary (Pheripheral):
Lymph nodes
Spleen
Peyer’s patches
Lymph node
distribution
in the body
Common Immune System of Mucous Membranes
MALT (Mucous Associated
Lymphoid Tissue)
GALT: tecido linfóide associado ao trato
gastrointestinal. Possui as placas de Peyers no íleo.
BALT: tecido linfóide associado aos brônquios
NALT: tecido linfóide associado ao nariz
SALT: tecido linfóide associado à pele
VALT: tecido linfóide associado aos vasos sangüíneos
CALT: tecido linfóide associado ao olho (à conjuntiva)
Mucosa Associated Lymphoid Tissue (MALT)
Cross-sectional diagram of the mucous membrane ling the intestine, showing a
Peyer’s patch lymphoid nodule in the submucosa. The intestinal lamina contains
loose clusters of lymphoid cells and diffuse follicles.
Gut Associated Lymphoid Tissue
(GALT)
Structure of the M cells and production of Ig A at inductive sites. M cells,situated in
mucous membranes, endocytose antigen from the lumen of the digestive,
respiratory, and urogenital tracts. The antigen is transported into the large
basolateral pocket.
Bronchus Associated Lymphoid Tissue
(BALT)
Cutaneous Associated Lymphoid Tissue
(CALT)
The skin is the largest organ in the body and
plays an important role in nonspecific (innate )
defences. The epidermal (outer) layer of the
skin is composed of specialized cells called
keratinocytes. These cells secrete a number
of cytokines that may function in local
inflammatory reaction. Scattered among the
epithelial-cell matrix of the epidermis are
Langerhann’s cells, atype of dendritic cell,
which internalize antigen by phagocytosis or
endocytosis. They undergo maturation and
migrate from the epidermis to regional lymph
nodes, where they function as potent
activators of naïve TH cells. In addition to
Langerhans cells, the epidermis also
contaions so-called intraepidermal
lymphocytes, which are mostly T cells. The
underlying dermal layer of the skin also
contains scattered T cells and macrophages.
Most of these dermal cells appear to be either
previously activated cells or memory cells.
Innate vs. adaptive immunity
Cells of innate immunity
Figure 13-9 Dendritic cells initiate immune responses. Immature dendritic cells constantly internalize and process proteins, debris, and microbes, when present. Binding
of microbial components to Toll-Like Receptors (TLRs) activates the maturation of the DC so that it ceases to internalize any new material, moves to the lymph node, upregulates MHC II, B7 and B7.1 molecules for antigen presentation, and produces cytokines to activate T cells. Release of IL-6 inhibits release of TGF β and IL-10
by T regulatory cells. The cytokines produced by DC and its interaction with TH0 cells initiate immune responses. IL-12 and IL-2 promote TH1 responses while IL-4
promotes TH2 responses. Most of the T cells divide to enlarge the response, but some remain as memory cells. Memory cells can be activated by DC, macrophage, or B
cell presentation of antigen for a secondary response.