Lymphoid organs are essential for the production, maturation, and proliferation of lymphocytes, and are categorized into primary (thymus, bone marrow) and secondary (spleen, lymph nodes) organs. The thymus is a critical site for T cell maturation, while bone marrow is responsible for the formation of blood cells and the maturation of B cells. Both organs play vital roles in immunological responses and the development of immune competence.

1. LYMPHOID ORGANS
Dr.R.RENUKA
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V.V.VANNIAPERUMALCOLLEGEFORWOMEN
VIRUDHUNAGAR,TAMILNADU.

2. DEFINITION
The organs concerned with the
production, maturation and
proliferation of lymphocytes are
called as lymphoid organs.

3. Types of Lymphoid organs
 There are two types of lymphoid organs:
Primary or central lymphoid organs
and
Secondary or peripheral lymphoid organs.

4. 1°(central) lymphoid organs
These are the major site for the formation of lymphocytes.
Here, the T lymphocytes and В lymphocytes mature into
immunocompetent cells by acquiring their antigen-specific receptors
(in the absence of any antigenic stimulation). After maturation, the
lymphocytes migrate to secondary lymphoid organs through blood and
lymph.
Primary lymphoid organs include thymus and bone marrow.
T cells mature in thymus and B cells in bone marrow.

5. 2° or peripheral lymphoid organ
These organs are those where antigen is concentrated and
allowed to interact with immune cells for proper immune
response.
These include spleen, lymph node and MALT that includes
NALT, GALT, BALT and genitourinary system.
In addition, there are 3° lymphoid organ that consists of loose
aggregates of lymphocytes often found in connective tissue.
The most important 3° lymphoid tissues are the cutaneous-
associated lymphoid tissues.

6. Thymus
 Thymus is a specialized primary lymphoid organ of the
immune system. Within the thymus, T cells mature.
 It resembles a thyme leaf and hence the name.
 It is situated below the thyroid gland and above the heart in the thorax region
as a derivative of pharynx.
 It begins its development on the 6th week of gestation.
 It is quite large in size at the time of birth but keeps reducing with age.
 It is a bilobed structure and the two lobes are covered by a fibrous capsule.
 Each lobe is organised into lobules which are separated from one another by
septae called as trabeculae.

7. THYMUS-CORTEX
Each lobule is organized into 2 compartments - outer cortex and inner medulla criss -crossed
by the network of stroma.
The cortex consists of immature T lymphocytes (thymocytes), reticular epithelial cells and
macrophages.
As the T cells mature, they move from cortex to medulla.
The reticular cells form a three dimensional network and in the meshes of which the
lymphocytes are found. These reticular cells are also called as ‘nurse cells’.
In thymus, thymocytes undergo rapid proliferation.
Rapid cell division is coupled with a high rate of cell death so that finally a very small subset
of thymocytes migrate from cortex to medulla after thymic education.
On the way to medulla, thymocytes begins to express receptors for Ags and surface markers.

8. THYMUS - CORTICOMEDULLARY JUNCTION
It is the site where the selection of right T-cells occurs. T cells
undergo thymic education through positive and negative
selection.
T cells either self- reactive or do not recognize foreign Ag
displayed on self MHC are deleted (negative selection).
Only the T cells whose receptor recognizes a foreign Ag plus
self- MHC molecule are allowed to mature and enter the medulla
(positive selection).

9. Cross section of part of thymus

10. Thymus- Medulla
The medulla consists of vascular structure, more reticular
epithelial cells and scattered mature T lymphocytes.
There are also some interdigitating cells associated with the
epithelial network and these cells are rich in MHC class II
antigens.
Hassall’s corpuscles - the whorls of epithelial cells around a
central core of degenerating epithelial cells with unknown
functions are also present.

11. Thymus… contd…
 The T lymphocytes are mature but nonfunctional or naive when they are
inside the thymus, as they do not come in contact with the antigens. This is
mainly because of the presence of epithelial barrier which prevents the
antigens of the blood to contact the lymphocytes of the thymus.
 These non-functional T lymphocytes travel through blood and lymph
circulation and finally reach the thymus dependent areas of the secondary
lymphoid organs (spleen) and get colonized there.
 In the secondary lymphoid organs, these T cells become functional by
antigenic stimulation.

12. Functions of Thymus
Thymus is the site for T cell maturation and proliferation.
It brings about cell mediated immunity.
It is responsible for graft rejection.

13. Thymectomy
Removal of thymus in mice results in
Impairment of cell mediated immunity
 drastic change in allograft rejection
delayed type hypersensitivity reaction
Wasting syndrome (hunched appearance, ruffled fur and
diarrhoea.)
Immunodeficiency
In humans,
Congenital Birth Defect- Di George Syndrome (delays in learning
to walk or talk and other developmental and learning delays).

14. Bone marrow (BM)
 BM is a major haematopoietic cellular connective
tissue within the bone that produces all immune
cells and delivers them into the circulation.
 There are 3 types of bone marrow tissues:
Red Marrow
White Marrow
Stroma
 BM is richly supplied with blood vessels and
capillaries but it lacks lymphatic vessels.

15. Bone marrow…contd…
 Red marrow-red because of the presence of RBC and its precursors.
- Site of active haematopoiesis found in the flat bones.
- At birth all bone marrow are red. With increasing age almost half of it is
converted into yellow marrow.
 Yellow marrow- yellow because of the presence of fat cells found in the
interior of long bones.
- Reduced haematopoiesis.
- These two forms are interconvertible as demands change.
 Stroma- tissue of BM provides microenvironment for haematopoiesis.
Cytokines and growth factors are produced. Supply ion to RBC.

16. FUNCTIONS OF BONE MARROW
 Site of stem (blood) cell formation.
 Functions both as primary and secondary lymphoid organs
 B cells origin and mature
 Selection process occurs that deletes those B cells which are self – reactive
 Mature B cells exit via vascular sinuses and enter the blood stream
 Some of the lymphoid stem cells migrate to thymus for their maturation into T cells.
 Cytokines and growth factors are secreted by stroma of bone marrow.
 Bone marrow is also a source of antibody synthesis

17. BM functions as both primary and
secondary lymphoid organs
 Bone marrow functions as primary lymphoid organ as the B cells are
developed and differentiated into immunocompetent B cells in it.
 In mammals, few days after secondary response, activated memory B
cells migrate into bone marrow where they mature into plasma cells. By
producing specific antibodies to antigens, the bone marrow acts as a
secondary lymphoid organ.