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Dr. P. Saranraj M.Sc., M.Phil., Ph.D., NET
Assistant Professor
Department of Microbiology
Sacred Heart College (Autonomous)
Tirupattur, Vellore District.
Mobile: 9994146964
E.mail: microsaranraj@gmail.com
LYMPHOID SYSTEM
LYMPHOID SYSTEM
The Immune system consists of Immune cells that
continuously circulate between the Blood and
Lymphoid organs.
The Lymphatic system consists of Lymphatic organs
and the circulating clear fluid called Lymph.
The Lymphatic system was first described in the seventeenth
century independently by Olaus Rudbeck and Thomas
Bartholin.
Lymphoid organs are classified into 2 types.They are:
(1) Primary Lymphoid Organs
(2) Secondary Lymphoid Organs.
PRIMARY & SECONDARY LYMPHOID ORGANS
PRIMARY LYMPHOID
ORGANS
SECONDARY LYMPHOID
ORGANS
Central lymphoid organs. Peripheral lymphoid organs.
Primary lymphoid organs are the
sites where Leukocytes (WBCs)
are generated. It includes: (i)
Bone marrow and (ii) Thymus.
Secondary lymphoid organs are the
sites where Adaptive immune
responses (Acquired immunity)
are initiated. It includes: (i)
Spleen, (ii) Lymph Node and (iii)
MALT.
The Lymphoid stem cells
proliferate (rapid increase),
differentiate and mature.
The Lymphoid stem cells becomes
functional.
Contains either B – cell or T – cell. Contains both B – cell or T – cell.
Antigens cannot enter in. Antigens enter in and stimulates
the Lymphoid cells.
PRIMARY & SECONDARY LYMPHOID ORGANS
PRIMARY LYMPHOID
ORGANS
SECONDARY LYMPHOID
ORGANS
Site of Maturation for T & B cells. Site of Cell function for T & B cells.
Develop before Secondary
lymphoid organs during Ontogeny
(Organization and development of
organs).
Develop after Primary lymphoid
organs during Ontogeny.
Differentiation of
Lymphocytes (Type of WBCs,
E.g., T cells, B cells & NK cells)
is taken place in Primary lymphoid
organs.
Interaction of immune cells
with each other and antigen
processing are taken place in
Secondary lymphoid organs.
Decrease in size and strength with
age.
Increase in size with age.
Lymphoid organs
THYMUS
Growth and maturation of T - lymphocytes takes
place inThymus only.
It is large at the time of birth (70 g) but with age, the
size keep on reducing and becomes very small by
attaining puberty (3 g). 
It is a flat, bilobed organ situated above the heart.
Each Lobe is surrounded by a Capsule and is divided into
Lobules, which are separated from each other by strands of
Connective tissue called Trabeculae.
Each lobule is organized into 2 compartments: the outer
compartment, or cortex, is densely packed with
immature T cells, called Thymocytes, whereas the
inner compartment, or medulla, is sparsely populated
with Thymocytes.
Both the cortex and medulla of the thymus are
crisscrossed (moved or travel around) by a three
-dimensional stromal - cell network composed of
Epithelial cells, Dendritic cells and Macrophages,
which make up the framework of the organ and contribute
to the growth and maturation of Thymocytes.
Some thymic epithelial cells in the outer cortex, called
Nurse cells, have long membrane extensions that
surround as many as 50 Thymocytes, forming large
multicellular complexes.
Hassall corpuscles are a characteristic morphologic
feature located within the medullary region of the
thymus.
FUNCTIONS OF THYMUS
The main function of the Thymus is to release Thymosin
hormone that will stimulate the maturation of T -
cells.
Failure ofThymus development shows dramatic decrease
in circulating Lymphocytes of the T-cell lineage and
absence of Cell - mediated immunity.
Aging is accompanied by a decline inThymic function.
BONE MARROW
Bone marrow is the soft, flexible connective
tissue present within the bone cavities.
In humans and Mice, bone marrow is the site of B - cell
origin and development.
Bone marrow forms around 4 % of total body weight.
There are two categories of bone marrow tissue: Red
marrow and Yellow marrow. From birth to early
adolescence, the majority of our bone marrow is red
marrow. As we grow and mature, increasing amounts of red
marrow is replaced by yellow marrow.
Bone marrow can generate 200 billions of new blood
cells every day.  
RED AND YELLOW BONE MARROW
RED BONE MARROW YELLOW BONE MARROW
Also known as Myeloid tissue. Also known as Fatty tissue.
Hematopoietic (formation of
blood cell components) in nature
and produces RBC, WBC &
Platelets.
Multipotent Stromal
(connective tissue cell of any organ)
in nature and produces Fat,
Cartilage and Bone.
Gets its red color from the
hemoglobin in the erythroid
cells.
Gets its yellow color from the
carotenoids in the fat droplets in
the high number of fat cells.
High Vascular supply. Poor Vascular supply.
Function - Helps to remove old
cells from circulation.
Function - When blood supply is
extremely low, yellow marrow can
be converted to red marrow in
order to produce more blood cells.
FUNCTIONS OF BONE MARROW
Bone marrow is the site of B - cell origin and
development.
A bone marrow transplant can save the lives of people
battling leukemia, lymphoma and other blood
cancers.
Bone marrow generates RBCs which carry oxygen to the
tissues.
Bone marrow generates Platelets or Thrombocytes
help prevent bleeding and aid in clotting of blood.
Granulocytes (Neutrophils, Basophils & Eosinophils) and
Macrophages fight against microbial infections. They also
remove dead cells and remodel tissue and bones.
LYMPH NODE
Lymph nodes are a group of small, bean-shaped organs
(2.6 cm in length) found mainly in the neck and
trunk of the human body.
They play vital roles in the filtration of antigens and
debris from Lymph (circulating colourless watery fluid)
and in the generation of immune responses to
pathogens.
Lymph nodes are often removed from cancer patients
as their filtration function catches tumor cells metastasized
(spread to other sites in the body) from primary tumors.
Lymphoid organs
STRUCTURE OF LYMPH NODE
The Capsule is made of Collagen and has a sub-capsular
Sinus.
The Lymph flows into the Sinus carrying Lymphocytes,
Antigen processing macrophages and Dendritic
cells to the node Cortex, Paracortex and Medulla.
Morphologically, Lymph node can be divided into 3 roughly
concentric regions: (1) Cortex, (2) Paracortex and (3)
Medulla.
The outermost layer, Cortex contains Lymphocytes
(mostly B - cells), Macrophages and Follicular
dendritic cells arranged in Primary follicles.
The Primary follicles enlarge into Secondary follicles,
each containing a Germinal center.
STRUCTURE OF LYMPH NODE
Beneath the cortex is the Paracortex, which is populated
largely by T - lymphocytes and also contains
Interdigitating dendritic cells thought to have
migrated from tissues to the node.  
The innermost layer of a lymph node, the Medulla is more
sparsely populated with Lymphoid-lineage cells of those
present, many are Plasma cells actively secreting antibody
molecules.
The Medulla in the core of the lymph node mainly
processesT - lymphocytes.  
FUNCTIONS OF LYMPH NODE
Drainage of fluid from blood stream into the tissues.
Filtration of the lymph at the lymph nodes.
Filtering blood.
Raise an immune reaction and fight against microbial
infections.
SPLEEN
The Spleen is an organ found in virtually all Vertebrates.
The spleen is an organ located in the left side of
the Abdominal cavity under the Diaphragm, the
muscular partition between the Abdomen and the Chest.
Similar to a Lymph node, it acts primarily as a blood
filter.
Old RBCs are recycled in the Spleen.
Platelets andWBCs are stored in Spleen.
The spleen also helps to fight against certain kinds of
bacteria that cause Pneumonia and Meningitis.
Lymphoid organs
STRUCTURE OF SPLEEN
The spleen varies in size and shape between people, but
it’s commonly Ovoid shaped and Reddish brown in
colour.
The spleen, in healthy adult humans, is approximately 7 cm
(2.8 in) to 14 cm (5.5 in) in length. It usually weighs
between 150 g and 200 g.
The spleen is surrounded by a Capsule that extends a
number of projections (Trabeculae) into the interior to
form a compartmentalized structure. 
The compartments are of two types, the (i) Red pulp &
(ii)White pulp, which are separated by a diffuse marginal
zone. 
STRUCTURE OF SPLEEN
The splenic Red pulp consists of a network of sinusoids
populated by Macrophages and numerous RBCs and
few Lymphocytes. It is the site where old and
defective red blood cells are destroyed and
removed.
The splenic White pulp surrounds the branches of the
Splenic artery, forming a Periarteriolar lymphoid
sheath (PALS) populated mainly by T -
lymphocytes.
Primary lymphoid follicles - Rich in B - cells and
some of them contain Germinal centers.
STRUCTURE OF SPLEEN
The Marginal zone, located peripheral to the PALS, is
populated by Lymphocytes and Macrophages.
In the marginal zone, antigen is trapped by
Interdigitating dendritic cells, which carry it to the
PALS.
The initial activation of B and T - cells takes place in
theT- cell - rich PALS.
Like Lymph node, the Primary follicles enlarge into
Secondary follicles, each containing a Germinal
center.
FUNCTIONS OF SPLEEN
Similar to a Lymph node, Spleen acts primarily as a
blood filter.
Old RBCs are recycled in the Spleen.
Platelets andWBCs are stored in Spleen.
The spleen also helps to fight against certain kinds of
bacteria that cause Pneumonia and Meningitis.
SPLENOMEGALY
Splenomegaly – Enlargement of Spleen.
 A spleen weight of 400 - 500 g indicates Splenomegaly,
while a weight of more than 1000 g is Labelled Massive
Splenomegaly.
Causes - Viral infections, Bacterial infections (Syphilis),
Parasitic infections (Malaria), Cirrhosis (Chronic liver
damage), Hemolytic anaemia, Blood cancers, Metabolic
disorders, Inflammatory disease (Rheumatoid arthritis) and
Pressure on veins.
Symptoms – Enlarged abdomen, Painful abdomen and
shoulder, Weakness, Feeling stomach full without eating,
Anaemia, Fatigue, Frequent infections and Easy bleeding.
MALT
The mucous membranes lining the digestive, respiratory,
and urogenital systems have a combined surface area of
about 400 m2 and are the major sites of entry for most
pathogens.
These vulnerable membrane surfaces are defended by a
group of organized lymphoid tissues mentioned earlier and
known collectively as Mucosal-associated lymphoid tissue
(MALT).
MALT can be further classified as Gut-associated lymphoid
tissue (GALT) or Bronchus-associated lymphoid tissue
(BALT).
The Tonsils,Appendix and Peyer’s patches are representative
of lymphoid tissue found in and around mucosal epithelia.
Tonsils
Tonsils are collections of Lymphoid tissue facing into the
Aerodigestive tract.
The Tonsils play a role in protecting the body against
Respiratory and Gastrointestinal infections.
Each tonsil consists of a network of crypts (pits) that store
cells used to fight infection.
The tonsils contain B & T- cells, that fights against infections.
Tonsils also produce Antibodies against Polio, Streptococcal
pneumonia, Influenza, and numerous infections.
Tonsillitis occurs when bacterial or viral organisms cause
inflammation of the Tonsillar tissue. This results in fever,
difficulty swallowing, sore throat, ear pain, loss of voice and
throat tenderness. 
Lymphoid organs
Lymphoid organs
Appendix
Appendix - Blind - ended tube connected to the Cecum.
The Cecum is a pouch like structure of the Colon, located at
the junction of the Small and the Large intestines.
The human appendix averages 9 cm in length but can range
from 2 to 20 cm. The diameter of the appendix is usually
between 7 and 8 mm.
Appendicitis – Inflammation ofAppendix.
Functions of Appendix - 1) Maintaining Gut flora, 2)
Important component of Mucosal immune function and 3)
Storehouse for good bacteria, “rebooting” the digestive
system after diarrheal illnesses.
Surgical removal of the appendix causes no observable
health problems.
Lymphoid organs
Peyer’s Patches
Peyer’s patches are small masses of lymphatic tissue found
throughout the Ileum region of the Small intestine.
Peyer’s patches are roughly egg-shaped lymphatic tissue
nodules that are similar to lymph nodes in structure, except
that they are not surrounded by a connective tissue capsule.
Important part of the immune system by monitoring
intestinal bacteria populations and preventing the growth of
pathogenic bacteria in the intestines.
Peyer’s patches also playing an important role in traping
antigens from pathogens and destroying them.
Lymphoid organs

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Lymphoid organs

  • 1. Dr. P. Saranraj M.Sc., M.Phil., Ph.D., NET Assistant Professor Department of Microbiology Sacred Heart College (Autonomous) Tirupattur, Vellore District. Mobile: 9994146964 E.mail: microsaranraj@gmail.com LYMPHOID SYSTEM
  • 2. LYMPHOID SYSTEM The Immune system consists of Immune cells that continuously circulate between the Blood and Lymphoid organs. The Lymphatic system consists of Lymphatic organs and the circulating clear fluid called Lymph. The Lymphatic system was first described in the seventeenth century independently by Olaus Rudbeck and Thomas Bartholin. Lymphoid organs are classified into 2 types.They are: (1) Primary Lymphoid Organs (2) Secondary Lymphoid Organs.
  • 3. PRIMARY & SECONDARY LYMPHOID ORGANS PRIMARY LYMPHOID ORGANS SECONDARY LYMPHOID ORGANS Central lymphoid organs. Peripheral lymphoid organs. Primary lymphoid organs are the sites where Leukocytes (WBCs) are generated. It includes: (i) Bone marrow and (ii) Thymus. Secondary lymphoid organs are the sites where Adaptive immune responses (Acquired immunity) are initiated. It includes: (i) Spleen, (ii) Lymph Node and (iii) MALT. The Lymphoid stem cells proliferate (rapid increase), differentiate and mature. The Lymphoid stem cells becomes functional. Contains either B – cell or T – cell. Contains both B – cell or T – cell. Antigens cannot enter in. Antigens enter in and stimulates the Lymphoid cells.
  • 4. PRIMARY & SECONDARY LYMPHOID ORGANS PRIMARY LYMPHOID ORGANS SECONDARY LYMPHOID ORGANS Site of Maturation for T & B cells. Site of Cell function for T & B cells. Develop before Secondary lymphoid organs during Ontogeny (Organization and development of organs). Develop after Primary lymphoid organs during Ontogeny. Differentiation of Lymphocytes (Type of WBCs, E.g., T cells, B cells & NK cells) is taken place in Primary lymphoid organs. Interaction of immune cells with each other and antigen processing are taken place in Secondary lymphoid organs. Decrease in size and strength with age. Increase in size with age.
  • 7. Growth and maturation of T - lymphocytes takes place inThymus only. It is large at the time of birth (70 g) but with age, the size keep on reducing and becomes very small by attaining puberty (3 g).  It is a flat, bilobed organ situated above the heart. Each Lobe is surrounded by a Capsule and is divided into Lobules, which are separated from each other by strands of Connective tissue called Trabeculae. Each lobule is organized into 2 compartments: the outer compartment, or cortex, is densely packed with immature T cells, called Thymocytes, whereas the inner compartment, or medulla, is sparsely populated with Thymocytes.
  • 8. Both the cortex and medulla of the thymus are crisscrossed (moved or travel around) by a three -dimensional stromal - cell network composed of Epithelial cells, Dendritic cells and Macrophages, which make up the framework of the organ and contribute to the growth and maturation of Thymocytes. Some thymic epithelial cells in the outer cortex, called Nurse cells, have long membrane extensions that surround as many as 50 Thymocytes, forming large multicellular complexes. Hassall corpuscles are a characteristic morphologic feature located within the medullary region of the thymus.
  • 9. FUNCTIONS OF THYMUS The main function of the Thymus is to release Thymosin hormone that will stimulate the maturation of T - cells. Failure ofThymus development shows dramatic decrease in circulating Lymphocytes of the T-cell lineage and absence of Cell - mediated immunity. Aging is accompanied by a decline inThymic function.
  • 10. BONE MARROW Bone marrow is the soft, flexible connective tissue present within the bone cavities. In humans and Mice, bone marrow is the site of B - cell origin and development. Bone marrow forms around 4 % of total body weight. There are two categories of bone marrow tissue: Red marrow and Yellow marrow. From birth to early adolescence, the majority of our bone marrow is red marrow. As we grow and mature, increasing amounts of red marrow is replaced by yellow marrow. Bone marrow can generate 200 billions of new blood cells every day.  
  • 11. RED AND YELLOW BONE MARROW RED BONE MARROW YELLOW BONE MARROW Also known as Myeloid tissue. Also known as Fatty tissue. Hematopoietic (formation of blood cell components) in nature and produces RBC, WBC & Platelets. Multipotent Stromal (connective tissue cell of any organ) in nature and produces Fat, Cartilage and Bone. Gets its red color from the hemoglobin in the erythroid cells. Gets its yellow color from the carotenoids in the fat droplets in the high number of fat cells. High Vascular supply. Poor Vascular supply. Function - Helps to remove old cells from circulation. Function - When blood supply is extremely low, yellow marrow can be converted to red marrow in order to produce more blood cells.
  • 12. FUNCTIONS OF BONE MARROW Bone marrow is the site of B - cell origin and development. A bone marrow transplant can save the lives of people battling leukemia, lymphoma and other blood cancers. Bone marrow generates RBCs which carry oxygen to the tissues. Bone marrow generates Platelets or Thrombocytes help prevent bleeding and aid in clotting of blood. Granulocytes (Neutrophils, Basophils & Eosinophils) and Macrophages fight against microbial infections. They also remove dead cells and remodel tissue and bones.
  • 13. LYMPH NODE Lymph nodes are a group of small, bean-shaped organs (2.6 cm in length) found mainly in the neck and trunk of the human body. They play vital roles in the filtration of antigens and debris from Lymph (circulating colourless watery fluid) and in the generation of immune responses to pathogens. Lymph nodes are often removed from cancer patients as their filtration function catches tumor cells metastasized (spread to other sites in the body) from primary tumors.
  • 15. STRUCTURE OF LYMPH NODE The Capsule is made of Collagen and has a sub-capsular Sinus. The Lymph flows into the Sinus carrying Lymphocytes, Antigen processing macrophages and Dendritic cells to the node Cortex, Paracortex and Medulla. Morphologically, Lymph node can be divided into 3 roughly concentric regions: (1) Cortex, (2) Paracortex and (3) Medulla. The outermost layer, Cortex contains Lymphocytes (mostly B - cells), Macrophages and Follicular dendritic cells arranged in Primary follicles. The Primary follicles enlarge into Secondary follicles, each containing a Germinal center.
  • 16. STRUCTURE OF LYMPH NODE Beneath the cortex is the Paracortex, which is populated largely by T - lymphocytes and also contains Interdigitating dendritic cells thought to have migrated from tissues to the node.   The innermost layer of a lymph node, the Medulla is more sparsely populated with Lymphoid-lineage cells of those present, many are Plasma cells actively secreting antibody molecules. The Medulla in the core of the lymph node mainly processesT - lymphocytes.  
  • 17. FUNCTIONS OF LYMPH NODE Drainage of fluid from blood stream into the tissues. Filtration of the lymph at the lymph nodes. Filtering blood. Raise an immune reaction and fight against microbial infections.
  • 18. SPLEEN The Spleen is an organ found in virtually all Vertebrates. The spleen is an organ located in the left side of the Abdominal cavity under the Diaphragm, the muscular partition between the Abdomen and the Chest. Similar to a Lymph node, it acts primarily as a blood filter. Old RBCs are recycled in the Spleen. Platelets andWBCs are stored in Spleen. The spleen also helps to fight against certain kinds of bacteria that cause Pneumonia and Meningitis.
  • 20. STRUCTURE OF SPLEEN The spleen varies in size and shape between people, but it’s commonly Ovoid shaped and Reddish brown in colour. The spleen, in healthy adult humans, is approximately 7 cm (2.8 in) to 14 cm (5.5 in) in length. It usually weighs between 150 g and 200 g. The spleen is surrounded by a Capsule that extends a number of projections (Trabeculae) into the interior to form a compartmentalized structure.  The compartments are of two types, the (i) Red pulp & (ii)White pulp, which are separated by a diffuse marginal zone. 
  • 21. STRUCTURE OF SPLEEN The splenic Red pulp consists of a network of sinusoids populated by Macrophages and numerous RBCs and few Lymphocytes. It is the site where old and defective red blood cells are destroyed and removed. The splenic White pulp surrounds the branches of the Splenic artery, forming a Periarteriolar lymphoid sheath (PALS) populated mainly by T - lymphocytes. Primary lymphoid follicles - Rich in B - cells and some of them contain Germinal centers.
  • 22. STRUCTURE OF SPLEEN The Marginal zone, located peripheral to the PALS, is populated by Lymphocytes and Macrophages. In the marginal zone, antigen is trapped by Interdigitating dendritic cells, which carry it to the PALS. The initial activation of B and T - cells takes place in theT- cell - rich PALS. Like Lymph node, the Primary follicles enlarge into Secondary follicles, each containing a Germinal center.
  • 23. FUNCTIONS OF SPLEEN Similar to a Lymph node, Spleen acts primarily as a blood filter. Old RBCs are recycled in the Spleen. Platelets andWBCs are stored in Spleen. The spleen also helps to fight against certain kinds of bacteria that cause Pneumonia and Meningitis.
  • 24. SPLENOMEGALY Splenomegaly – Enlargement of Spleen.  A spleen weight of 400 - 500 g indicates Splenomegaly, while a weight of more than 1000 g is Labelled Massive Splenomegaly. Causes - Viral infections, Bacterial infections (Syphilis), Parasitic infections (Malaria), Cirrhosis (Chronic liver damage), Hemolytic anaemia, Blood cancers, Metabolic disorders, Inflammatory disease (Rheumatoid arthritis) and Pressure on veins. Symptoms – Enlarged abdomen, Painful abdomen and shoulder, Weakness, Feeling stomach full without eating, Anaemia, Fatigue, Frequent infections and Easy bleeding.
  • 25. MALT The mucous membranes lining the digestive, respiratory, and urogenital systems have a combined surface area of about 400 m2 and are the major sites of entry for most pathogens. These vulnerable membrane surfaces are defended by a group of organized lymphoid tissues mentioned earlier and known collectively as Mucosal-associated lymphoid tissue (MALT). MALT can be further classified as Gut-associated lymphoid tissue (GALT) or Bronchus-associated lymphoid tissue (BALT). The Tonsils,Appendix and Peyer’s patches are representative of lymphoid tissue found in and around mucosal epithelia.
  • 26. Tonsils Tonsils are collections of Lymphoid tissue facing into the Aerodigestive tract. The Tonsils play a role in protecting the body against Respiratory and Gastrointestinal infections. Each tonsil consists of a network of crypts (pits) that store cells used to fight infection. The tonsils contain B & T- cells, that fights against infections. Tonsils also produce Antibodies against Polio, Streptococcal pneumonia, Influenza, and numerous infections. Tonsillitis occurs when bacterial or viral organisms cause inflammation of the Tonsillar tissue. This results in fever, difficulty swallowing, sore throat, ear pain, loss of voice and throat tenderness. 
  • 29. Appendix Appendix - Blind - ended tube connected to the Cecum. The Cecum is a pouch like structure of the Colon, located at the junction of the Small and the Large intestines. The human appendix averages 9 cm in length but can range from 2 to 20 cm. The diameter of the appendix is usually between 7 and 8 mm. Appendicitis – Inflammation ofAppendix. Functions of Appendix - 1) Maintaining Gut flora, 2) Important component of Mucosal immune function and 3) Storehouse for good bacteria, “rebooting” the digestive system after diarrheal illnesses. Surgical removal of the appendix causes no observable health problems.
  • 31. Peyer’s Patches Peyer’s patches are small masses of lymphatic tissue found throughout the Ileum region of the Small intestine. Peyer’s patches are roughly egg-shaped lymphatic tissue nodules that are similar to lymph nodes in structure, except that they are not surrounded by a connective tissue capsule. Important part of the immune system by monitoring intestinal bacteria populations and preventing the growth of pathogenic bacteria in the intestines. Peyer’s patches also playing an important role in traping antigens from pathogens and destroying them.