The document discusses the lymphoid organs, which are divided into primary and secondary organs. The primary organs, including the bone marrow, thymus, and bursa of fabricius, are where lymphocytes develop and mature. The secondary organs, such as lymph nodes, spleen, and mucosal associated lymphoid tissue (MALT), are where lymphocytes encounter antigens and carry out immune responses. Primary organs are large at birth and shrink with age, while secondary organs are small at birth and grow with age.

1. Shri Shankaracharya Mahavidyalaya, Junwani , Bhilai
Lymphoid Organ
Dr. Sonia Bajaj
(Head of Department)

2. Introduction
The organs concerned with immune reactions are called lymphoid organs. They contain
lymphoid cells (Lymphocytes). The lymphoid organs and the lymphoid cell constitute the
lymphoid system.
are two types.
They are:
1. The primary lymphocyte organ (Central)
2. The secondary lymphocyte organ(Peripheral)
1. Primary Lymphoid Organs
Primary lymphocyte organs are the major site of lymphopoiesis. Here the lymphoid stem cell
proliferate, differentiate and mature into immunocompetent cells in the absence of antigenic
stimulation and after maturation, lymphocytes migrate through the blood or lymph and
accumulate in the secondary lymphoid organs. The thymus, the bursa of fabricius in birds and
bone marrow in mammal are the primary lymphocyte organs.
“The primary lymphoid organ a large at birth and they atrophy with age.”

3. These are of following types:
1. Bone marrow
Bone marrow is a primary lymphoid organ. It is a soft
tissue within the cavity of bones. In mammal it is the
largest issue with a total weight of about 3 kg in an
average human adult.
Marrow is divisible into two reasons, namely
1. Vascular and adipose region
2. haemopoietic region
Function of bone marrow
• hematopoietic stem cell arise in the bone marrow.
• From the hematopoietic stem cell of bone marrow
• all the different type of blood cells namely RBC,
WBC and platelets are developing.
• Majority of lymphoid progenitors develop into B
lymphocyte in the bone marrow.
• Some of the lymphoid progenitors migrate into the
thymus, where they develop in the t lymphocyte.
• Bone marrow functions both as a primary lymphocyte
and as a secondary lymphocyte organ.

4. 2. Thymus
Thymus is a primary lymphoid organ. In mammals, the thymus is a parapharyngeal
derivative arising from the epithelial of the third and fourth pharyngeal pouches at
about the six week of gestation.
In human being the thymus occurs as two oval lobes just behind the top of the
sternum, below the thyroid gland.
Function of thymus-
• It produces T Lymphocytes.
• It brings about cell mediated immunity.
• It brings about graft rejection.
•
.

5. 3.Bursa of Fabricius
Bursa of fabricius is a primary lymphocyte organ in birds. This is a sac -like
lymphoepithelial structure arising as dorsal diverticula from the cloaca in birds. This
organ originates from the highgut epithelium of chick embryo. The bursa is made up
of three types of cells,namely lymphocyte, macrophages and plasma cell. The
lymphocytes of bursa are called B (bursal) cells.
They have an unique antigen the B or beta antigen.
Function of bursa-
• It produces B -lymphocyte macrophages and plasma cells.
• It brings about humoral immunity

6. 2. Secondary Lymphoid Organs
The secondary lymphoid organ is concerned with immune reactions. in a
secondary lymphoid organ the lymphocyte are made functional. Here the cells
are exposed to antigen and they produce antibodies. The secondary lymphocyte
organs are small and poorly developed at birth and they grow progressively with
age.

7. These are of following types:
1. Lymph nodes
Lymph nodes are secondary lymphoid organs. These are
small solid structures located at different points along the
lymphatic system. They are complex, cellular, Spherical and
ovoid structure present along the lymphatic ducts. Human
lymph nodes are bean shaped. It’s about 1.25 mm in
diameter. Several afferent lymphatic ducts carry lymph into
the gland a single efferent lymphatic duct leaves the node.
Lymph node is covered by a capsule. Each lymph node made
is made up of three regions namely outer carterx, middle
paracortex and inner medulla. The cortex contents B cells,
Para cortex contents T cell and medulla content B and T cells.
Function of lymphoid notes
• They function is important centers of phagocytosis.
• Lymph nodes are also responsible for the initiation and
development of humoral and cell mediated immune
responses by processing both the t dependent and be
depended arise with germinal center's.

8. 2. Spleen
Spleen is a secondary lymphoid organ. It is a solid, encapsulated organ located in
the upper part of the abdominal cavity behind the stomach and close to the
diaphragm. It is a large bean-shaped organ containing lymphocytes and
phagocytes. It filters the blood by trapping the pathogens in it.
Function of spleen
• The graveyard for worn out blood cells.
• A reserve tank in the formation of RBC at times of necessity

9. 3. Mucosal associated Lymphoid
Tissue (MALT)
M A L T is the mucosa associated lymphoid
tissue. The mucosal layer of the alimentary canal,
respiratory and urinary genital tract is provided
with dispersed group of lymphoid tissue known
as MALT.
These tissues are usually without a capsule. in
human being the Peyer’s patches, the tonsil and
appendix a good example of lymphoid tissues
found in the mucosal layer of the alimentary
canal .hence these tissues are also referred as to
GULT(Gut Associated Lymphoid Tissue.)
A. Peyer’s patches - Peyer’s patches of small
intestine and appendix are also some of the
secondary lymphoid tissue. They are MALT.
B. Tonsils- Tonsils are lymphoid tissues encircling
the pharynx. They are secondary lymphoid tissue.
They from MALT

11. References-
Immunology –D.Fatima
Immunology – Kuby
Jantu Vigyan- S.M. sexsena
Jantu Vigyan- Dr.H.N. Baijal