Anatomy of the Immune System and lymphatic system
The body’s immune system is made up of individual parts which work together to
find and destroy bacteria, viruses, fungi, and tumors. Each part of the immune system
must be functioning properly in order to detect and differentiate the unhealthy
organisms from healthy tissues. Together, each of the six individual parts of the
immune system work to keep the person healthy and free from disease, bacteria and
viruses. The lymphatic system is part of the circulatory system and a vital part of the
immune system, comprising a network of lymphatic vessels that carry a clear fluid
called lymph (from Latin, lympha meaning water) directionally towards the heart.
One of the main functions of the lymph system is to provide an accessory return route
to the blood for the surplus three liters. The other main function is that of defense in
the immune system. Lymph is very similar to blood plasma: it contains lymphocytes
and other white blood cells. It also contains waste products and cellular debris
together with bacteria and proteins. Associated organs composed of lymphoid tissue
are the sites of lymphocyte production. Lymphocytes are concentrated in the lymph
nodes. The spleen and the thymus are also lymphoid organs of the immune system.
The tonsils are lymphoid organs that are also associated with the digestive system.
Lymphoid tissues contain lymphocytes, and also contain other types of cells for
support. The system also includes all the structures dedicated to the circulation and
production of lymphocytes (the primary cellular component of lymph), which also
includes the bone marrow, and the lymphoid tissue associated with the digestive
Structure of the lymphatic system and its role in immunity consists of lymphatic
organs, a conducting network of lymphatic vessels, and the circulating lymph
1. Bone Marrow
The primary point of production of the cells of the immune system, bone marrow is a
substance found inside the bones primarily in the hips and thighs. Bone marrow is
made up of white blood cells, red blood cells and platelets. Bone marrow is
responsible for both the creation of T cells and the production and maturation of
B cells. From the bone marrow, B cells immediately join the circulatory system and
travel to secondary lymphoid organs in search of pathogens. T cells, on the other
hand, travel from the bone marrow to the thymus, where they develop further. Mature
T cells join B cells in search of pathogens.
The thymus is the organ responsible for T-cell maturity and release. This is where the
T-cells which are critical to the adaptive immune system develop self-tolerance before
being released into the body’s system. The thymus is a primary lymphoid organ and
the site of maturation for T cells, the lymphocytes of the adaptive immune system.
The thymus increases in size from birth in response to postnatal antigen stimulation,
then to puberty and regresses thereafter. The loss or lack of the thymus results in
severe immunodeficiency and subsequent high susceptibility to infection. In most
species, the thymus consists of lobules divided by septa which are made up of
epithelium and is therefore an epithelial organ. T cells mature from thymocytes,
proliferate and undergo selection process in the thymic cortex before entering the
medulla to interact with epithelial cells.
The thymus provides an inductive environment for development of T cells from
hematopoietic progenitor cells. In addition, thymic stromal cells allow for the
selection of a functional and self-tolerant T cell repertoire. Therefore, one of the most
important roles of the thymus is the induction of central tolerance.
The thymus is largest and most active during the neonatal and pre-adolescent periods.
By the early teens, the thymus begins to atrophy and thymic stroma is mostly replaced
by adipose tissue. Nevertheless, residual T lymphopoiesis continues throughout adult
a) Anatomy: The thymus is responsible for producing a particular type of white blood
cell known as the T-cell. It can be found just below the chest bone.
b) Histology: the thymus consists of lymphoid tissues and lymphocytes. Two distinct
structures, the cortex and the medulla work to push lymphoid cells from maturity into
circulation within the body.
3. Lymph nodes
Lymph nodes are part of the lymphatic system that can be found widely distributed
throughout the entire body. They are responsible for trapping foreign particles and
filtering pathogens found within the body. A lymph node is an organized collection of
lymphoid tissue, through which the lymph passes on its way back to the blood.
Lymph nodes are located at intervals along the lymphatic system. Several afferent
lymph vessels bring in lymph, which percolates through the substance of the lymph
node, and is then drained out by an efferent lymph vessel. There are between five and
six hundred lymph nodes in the human body, many of which are grouped in clusters
in different regions as in the underarm and abdominal areas. Lymph node clusters are
commonly found at the base of limbs (groin, armpits) and in the neck, where lymph is
collected from regions of the body likely to sustain pathogen contamination from
a) Structure: A fibrous capsule extends from outside the lymph node to the inner
substance which includes the cortex and medulla to make up the lymph node.
b) Cortex: B cells arranged as follicles make up the outer cortex and the inner cortex
is made up of t-cells.
c) Medulla: the medullary cords are made up of plasma, macrophages and B cells.
The medullary sinuses separate the medullary cords and contain histiocytes and
reticular cells. The large blood vessels, sinuses and medullary cords make up the
d) Passage of lymph: lymphatic circulation begins in the nodes and passes through the
marginal sinus into the cortical sinuses. The passage of lymph continues until the
lymph reaches the medullary sinuses and then exits the efferent lymphatic.
Located in the upper left abdominal section, the spleen is structured similar to an
oversize lymph node and works as a blood filter.
The main functions of the spleen are:
1. to produce immune response against blood-borne antigens
2. to remove particulate matter and aged blood cells, mainly erythrocytes
3. to produce blood cells during fetal life
The spleen synthesizes antibodies in its white pulp and removes antibody-coated
bacteria and antibody-coated blood cells by way of blood and lymph node circulation.
A study published in 2009 using mice found that the spleen contains, in its reserve,
half of the body's monocytes within the red pulp. These monocytes, upon moving
to injured tissue (such as the heart), turn into dendritic cells and macrophages while
promoting tissue healing. The spleen is a center of activity of the
mononuclear phagocyte system and can be considered analogous to a large lymph
node, as its absence causes a predisposition to certain infections. Like the thymus, the
spleen has only efferent lymphatic vessels. Both the short gastric arteries and the
splenic artery supply it with blood.
a) Structure: made up of two distinct parts known as the red pulp and the white pulp,
the spleen filters foreign bodies out of the blood keeping the person healthy.
b) Red pulp: this is where the filtration of red blood cells takes place removing
damaged cells from the body.
c) White pulp: responsible for immune response, white pulp includes T cells and B
cells which fight antigens in the blood stream for improved health.
5. Mucosa-associated lymphoid tissue (MALT)
A diffusion system made up of small amounts of lymphoid tissue located in the
body’s mucosal linings, the mucosa-associated lymphoid tissue is the largest part of
lymphatic tissue. The MALT protects the body from various antigens and has a
differential naming structure which refers to various locations of the tissue within the
body such as:
GALT (gut-associated lymphoid tissue. Peyer's patches are a component of
GALT found in the lining of the small intestines.)
BALT (bronchus-associated lymphoid tissue)
NALT (nasal-associated lymphoid tissue)
CALT (conjunctival-associated lymphoid tissue)
O-MALT (organized mucosa-associated lymphatic tissue); specifically, the
tonsils of Waldeyer's tonsillar ring are O-MALT.
D-MALT (diffuse mucosa-associated lymphatic tissue); MALT that is not
organized as a separately macroscopically anatomically identifiable mass,
tissue or organ (such as the aforementioned O-MALT) is diffuse MALT.
LALT (larynx-associated lymphoid tissue)
SALT (skin-associated lymphoid tissue)
VALT (vulvo-vaginal-associated lymphoid tissue)
6. Lymphocyte recirculation
The cycle in which lymphocytes circulate throughout the body, in both lymphoid and
non-lymphoid tissues, to remove antigens from the body and keep the person free
from disease, viruses and bacteria.
The lymphatic vessels, also called lymph vessels, conduct lymph between different
parts of the body. They include the tubular vessels of the lymph capillaries, and the
larger collecting vessels–the right lymphatic duct and the thoracic duct (the left
lymphatic duct). The lymph capillaries are mainly responsible for the absorption of
interstitial fluid from the tissues, while lymph vessels propel the absorbed fluid
forward into the larger collecting ducts, where it ultimately returns to the bloodstream
via one of the subclavian veins. These vessels are also called the lymphatic channels
or simply lymphatics.
The lymphatics are responsible for maintaining the balance of the body fluids. Its
network of capillaries and collecting lymphatic vessels work to efficiently drain and
transport extravasated fluid, along with proteins and antigens, back to the circulatory
system. Numerous intraluminal valves in the vessels ensure a unidirectional flow of
lymph without reflux. Two valve systems are used to achieve this one directional
flow—a primary and a secondary valve system. The capillaries are blind-ended,
and the valves at the ends of capillaries use specialised junctions together with
anchoring filaments to allow a unidirectional flow to the primary vessels. The
collecting lymphatics, however, act to propel the lymph by the combined actions of
the intraluminal valves and lymphatic muscle cells.
Major lymphatic ducts and trunks in relation to veins (anterior view of the thoracic and abdomnial
Functions of lymphatic system
The lymphatic system has multiple interrelated functions:
It is responsible for the removal of interstitial fluid from tissues
It absorbs and transports fatty acids and fats as chyle from the digestive
It transports white blood cells to and from the lymph nodes into the bones
The lymph transports antigen-presenting cells, such as dendritic cells, to the
lymph nodes where an immune response is stimulated.
The lymphatic system returns fluids that have leaked from the blood (vascular
system) back to the blood. Without it, our cardiovascular and immune systems
would begin to shut down. The lymphatic system contains three parts, a
network of lymphatic vessels, a fluid inside of the vessels called lymph, and
lymph nodes that cleanse the lymph while it passes through.
Immune and Lymphatic Systems
The immune and lymphatic systems are two closely related organ systems that share
several organs and physiological functions. The immune system is our body’s defense
system against infectious pathogenic viruses, bacteria, and fungi as well as parasitic
animals and protists. The immune system works to keep these harmful agents out of
the body and attacks those that manage to enter.
The lymphatic system is a system of capillaries, vessels, nodes...
and other organs that transport a fluid called lymph from the tissues as it returns to the
bloodstream. The lymphatic tissue of these organs filters and cleans the lymph of any
debris, abnormal cells, or pathogens. The lymphatic system also transports fatty acids
from the intestines to the circulatory system.
Immune and Lymphatic System Anatomy
All of the leukocytes, or white blood cells, of the immune system are produced by red
bone marrow. Leukocytes can be further broken down into 2 groups based upon the
type of stem cells that produces them: myeloid stem cells and lymphoid stem cells.
Myeloid stem cells produce monocytes and the granular leukocytes—eosinophils,
basophils, and neutrophils.
Monocytes. Monocytes are agranular leukocytes that can form 2 types of
cells: macrophages and dendritic cells.
1. Macrophages. Monocytes respond slowly to infection and once present
at the site of infection, develop into macrophages. Macrophages are
phagocytes able to consume pathogens, destroyed cells, and debris by
phagocytosis. As such, they have a role in both preventing infection as
well as cleaning up the aftermath of an infection.
2. Dendritic cells. Monocytes also develop into dendritic cells in healthy
tissues of the skin and mucous membranes. Dendritic cells are
responsible for the detection of pathogenic antigens which are used to
activate T cells and B cells.
1. Eosinophils. Eosinophils are granular leukocytes that reduce allergic
inflammation and help the body fight off parasites.
2. Basophils. Basophils are granular leukocytes that trigger inflammation
by releasing the chemicals heparin and histamine. Basophils are active
in producing inflammation during allergic reactions and parasitic
3. Neutrophils. Neutrophils are granular leukocytes that act as the first
responders to the site of an infection. Neutrophils use chemotaxis to
detect chemicals produced by infectious agents and quickly move to
the site of infection. Once there, neutrophils ingest the pathogens via
phagocytosis and release chemicals to trap and kill the pathogens.
Lymphoid stem cells produce T lymphocytes and B lymphocytes.
T lymphocytes. T lymphocytes, also commonly known as T cells, are cells
involved in fighting specific pathogens in the body. T cells may act as helpers
of other immune cells or attack pathogens directly. After an infection, memory
T cells persist in the body to provide a faster reaction to subsequent infection
by pathogens expressing the same antigen.
B lymphocytes. B lymphocytes, also commonly known as B cells, are also
cells involved in fighting specific pathogens in the body. Once B cells have
been activated by contact with a pathogen, they form plasma cells that produce
antibodies. Antibodies then neutralize the pathogens until other immune cells
can destroy them. After an infection, memory B cells persist in the body to
quickly produce antibodies to subsequent infection by pathogens expressing
the same antigen.
Natural killer cells. Natural killer cells, also known as NK cells, are
lymphocytes that are able to respond to a wide range of pathogens and
cancerous cells. NK cells travel within the blood and are found in the lymph
nodes, spleen, and red bone marrow where they fight most types of infection.
As blood passes through the tissues of the body, it enters thin-walled capillaries to
facilitate diffusion of nutrients, gases, and wastes. Blood plasma also diffuses through
the thin capillary walls and penetrates into the spaces between the cells of the tissues.
Some of this plasma diffuses back into the blood of the capillaries, but a considerable
portion becomes embedded in the tissues as interstitial fluid. To prevent the
accumulation of excess fluids, small dead-end vessels called lymphatic capillaries
extend into the tissues to absorb fluids and return them to circulation.
The interstitial fluid picked up by lymphatic capillaries is known as lymph. Lymph
very closely resembles the plasma found in the veins: it is a mixture of about 90%
water and 10% solutes such as proteins, cellular waste products, dissolved gases, and
hormones. Lymph may also contain bacterial cells that are picked up from diseased
tissues and the white blood cells that fight these pathogens. In late-stage cancer
patients, lymph often contains cancerous cells that have metastasized from tumors and
may form new tumors within the lymphatic system. A special type of lymph, known
as chyle, is produced in the digestive system as lymph absorbs triglycerides from the
intestinal villi. Due to the presence of triglycerides, chyle has a milky white coloration
Lymphatic capillaries merge together into larger lymphatic vessels to carry lymph
through the body. The structure of lymphatic vessels closely resembles that of veins:
they both have thin walls and many check valves due to their shared function of
carrying fluids under low pressure. Lymph is transported through lymphatic vessels
by the skeletal muscle pump—contractions of skeletal muscles constrict the vessels to
push the fluid forward. Check valves prevent the fluid from flowing back toward the
Lymph nodes are small, kidney-shaped organs of the lymphatic system. There are
several hundred lymph nodes found mostly throughout the thorax and abdomen of
the body with the highest concentrations in the axillary (armpit) and inguinal (groin)
regions. The outside of each lymph node is made of a dense fibrous connective tissue
capsule. Inside the capsule, the lymph node is filled with reticular tissue containing
many lymphocytes and macrophages. The lymph nodes function as filters of lymph
that enters from several afferent lymph vessels. The reticular fibers of the lymph node
act as a net to catch any debris or cells that are present in the lymph. Macrophages and
lymphocytes attack and kill any microbes caught in the reticular fibers. Efferent
lymph vessels then carry the filtered lymph out of the lymph node and towards the
All of the lymphatic vessels of the body carry lymph toward the 2 lymphatic ducts:
the thoracic duct and the right lymphatic ducts. These ducts serve to return lymph
back to the venous blood supply so that it can be circulated as plasma.
Thoracic duct. The thoracic duct connects the lymphatic vessels of the legs,
abdomen, left arm, and the left side of the head, neck, and thorax to the left
Right lymphatic duct. The right lymphatic duct connects the lymphatic
vessels of the right arm and the right side of the head, neck, and thorax to the
right brachiocephalic vein.
Outside of the system of lymphatic vessels and lymph nodes, there are masses of non-
encapsulated lymphatic tissue known as lymphatic nodules. The lymphatic nodules
are associated with the mucous membranes of the body, where they work to protect
the body from pathogens entering the body through open body cavities.
Tonsils. There are 5 tonsils in the body—2 lingual, 2 palatine, and 1
pharyngeal. The lingual tonsils are located at the posterior root of the tongue
near the pharynx. The palatine tonsils are in the posterior region of the mouth
near the pharynx. The pharyngeal pharynx, also known as the adenoid, is
found in the nasopharynx at the posterior end of the nasal cavity. The tonsils
contain many T and B cells to protect the body from inhaled or ingested
substances. The tonsils often become inflamed in response to an infection.
Peyer’s patches. Peyer’s patches are small masses of lymphatic tissue found
in the ileum of the small intestine. Peyer’s patches contain T and B cells that
monitor the contents of the intestinal lumen for pathogens. Once the antigens
of a pathogen are detected, the T and B cells spread and prepare the body to
fight a possible infection.
Spleen. The spleen is a flattened, oval-shaped organ located in the upper left
quadrant of the abdomen lateral to the stomach. The spleen is made up of a
dense fibrous connective tissue capsule filled with regions known as red and
white pulp. Red pulp, which makes up most of the spleen’s mass, is so named
because it contains many sinuses that filter the blood. Red pulp contains
reticular tissues whose fibers filter worn out or damaged red blood cells from
the blood. Macrophages in the red pulp digest and recycle the hemoglobin of
the captured red blood cells. The red pulp also stores many platelets to be
released in response to blood loss. White pulp is found within the red pulp
surrounding the arterioles of the spleen. It is made of lymphatic tissue and
contains many T cells, B cells, and macrophages to fight off infections.
Thymus. The thymus is a small, triangular organ found just posterior to the
sternum and anterior to the heart. The thymus is mostly made of glandular
epithelium and hematopoietic connective tissues. The thymus produces and
trains T cells during fetal development and childhood. T cells formed in the
thymus and red bone marrow mature, develop, and reproduce in the thymus
throughout childhood. The vast majority of T cells do not survive their
training in the thymus and are destroyed by macrophages. The surviving T
cells spread throughout the body to the other lymphatic tissues to fight
infections. By the time a person reaches puberty, the immune system is mature
and the role of the thymus is diminished. After puberty, the inactive thymus is
slowly replaced by adipose tissue.
1) "Lymph - Definition and More from the Free Merriam-Webster Dictionary".
www.merriam-webster.com. Retrieved 2010-05-29.
2) Human Physiology: From Cells to Systems, by Lauralee Sherwood
3) Tak W. Mak; Mary E. Saunders (Ph.D.); Mary E. Saunders (2008). Primer to
the immune response. Academic Press. pp. 28–. ISBN 978-0-12-374163-9.
Retrieved 12 November 2010.
4) Warwick, Roger; Peter L. Williams. "Angiology (Chapter 6)". Gray's anatomy.
illustrated by Richard E. M. Moore (Thirty-fifth ed.). London: Longman.
5) Miller, J. F. (2002). "The discovery of thymus function and of thymus-derived
lymphocytes". Immunol Rev. 185 (1): 7–14. doi:10.1034/j.1600-
065X.2002.18502.x. PMID 12190917.
6) Swirski, FK; Nahrendorf, M; Etzrodt, M; Wildgruber, M; Cortez-Retamozo, V;
Panizzi, P; Figueiredo, JL; Kohler, RH; Chudnovskiy, A; Waterman, P; Aikawa,
E; Mempel, TR; Libby, P; Weissleder, R; Pittet, MJ (2009). "Identification of
splenic reservoir monocytes and their deployment to inflammatory sites".
Science. 325 (5940): 612–6. doi:10.1126/science.1175202. PMC 2803111 .
7) Jia, T; Pamer, EG (2009). "Immunology. Dispensable but not irrelevant".
Science. 325 (5940): 549–50. doi:10.1126/science.1178329. PMC 2917045 .
8) Finally, the Spleen Gets Some Respect By NATALIE ANGIER, The New York
Times, August 3, 2009
9) Blackbourne, Lorne H (2008-04-01). Surgical recall. Lippincott Williams &
Wilkins. p. 259. ISBN 978-0-7817-7076-7.
10) "Definition of lymphatics". Webster's New World Medical Dictionary.
medicineNet.com. Retrieved 2008-07-06.
11) Vittet D. Lymphatic collecting vessel maturation and valve morphogenesis.
Microvasc Res. 2014 Jul 12. pii: S0026-2862(14)00100-9. PMID 25020266
12) Heppell C1, Richardson G, Roose T. A model for fluid drainage by the
lymphatic system. PMID 23161129
13) Bazigou E, Wilson J, Moore JE Primary and secondary lymphatic valve
development: Molecular, functional and mechanical insights.Microvasc Res.
2014 Jul 30. pii: S0026-2862(14)00112-5. PMID 25086182