Primary lymphoid organs

  • Lymphocytes, which are immature and generated in hematopoiesis gets mature and become committed to a particular antigenic specificity within the primary lymphoid organs.
  • The cell becomes immune-competent only after the lymphocytes have matured in the primary lymphoid organs.
  • T cells arise in the bone marrow and develop in the thymus.
  • In many mammals including human and mice, B cells originate in bone marrow.
  • Both thymus and bone marrow are responsible for promoting T- cell and B-cell development respectively.
  • These two are considered to be primary lymphoid organs because initial lymphocyte differentiation in these organs is essential to the development of immune system.

Thymus

  • It is a lympho-epithelial structure derived from the epithelium of 3rd and 4th pharyngeal pouches at about 6th week of intrauterine life.
  • Finally, by 8th week it grows into a compact epithelial structure.
  • Maximum size is attained by the thymus at birth but gradually its size decreases by aging.
  • It is a flat, bi-lobed organ situated above the heart and behind upper part of sternum.
  • Each lobe is surrounded by a capsule and divided into lobules.
  • Strands of connective tissue called trabeculae separate these lobules from each other.
  • There are two compartments organized for each lobule: cortex being the outer compartment and medulla the inner one.
  • Cortex is densely packed with immature T-cells called thymocytes whereas medulla is sparsely populated with thymocytes.

Image result for thymus                         Image result for thymus

  • The cortex and medulla of the thymus are crisscrossed by three-dimensional stromal cell network.
  • These networks are composed of epithelial cells, dendritic cells, and macrophages, which make up the framework of the organ.
  • All these structures contribute to the growth and maturation of thymocytes.
  • Many of these stromal cells interact physically with the developing thymocytes.
  • The function of thymus is to generate and select a repertoire of T cells that will protect the body from infection.
  • Large number of different T-cell receptors is generated by gene arrangement as the thymocytes develop, which produces some T cells with receptors capable of recognizing antigen- MHC complexes.
  • However, among those receptors generated by gene rearrangement process, most of them are incapable of recognizing antigen-MHC complexes.
  • But, a small portion reacts with combinations of self antigen-MHC complexes.
  • Using some mechanism, the thymus induces the death of those T cells that cannot recognize antigen-MHC complexes.
  • Those T cells that react with self antigen-MHC strongly enough can pose a danger of causing auto-immune disease.
  • More than 95% of all the thymocytes die by apoptosis in the thymus without ever reaching maturity.
  • However, minority of cells around 2% of the cells leave the thymus as mature T cells and migrate to the secondary lymphoid organs during foetal and early postnatal life and possibly over a longer period.
  • Thymus attains its maximum size at puberty and then atrophies.
  • This results in the significant decrease in both cortical and medullary cells and an increase in the total fat content of the organ.
  • The weight of thymus is 30 grams in human infants which get reduced to about 3 grams in elderly as it is dependent to age factor.
  • This loss in mass of thymus is accompanied by a decline in T cell output.
  • By age 35, the thymic generation of T cells has dropped to 20% and by age of 65, the output has fallen to only 2%of the new born rate.
  • The life span of immunologically competent T cells is believed to be months or years, much longer than that of B cells (days or weeks).

Bone marrow

  • Bone marrow is a complex tissue where hematopoiesis and a fat depot occur.
  • 50% or more of the marrow compartment will be filled by fat with the passage of the time.
  • Bone marrow generates hematopoietic cells which move through the walls of blood vessels and enter the blood stream
  • Blood carries them out of the marrow and distributes these various cell types to the rest of the body.
  • In human and mice, B cell originates and develops in the bone marrow.
  • Arising from lymphoid progenitor, B cells which are immature, proliferates and differentiates within the bone marrow which interact directly with the stromal cells within the bone marrow.
  • Various cytokines are released by this interaction that is required for development.
  • Bone marrow B cells are the source of 90% of the immunoglobulins IgG and IgA in plasma.
  • During T cell maturation, thymic selection occurs which is similar to the selection process in bone marrow for B cells that possess self-reactive antibody receptors.

Image result for bone marrow                                             Image result for bone marrow

  • Bone marrow might not be the site of B cell development for all species as in human and mice.
  • In birds, bursa of Fabricius which is a lymphoid organ associated with the gut works as the primary site of B cell maturation.
  • In mammals such as primates and rodents, there is not such organ which works as primary lymphoid organ.
  • In cattle and sheep, fetal spleen plays a vital role as a primary lymphoid tissue hosting the maturation, proliferation and diversification of B cells early in gestation.
  • Later in gestation, this function is assumed by tissues that are embedded in the wall of intestine called the ileal Peyer’s patch.
  • These tissues contain a large number of B cells as well as T cells.
  • The rabbit also uses gut associated tissues i.e. appendix as primary lymphoid tissue for important steps in the proliferation and diversification of B cells.

Primary lymphoid organs

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