organs of immune system for bsc students

1. ORGANS OF IMMUNE SYSTEM
Based on their function they are of two types
1.PRIMARY LYMPHOID ORGANS :
• Also known as central lymphoid organs
• Process of formation and development of blood cells,mature and become
committed to a particular antigenic specificity within the primary lymphoid
organs
• They are transported to secondary lymphoid organs via circulation
• Thymus,bursa of fabricus ,bone marrow are the primary lymphoid organs
2.SECONDARY LYMPHOID ORGANS
• Also known as peripheral lymphoid organs
• Here they bing to appropriate antigen and undergo further antigen
dependent differentiation
• Lymph nodes spleen are the secondary lymphoid organs
PRIMARY LYMPHOID ORGANS
• THYMUS:
• It is a flat, bilobed organ situated above the heart and below the thyroid gland.
• Each lobe is surrounded by a capsule and is divided into lobules, which are
separated from each other by strands of connective tissue called trabeculae.
• Each lobule is organized into two compartments: the cortex (outer compartment)
the medulla (inner compartment).
• The progenitor T cells formed during hematopoiesis in bone marrow enter into
the thymus where they multiply rapidly in the cortex. Cortical thymocytes are
phenotypically and functionally immature but progressively differentiate into T
cells within the thymic cortex.
• Thymocytes then migrate to the medulla where selection and maturation are
completed and finally enter into circulation. Only 2% of thymocytes leave the
thymus each day, the remaining cells die.

2. • Both the cortex and medulla of the thymus are criss crossed by a three
dimensional network consisting of epithelial cells ,dendrite cells and macrophages
which helps in the growth and maturation of thymocyctes .
• some epithelial cells of the cortex have long interconnecting cytoplasmic
extensions that surrounds as many as 50 thymocytes these cells are called nurse
cells .
• For the differentiation and maturation of thymocytes several hormonal f.
actors are required. The epithelial cells of the thymus serve as the source of these
hormones namely, α1-thymosin, β4-thymosin, thymopoietin and thymulin..
• T-lymphocytes are so educated in the thymus becine capable of mounting cell
mediated immune system against appropriate antigen.
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• BONE MARROW
• All the cells of the immune system are initially derived from the bone marrow
through a process called hematopoiesis
• Bone marrow is the soft tissue in the hollow shafts of the flat bones
• Immature B cells proliferate and differentiate within the bone marrow by
interacting with the various cytokines secreted by the stromal cells.

3. • During this maturation process, those B cells that develop self reactive antibody
receptors or fail to develop functional antibody molecules are eliminated by the
selection process.
• About 90% of differentiating B cells are believed to have this fate. B cells that
survive this selection process leave the bone marrow through efferent blood
vessels.
• BURSA OF FABRICUS:
• Bursa of fabricus is a primary lymphoid organ in birds where stens cells from
yolk sac ,foetal lliver and bone marrow mature proliferate and differentiate into
burs derived lymphocytes called B lymphocytes or Bcells then migrates from it
and reaches secondary lymphoid organs and further antigenic stimulation takes
place.
• Bursa of fabricus arises as a pouch from the dorsl part of cloaca in birds.
• SECONDARY LYMPHOID ORGANS:
• LYMPH NODES:
• Lymph nodes are encapsulated bean-shaped structures, normally packed with
lymphocytes, macrophages, and dendritic cells (Fig. 15). Lymph nodes are
present at the junctions of lymphatic vessels and serve as the first organized
structures to encounter most antigens
• a lymph node can be divided into three regions: the cortex, paracortex and
medulla, each of it providing a different microenvironment.
• Cortex is the outermost layer contains mostly B lymphocytes, follicular
dendritic cells and macrophages all arranged in clusters called primary follicles.
• Paracortex is the region just beneath the cortex which is largely populated
with T lymphocytes and also contains interdigitating dendritic cells. Due to the
presence of more T-lymphocytes Paracortex is a thymus dependent area as
compared to the thymus independent cortex region.
• Medulla is the inner most region, more sparsely populated by cells. Many of
the cells are plasma cells; activated TH and TC cells are also present.
• Lymph enters the lymph node via afferent lymphatic vessel. The lymph
now percolates slowly inward through the cortex, paracortex and medulla
allowing phagocytic cells and dendrite cells to trap pathogens and antigens carried
by lymph.

4. • the lymph then is drained into a simgle large lymph vessel called efferent
lymphatic vessel that carries the lymph to the thoracic cavity .
•
• SPLEEN:
• The spleen is a large, ovoid filtering organ situated high in the left abdominal
cavity.
• spleen filters blood and trap blood borne antigen and thus responds to systemic
infections.
• The spleen is surrounded by a capsule that sends projections known as
trabeculae into the compartments. The compartment is of two types-red pulp
and white pulp separated by diffused marginal zone.
• The red pulp consists of erythrocyte rich blood intermingled with many dendritic
cells, macrophages, a few lymphocytes and plasma cells.
• The white pulp surrounds the splenic arteries forming a periarteriolar lymphoid
sheath (PALS) populated chiefly by T lymphocytes

5. • . Blood borne lymphocytes and antigens enter the spleen through splenic artery
Sooner the antigen enters the marginal zone, the interdigitating dendritic cells
traps them and presents it with class II MHC molecules to carry it to TH cells.
• This further leads to the activation of B cells.
• The activated B cells, together with some TH cells migrate to the primary follicle
and develop into secondary follicles containing germinal centers like those in the
lymph nodes.