Biomedical Engineering Reference
In-Depth Information
sinuses encounter marginal zone and metallophilic macrophages as well as dendritic
cells [
220
]. Marginal zone dendritic cells then migrate to the T-cell region — the
peri-arteriolar lymphatic sheath
— of the white pulp as well as a B-cell follicle
encapsulated by the peri-arteriolar lymphatic sheath. T lymphocytes enter the spleen
from blood at the marginal zone, then migrate in channels formed by fibroblastic
reticular cells to reach the peri-arteriolar lymphatic sheath. Spleen-resident dendritic
cells acquire blood-borne antigens either directly from marginal sinuses or indirectly
from macrophages of the marginal sinuses. The marginal zone also contains a
population of non-circulating B lymphocytes that transport antigens from marginal
sinuses to follicular dendritic cells in the B-cell follicle. Activated lymphocytes
leave the spleen and enter the blood stream.
Human dendritic cells in blood and spleen can be classified into 3 known subpop-
ulations [
220
]: (1) CD11c
high
,
105
,
106
neg
/
low
HLADR
+
IL3R
α
(CD123) cells that
);
107
are blood dendritic cell antigen BDCA2
+
(CD1c
+
)orBDCA3
+
(CD141
+
α
X
Itg
neg
/
low
, HLADR
high
, BDCA2
(2)
+
,IL3R
α
+
cells; and (3) BDCA1
+
den-
dritic cells.
Antigen-presenting cells foster tolerance or immunity according to the availabil-
ity of danger signals. Antigen presentation by dendritic cells indeed provokes naive
T lymphocytes to differentiate into effector and memory T lymphocytes or leads
to different forms of tolerance according to the functional status of these dendritic
cells [
220
].
Stromal cells of these secondary lymphoid organs coordinate the interaction
between T lymphocytes and dendritic cells, as they recruit dendritic cells to
T-lymphocyte zones [
220
]. Resident stromal cells indeed guide migrating dendritic
cells within secondary lymphoid organs.
3.10.2.2
Immunological Synapse
T lymphocytes operate in immune responses either via secreted soluble mediators
or via cell contact. T lymphocytes release soluble agents for intercellular communi-
cation and target killing. The adaptive immune response requires the formation of
immunological synapses between mature antigen-presenting cells such as dendritic
cells and CD4
T cells in the lymph node.
108
+
105
I.e.,
X
-integrin.
106
The HLADR molecules are aimed at presenting peptidic antigens to the immune system to
trigger or suppress helper T-cell responses. These cell-surface receptor are HLADR
α
β
heterodimer. The HLADR molecules are encoded by several loci; each locusis composed of several
genes.
107
Expression of MHC (HLA) class-2 molecules (DR subunits) allows identification of an effector
cell subset (DR
α
-HLADR
).
108
Activated T cells leave the lymph node and return to the blood circulation, whereas dendritic
cells die in the lymph node.
+
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