The lymphatic system is part of the
Two key roles in the body:
Carries fluid from the extracellular
environment into the bloodstream, and
thereby helps to maintain fluid balance.
It performs the primary role in defending the
body against disease.
Specialized organs of Lymphatic
Lymph nodes, the
spleen, the thymus, the
tonsils, and Peyer’s
The Lymphatic Network
Interstitial fluid is formed when plasma
from the bloodstream is pushed through
This watery fluid serves as a transport
medium for gases, nutrients, and other
molecules that travel between cells and
Most of the interstitial fluid (about 90%) diffuses
back into capillaries for the return trip to the
heart, but some of it does not.
How is the remaining interstitial fluid returned?
The accumulation of interestitial fluid in the extra
cellular space generates a pressure gradient, which
creates enough force to literally push the remaining
interestitial fluid into the lymphatic network.
This series of veinlike tubes drains
interstitial fluid and returns it to the
bloodstream in a one-way flow that moves
slowly toward the heart.
Once the interstitial fluid enters the
lymphatic network, it is referred to as
Tiny vessels are found between cells and
among capillary beds of nearly all tissues
and organs in the body.
Areas where they are not found include
the brain and spinal cord, bone tissue, and
Their structure resembles veins, as their walls
consists of the same three layers of tissue with
the inner endothelium forming one-way valves.
The valves of lymphatic vessels help prevent the
backflow of lymph.
As lymphatic vessels extend toward the heart,
they gradually increase in size and merge to
form the larger lymphatic trunks and collecting
Along their course they lead to small, oval
organs called lymph nodes, and exit from
them to continue on their way.
Lymphatic Trunks and Collecting
Lymphatic trunks are formed from the
merging of numerous lymphatic vessels.
They extend a short distance toward the
heart until they converge to form one of
the two collecting ducts in the lymphatic
network: the thoracic duct or the right
The thoracic duct is the main collecting vessel
for the lymphatic network.
It drains lymph from the left side of the head,
neck, and thorax; the left upper limb; and the
entire body below the diaphragm.
It originates in the lower abdomen as an
expanded vessel (cysterna chyli) and travels
upward against the back wall to the upper chest.
The thoracic duct empties lymph into the
bloodstream where it unites with the left
The right lymphatic duct drains lymph from the
right side of the head, neck, and thorax and the
right upper limb.
It extends a short distance in the neck to empty
lymph into the right subclavian vein, with which it
Movement of lymph
Lymph is propelled through the lymphatic
network by pressure gradients.
Gradients are established by a number of
factors, including the accumulation of
protein in the interstitial fluid, skeletal
muscle contraction, and breathing
Interstitial fluid is formed by the movement of
blood plasma out of blood capillaries.
It is composed of water and dissolved
substances, but it lacks larger substances such
as cells and large proteins.
The concentration of small proteins in the
remaining interstitial fluid tends to rise, causing
the osmotic pressure of the fluid to rise as well.
Recall that osmotic pressure rises when
the solute concentrations rise; in this case,
the proteins are the solutes.
Flow of lymph
As lymph begins to accumulate in the
blind-ended capillaries, the pressure in
these tiny tubes rises.
This provides the force to move the lymph
into the larger lymphatic vessels, where
the pressure is lower.
Other Lymphatic Organs
Lymphatic organs other than the vessels consist
mostly of packed lymphocytes that perform
Lymphoid Tissue contains collections of
lymphocytes, a type of white blood cell.
Lymphocytes play a role in mounting a
defensive immune response to foreign materials
such as pathogenic microorganisms.
Lymph nodes are small, oval masses of
lymphoid tissue located along the
There are hundreds of nodes in the body:
nodes are usually found in clusters or
chains in the neck, armpit, groin, and deep
within the abdominal cavity.
Lymph Node Structure
Most are shaped like a kidney bean and
are less than 2.5 cm in length.
A lymph node is a receiving and sending
station for lymph.
Along its convex margin it receives
incoming lymph by way of lymphatic
vessels that merge with it.
These vessels are called afferent
At its concave margin, known as the
hilus, the node sends lymph on its way
toward the heart by way of efferent
The internal structure consists of two
An outer area, called the cortex, and an inner
part, known as the medulla.
The cortex consists of many lymph nodules,
where lymphocytes and macrophages are
It is also the site of lymphocyte maturation.
the medulla, the lymphocytes
are arranged in strands called
Lymph Node Function
Microorganisms and other foreign particles
carried by the lymph are filtered and
removed. (bacteria, toxins released by
bacteria, and viruses)
These are inactivated by large numbers of
lymphocytes that are packed within the
Macrophages are also
present; they engulf damaged
cells and cell debris.
Largest of the lymphatic organs.
It is enveloped by a fibrous capsule whose
protein fibers divide it internally into
compartments, many of which contain
lymphocytes and macrophages.
The spleen also contains venous sinuses
that are normally filled with large numbers of
red blood cells.
These areas are referred to as red pulp.
The regions of the spleen that contain white blood
cells are called the white pulp.
The spleen functions as a large filter for
removing foreign particles, old and defective
red blood cells, and platelets from the
These functions are accomplished by the
lymphocytes and macrophages Within the
The spleen also provides a blood
reservoir, as it stores a large volume
of blood that can be called upon
during emergency situations involving
The Thymus Gland also plays a role in the
In infants, it is a large, bilobed gland
located in the thoracic cavity, where it lies
behind the sternum.
In adults it is very much reduced in size
During infancy, the thymus cells promote the
maturity of lymphocytes into T lymphocytes (T
cells) and releases them into the bloodstream.
T cells play an important role in the immune
In adolescence, the thymus gland begins to
atrophy, and by adult age it has been replaced
by fibrous and fatty tissue.
Located in the mouth and throat, where
they lie partially embedded within the
They are named according to their
Palatine tonsils—on the back end of the
Pharyngeal tonsils—lie against the wall of
the upper throat (2)
Lingual tonsils---at the base of the tongue
These cells gather and engulf disease-
causing microorganisms in the mouth and
During an infection, the palatine tonsils in
particular may enlarge because of the
proliferation of bacteria and white blood cells.
Consequently, you have a sore throat!
Ifyou have frequent infections,
they may remove your tonsils by
way of a procedure called
Isolated clusters of lymphoid tissue in the
wall of the small intestine near its distal
They contain lymphocytes and
macrophages in large numbers.
They identify microorganisms and other
foreign particles that reside in the
The macrophages provide a
defensive barrier, preventing the
movement of bacteria across the
intestinal wall and into the
The Defense Mechanisms of the
All self cells contain a similar distribution
of membrane receptors which can be read
by white blood cells as “code” and which
differs from the distribution of membrane
receptors found in nonself cells.
The specific distribution of membrane
receptors is determined by a group of
genes in the chromosomes of all self cells,
called major histocompatibility complex
Two general types of defensive strategies that
the body employs:
nonspecific mechanisms: nonspecific
defense mechanisms help prevent the
entrance of foreign material into the body.
An example is the unselective destruction of
all bacteria by phagocytosis.
Specific defense mechanisms: this keys
into the particular type of pathogen or
toxin that is infecting the body.
Once the invaders have been identified,
they may be inactivated by a variety of
The selective production of special
proteins, called antibodies, is one way of
inactivating the invaders.
The specific defense mechanisms
are collectively referred to as the
There are many defense
mechanisms that are
The most important include
physical barriers, Phagocytosis,
natural killer cells, proteins, and
Most important barrier is the skin.
Mucous membranes provide a second
important physical barrier.
Mucous membranes also produce
chemicals that help in the defensive
An invasion by a pathogen that
successfully penetrates the body’s
physical barriers is called an infection.
Phagocytosis is the ingestion and
destruction of particles by specialized
Once collected, the particle is digested by
vacuoles inside the cell.
A cell that performs this function is called
Phagocytosis is a type of nonspecific
defense because the white blood cells
performing this function do not
distinguish between different types of
Phagocytosis is performed by most
white blood cells.
The most active in this defensive activity are
monocytes and neutrophils, which squeeze
through blood vessel walls to reach sites of
infection and inflammation.
Monocytes give rise to their more mature form,
known as macrophages, which often become
attached to the walls of lymphatic vessels and
other lymphatic organs.
Macrophages may also wander freely
throughout the circulatory system, or they
may fix themselves to the walls of blood
vessels, the liver, and the lungs.
Attached macrophages constitute an
important part of the body’s defense
network, known as mononuclear
Natural Killer Cells
Natural killer cells are a type of white
blood cell that kills invading foreign cells
nonspecifically by a method other than
Natural killer cells are unique in that they
kill cancer cells and virus-infected cells by
puncturing a hole in their plasma
membrane (called cell lysis).
Itis currently thought that
natural killer cells are a type
Certain groups of proteins play a role in
One group called complement; it includes
more than 20 types of plasma proteins
that are normally in an inactive state.
COMPLEMENT becomes activated by the
onset of an infection.
Once activated, it labels the foreign
substance as unwanted, enabling their
identification by phagocytes.
A second group of proteins in nonspecific
defense are the interferons.
They are the body’s main defense
Interferons are secreted by cells that have
become infected by viruses.
They diffuse to nearby cells and bind to
their membranes. This causes an
interference in the ability of viruses to
proliferate in these cells.
Interferon therapy is used in the treatment
of hairy-cell leukemia.
Inflammation is a response to body stress,
or disruption of homeostasis, which often
follows an infection or physical injury.
It is a nonspecific mechanism because it
may occur in any tissue of the body.
The inflammatory responses are initiated
when a damaged cell releases substances
into the bloodstream.
These substances include histamine
They produce two main responses by
Vasodilation of blood vessels
Increased permeability of blood
These responses produce four
The leaking of fluids into the
extracellular space is also
known as EDEMA. (swelling)
Specific mechanisms: The
The immune response relies
upon the ability of lymphocytes
to recognize specific antigens.
The defense mechanisms that
are specific constitute the
immune response of the body,
Two types of immune responses:
provide the main defensive
antibodies as the main weapon
Components of Immunity
ANTIGENS—substances that provoke an
immune response when they enter the
They are recognized by white blood cells
Antigens include toxic molecules released
by microorganisms, or the whole
An antibody is a protein molecule that is
produced in response to a specific
It binds to the antigen to form an antigen-
antibody complex, which inactivates the
toxic effects of the antigen.
Antibodies belong to a family of proteins
known as immunoglobulins (Ig).
There are five classes of Ig:
IgG--- found in the blood, lymph, intestines
IgA---in tears, salvia, mucus, milk, blood, and
IgM and IgD---in blood and lymph, and
attached to surfaces of B cells
IgE---attached to surfaces of basophil cells,
mast cells, and B cells.
The most important type of white blood cell in
immunity is the lymphocyte.
Lymphocytes originate from the red bone
marrow before birth.
Soon after their production, about half of them
migrate to the thymus gland.
Once within the tissues of the thymus gland,
these cells mature into a form of lymphocyte
called T cells.
The rest migrate to the spleen or Peyer’s
patches, where they mature into B cells.
Soon after this maturation process, T cells
and B cells leave their site of development
and migrate to lymphoid tissue.
Here they become “programmed” to
recognize self cells and distinguish them
from nonself cells.
Once a lymphocyte has developed this
ability, it is called immunocompetent.
A T cell that has become sensitized
develops special properties that enable it
to react against the particular antigen.
These properties are passed on to its
descendent cells, producing a cell line with
identical properties, they are known as
Killer T cells are specialized in destroying
virus-infected cells, cancer cells, and
They bind to these cells once they are
recognized and release toxic chemicals
Killer T cells are also called cytolytic T
Helper T cells---when they identify an antigen,
they release proteins into the bloodstream that
signal other cells to attack the invader.
These proteins, called lymphokines, stimulate
killer T cells to grow and divide, attract
neutrophils and monocytes to the site of
intrusion, and enhance the ability of
macrophages to ingest and destroy nonself
Thehelper T cells orchestrate the
defensive symphony of the body.
Memory T cells----provide the immune response
with a “memory” of the specific antigen that
caused the initial sensitization.
B Cells---account for about 20% of the
lymphocyte population in a normal blood
In B cells, most of the descendents of the
sensitized cell become plasma cells.
Plasma cells swing into action to synthesize vast
amounts of antibodies at an incredible rate
(2000 antibody molecules per second).
The plasma cells are the only cells of the body
that produce antibodies.
Memory B cells---found mainly in the bone
marrow. They provide a “memory bank” of
sensitized cells that can respond to the same
antigen when it is encountered again.
Provided by the various types of T cells.
Initiated when a macrophage identifies an
antigen, phagocytizes it, and processes it.
Processed antigen is expressed on the
macrophage plasma membrane.
Macrophage then presents the antigen to a T
cell, resulting in T cell sensitization.
The sensitized T cell then proliferates into
various T-cell lines (killer, helper, suppressor,