Spleen
• Spleen has thick capsule, sends septa into substance of spleen.
• White pulp is aggregation of lymphocytes around the artery-periarterial lymphatic sheath (PALS) made of T lymphocytes. At some places it forms lymphatic nodules
with germinal centers called as Malpighian bodies. They contain B lymphocytes
Red pulp forms the major part of spleen. It is made of lymphocytes arranged like branching and anastomosing cords along the sinusoids.
• Spleen is the largest hemolymphoid organ.
Function--removes old red blood cells (RBCs), production of lymphocytes, removes blo0d-borne antigens. In fetal life it also acts as a site of erythrocyte production.
• Splenic circulation:
Central artery from white pulp enters red pulp and divides into straignt vessels called as penicill. It is then surrounded by sheath of macrophages and is called as
ellipsoid which has a narrow lumen. Further it dilates to form ampulla and blood enters sinusoids of red pulp.
In closed circulation theory blood directly enters into sinusoids Irom ampula. in open Circulation theory blood passes out of capillaries between the cords and then
enters sinusoids.
Tonsil
•Palatine tonsil is situated in tonsillar fossa, on lateral wall of oropharynx.
•It forms one of the constituents of Waldeyer's ring.
•It belongs to MALT (mucosa-associated lymphoid tissue). It is active during early life.
•Mucosal lining is non-keratinized stratified sqamous epithelium.
Mucosal lining invaginate to form crypt which are characteristic of palatine tonsil
•Lvmphocytes aggregated in the form of follicles with a central germinal center along the crypts. Folicles mainly contain B lymphocytes. T lymphocytes lie outside the
follicle.
•Mucus acini are seen in the deeper plane.
Thymus
•Thymus is a primary lymphoid organ. It grows in size up to puberty and regress during adulthood.
•Capsule sends incomplete septa into the parenchyma, dividing it into lobules.
Each lobule contains outer cortex and central medulla.
•Cortex is made of densely packed small lymphocytes
Also contains macrophages for phagocytosis of degenerating lymphocytes and epitheliocytes which provide supporting meshwork.
• Medulla of adjacent lobules are interconnected. It contains sparsely arranged larger lymphocytes and epitheliocytes.
•Degenerated epitheliocyte cell aggregations appear as concentric eosinophilic mass called Hassall's corpuscles in the medulla.
•Epithelioid cells form reticular system of thymus and forms blood-thymus barrier. Also produce hormones for proliferation and maturation of T lymphocytes.
•Function of thymus differentiation of T lymphocytes.
Lymph Node
•Lymph node has outer fibrous capsule. This sends trabeculae inside. There is distinct subcapsular space.
Afferent lymphatics around the gland pierces the capsule and drain into subcapsular space.
•Cortex contains lymphoid follicles with pale staining germinal center. Rim of the lollicles have densely packed inactive B lymphocytes. Germinal centers contain pale
staining large, rapidly dividing B lymphocytes.
•Paracortex, which is the junction between cortex and medulla contains T lymphocytes.
•In medulla lymphocytes arranged like branching cords along the sinuses called as medullary cords. These are mainly macrophages and plasma cells.
•Lymph will drain from the afferent lymphatics into subcapsular sinus, then passes along the trahacular sinus into medullary sinuses.
Finally drains out from the hilum through the efferent lymphatics.
As it passes through the sinuses it comes in contact with antigen presenting cells which present it to the lymphocytes.
•Functions of lymph node-filtration of lymph, proliferation of lymphocytes and forms a part of immune system.