Lymphoid organs include central lymphoid organs like the thymus and bone marrow where lymphocytes develop and peripheral lymphoid organs like the spleen and lymph nodes. The bone marrow contains stem cells and generates both B and T cells. The thymus promotes T cell differentiation. The spleen filters blood, stores red blood cells, and mounts immune responses. Lymph circulates through lymph vessels and nodes where it is cleaned by lymphocytes before returning to the blood. Mucosa-associated lymphoid tissue includes structures like the tonsils that initiate immune responses to antigens entering through mucosal surfaces.

1. LYMPHOID ORGANS

2. LYMPHOID
ORGANS
An organs where lymphocytes
are produced and develop

3. LYMPHOID ORGANS
Spleen
Bone Marrow
CENTRAL
LYMPHOID
ORGANS
Lymph
Lymph Node
Lymph Vessels
MALT
PERIPHERAL
LYMPHOID
ORGANS
Thymus

4. BONE MARROW
 Bone marrow is the soft, flexible connective tissue present within the
bone cavities.
 It contain pluripotent stem cells .
 Lymphocytes differentiates to become either T cells or B cells .
 There are two categories of bone marrow tissue: Red marrow and Yellow
marrow.
FUNCTIONS OF BONE MARROW
Bone marrow is the site of B – cells origin and development.
Bone marrow generates RBCs which carry oxygen to the tissues.

5. THYMUS
Function of the thymus
differentiation of lymphocytes called T cells, or T lymphocytes.
Produces hormones called thymosin that promote the differentiation and division of T cells,
making them immunocompetent.
 It is a flat, bilobed organ situated above the heart.
 It posses a fibrous capsule on periphery .
 Numerous septa arises from the capsule and pass into peripheral cortex dividing into lobules .
 In cortex , collection of lymphocytes are present while in medulla , lymphocytes are less in number .

6. SPLEEN
 LARGEST LYMPHOID ORGAN
 Lies in upper left quadrant of abdomen
 Above pancreas
 Dark red
 Oval shaped organ
 Roughly size of fist .
 The interior is the splenic pulp , contains
white and red pulp .
 White pulp contain masses of
lymphocytes while Red pulp contains
lymphocytes , plasma cells and
monocytes .

7. FUNCTIONS :-
 PHAGOCYTOSIS : erythrocytes are mainly destroyed in spleen and
breakdown products , bilirubin and iron , are transported to the liver .
Other cellular material eg, leukocytes , platelets and bacteria are
phagocytosed .
 STORAGE OF BLOOD : spleen contains upto 350 ml of blood and in
response to sympathetic stimulation can return most of the volume to
the circulation . Eg , hemorrrhage
 IMMUNE RESPONSE : spleen contains T and B lymphocytes , which
activate by the presence of antigens . Eg , in infection
 ERYTHROPOIESIS : important sites of fetal blood cell production

8. Lymph is the clear watery fluid that circulates throughout the lymphatic system.
Contains blood plasma , WBCs , antigens
Lymph is formed when the interstitial fluid is collected through lymph capillaries and then
transported through larger lymphatic vessels to lymph nodes , where it cleaned by
lymphocytes , and drain into subclavian vein where it mixes back with blood .
LYMPH
Lymph not present in
CNS
Cornea of eyes
Bones
Most superficial layers
of the skin .

9. LYMPH VESSELS
 Lymphatic vessels are thin walled vessels structured like blood vessels that
carry lymph .
 Lined by endothelial cells , have a thin layer of smooth muscle and adventitia
that bind the lymph vessels to surrounding tissue .
 Lymph vessels carry lymph to lymph node called afferent lymph vessels and
those carry it from lymph node called efferent lymph vessels .
 Lymph vessels have numerous cup shaped valves to ensure that lymph flows
in one way direction .

10. LYMPH NODE
 Oval or bean shaped organ
 Present throughout the body , most abundant
in the head , neck , abdomen , pelvis .
 Major sites of B and T lymphocytes
 Important for the proper functioning of
immune system .
 Help to remove and destroy antigens in the
blood and lymph .
 Enclosed in a fibrous capsule and divided it
into 3 compartments
1. superficial cortex : contains follicles of B cells
2. deep cortex : contains T cells
3. medulla : contains plasma cells that secrete
immunoglobins.

11. MUCOSA – ASSOCIATED LYMPHOID
TISSUE ( MALT )
 The main sites of entry for microbes into the body are through mucosal surfaces.
 Hence, the majority (>50%) of lymphoid tissue in the human body is located within the
lining of the respiratory, digestive and genitourinary tracts.
 Small concentrations of lymphoid tissue are also found in thyroid, lung, salivary
glands, eye, and skin.
 These lymphoid tissues collectively are referred to as mucosa associated lymphoid
tissues (MALT).
 Main function is to initiates immune responses to specific antigens
 Examples include tonsils, Peyer’s patches

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