Tuesday, 6 November 2012

Organs of the immune system

The immune system can be distinguished by function as the:
Primary Lymphoid organs:
  • Only after a lymphocyte has matured a primary lymphoid organ is the cell immune competent.
  • In mammals, T-cells mature in the "Thymus" and B-cells mature in the "Bone marrow” (in Bursa of Fabricus in birds).
  • There are TWO cells in the Primary Lymphoid Organs – Thymus and Bone Marrow.
Secondary lymphoid organs

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  • The thymus is a flat, bi-lobed organ situated above the heart.
  • Immature T-cells are simply referred as "Thymocyte", these are densely packed outer compartment (or) cortex of the thymus lobule.
  • Some of the thymic epithelial cells in the outer cortex, called "nurse cells", have long membrane extensions that surround as many as so thymocytes, forming large multicellular complexes
  • The average weight of the thymus is 70 grams in infants; its age dependent involution levels an organ with an average weight of only 3 grams in the elderly.

Explanations about Thymus:

  • The thymus covered by a fibrous capsule. It is formed of two lobes.
  • Each lobe of the thymus is organized into lobule which is separated from one another by septa called trabeculae.
  • With in each lobule, the cells are arranged into an outer cortex and an inner medulla.
  • The cortex is tightly packed with proliferating immature lymphocytes while the medulla contains more mature cells.
  • As the cells mature, they move from the cortex to the medulla.
  • The cortex consists of lymphocytes (thymocytes) and reticular cells.
  • The medulla consists of vascular structure, reticular epithelial cells and scattered lymphocytes.
  • There are also some interdigitating cells associated with the epithelial network and these cells are rich in MHC class II antigens.



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