Primary lymphoid organs/ Central lymphoid organs
It is the site of lymphocyte development (B cell and T cell development)
What happens in these organs?
- Immature lymphocytes generated via the process hematopoiesis will mature here.
- Lymphocytes were committed to a particular antigenic specificity within the primary lymphoid organs.
Primary lymphoid organs are of two types: Thymus and Bone marrow
The thymus is the site of T-cell development and maturation.
Development of thymus: From endoderm of the third pharyngeal pouches (arises during the fifth week)
Location of the organ: In the mediastinum between the sternum and the aorta
Composition: Mainly made up of epithelial cells, immature and mature lymphocytes and fat tissue
Size of the thymus:
- The thymus is largest and most active during the neonatal and pre-adolescent periods.
- Then the size of the thymus keeps on reducing as you get older due to hormonal changes.
- By late adulthood, most of the thymus is made up of fat tissue.
- In older adults, the thymus has atrophied. But still it produces new T cells for immune responses (Average weight of the thymus is 70 g in infants, average weight of thymus in an adult is only 3 g)
Structure of thymus: flat, bilobed (right lobe and left lobe), ductless gland
- Each lobe is surrounded by a capsule which protects the thymus.
- Each lobe is divided into smaller sections called lobules. These lobules are separated from each other by strands of connective tissue called trabeculae.
Each lobule is organized into two compartments:
1) The outer compartment: Cortex (densely packed with immature T cells called thymocytes) – deeply staining
Composition of the cortex: Large numbers of T cells, dendritic cells, epithelial cells, and macrophages.
(These cells produce thymic hormones and express high levels of class I and class II MHC molecules)
What happens in cortex: Immature T cells (pre-T cells) from red bone marrow migrate to the cortex where they proliferate and begin to mature.
- Dendritic cells assist in the maturation process.
- Specialized epithelial cells (thymic nurse cells) helps to “educate” the pre-T cells by a process known as positive selection
- They also produce thymic hormones that participate in the maturation of T cells.
- Only about 2% of developing T cells survives in the cortex and others will die via apoptosis. Thymic macrophages remove these dead cells and wastes.
- Finally the surviving T cells enter the medulla part of the thymus.
2) The inner compartment: medulla (sparsely populated with thymocytes) – lighter-staining
Composition of medulla: More mature T cells than those found in cortex region, epithelial cells, dendritic cells, and macrophages
Thymic (Hassall’s) corpuscles are found in this region. They may serve as sites of T cell death in the medulla.
Medulla connects to the venous bloodstream.
Function: Transport of mature, immunocompetent, inactive T cells to the lymph nodes, spleen and other lymphatic tissues via blood
Major functions of thymus
1) Lymphopoeisis: Maturation and proliferation of T lymphocytes
2) Central tolerance – The thymus sorts T cells so that they will be inactive towards host molecules. Loss of central tolerance leads to autoimmune disease
3) Secretion of several hormones related to immunity – Thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin etc which will promote the maturation of T cells.
Red bone marrow
- In this organ stem cells develop into the various types of blood cells, including lymphocytes (B cells and pre T cells).
- Bone marrow is the site of B-cell origin and development (In human and mice).
- Immature B cells proliferate and differentiate within the bone marrow and acquire immunocompetance from here.
- The pre-T cells generated in the bone marrow migrate to the thymus, where they become immunocompetent T cells.
- Bone marrow contains stromal cells. These cells interact directly with the B cells and secrete various cytokines that are required for development.
Bursa of Fabricius
- Sac like primary lymphoid organ present only in birds.
- It is located dorsal to the cloaca.
- Play an important role in the development and maturation of B cells. It is the site of hematopoiesis.