Primary lymphoid organs à maturation of
lymphocytes takes places.
Thymus
It is the site
for T cell development and maturation.
Bilobed organ
situated above the heart. Each lobe is surrounded by a capsule and divided into
lobules, which are separated from each other by strands of connective tissue
called trabeculae. Each lobule is organized into two compartments: cortex which
is densely packed with immature T cells called Thymocytes whereas the inner
compartment or medulla is sparsely populated with thymocytes.
Both the
compartments consist of epithelial cells, dendritic cells, and macrophages
which make up the framework of the organ and contribute to the growth and
maturation of thymocytes.
Function of thymus:
to generate and select a repertoire of T cells that will protect the body from
infection. As thymocytes develop, enormous diversity of T cell receptors is
generated by gene rearrangement.
It produces some T cells with receptors
capable of recognizing antigen MHC complexes. But only small portion of cells
survive and rest will undergo apoptosis.
Bone marrow
It is a complex
tissue that is the site of haematopoiesis and a fat depot. Hematopoietic cells
are generated here move through the walls of blood vessels and enter the
bloodstream, which carries them out of the marrow and distributes these various
cell types to the rest of the body.
It is site of B
cell origin and development. Immature B cells proliferate and differentiated
within the Bone marrow and stromal cells within the bone marrow interact
directly with the B cells and secrete various cytokines that are required for
development.
Secondary lymphoid organs à site for mature lymphocytes to interact
with antigen.
Lymphatic system
lymphoid
follicles: primary follicle à network of
follicular dendritic cells and small resting B cells. After antigenic
challenge, a primary follicle becomes secondary follicle à a ring of concentrically packed
B lymphocytes surrounding a center (the germinal center) in which one finds a
focus of proliferating B lymphocytes and an area that contains nondividing B
cells and some helper T cells interspersed with macrophages and follicular
dendritic cells.
Lymph nodes and
spleen à surrounded by
fibrous capsules.
MALT à less organized lymphoid tissue
found in various body sites.
Lymph node:
Encapsulated
bean shaped structures containing a reticular network packed with lymphocytes,
macrophages and dendritic cells. They are organised in such a way to encounter
antigens that enter the tissue spaces.
Cortex à outermost layer contains B cells
mostly, macrophages and follicular dendritic cells arranged in primary
follicles. After antigenic challenge, the primary follicles enlarge into
secondary follicles, each containing germinal center.
Paracortex à it is beneath cortex which is
populated largely by T cells and also contains dendritic cells that migrated
from tissue to the node.
Medulla à more sparsely populated with
lymphoid lineage cells, and of those present, many are plasma cells actively
secreting antibody molecules.
Lymph nodes
traps the antigens from local tissues.
Spleen:
Present in the high left abdominal cavity. It
is large ovoid, plays a major role in mounting immune responses to antigens in
the blood stream. Spleen filters the
blood and traps the blood borne antigens. So it responds to systemic
infections. It is not supplied by lymphatic vessels, instead antigens and cells
are carried into spleen through splenic artery.
It is surrounded by capsules from which a number of projections extend
into the interior to form a compartmentalized structure.
Two compartments:
Red pulp:
network of sinusoids populated by macrophages, numerous red blood cells, and
few lymphocytes. Defective and old RBCs are destroyed and removed.
White pulp: it
surrounds a branches of splenic artery forming a periarteriolar lymphoid sheath
(PALS) populated mainly by T cells. Primary lymphoid follicles are attached to
PALS.
Marginal zone
contains lymphocytes and macrophages. Antigens are trapped in marginal zone by
dendritic cells, which then carry it to the PALS. And also lymphocytes enter
through the same route as of the antigen.
MALT (mucosa associated lymphoid tissue)
The vulnerable
membrane surface of digestive, respiratory and urogenital systems are defended
by a group of organized lymphoid tissues called MALT.
Respiratory
epithelium à bronchus
associated lymphoid tissue (BALT)
Epithelium of
the digestive tract à Gut- associated
lymphoid tissue (GALT)
MALT includes
tonsils, and appendix.
M cells carry
the antigen from lumina of the tracts to the underlaying MALT. They are
flattened epithelial cells lacking the microvilli that characterize the rest of
the mucous epithelium. They have deep invagination, or pocket in the
basolateral, plasma membrane which is filled with a cluster of B cells, T cells
and Macrophages.
0 comments :
Post a Comment