Transcript The Lymphatic System and Immunity

The Lymphatic System and
Immunity
Functions of the Lymphatic System
• The lymphatic system has 3 main
functions:
– Drains excess interstitial fluid and returns it to
the blood
– Transports lipids and lipid-soluble vitamins (A,
D, E, and K) absorbed by the GI tract to the
blood
– Carries out specific immune responses
against certain foreign invaders (microbes,
etc.)
Lymphatic System
• The lymphatic system consists of a fluid
called lymph, lymphatic vessels,
lymphatic organs (such as red bone
marrow, the thymus, lymph nodes, and
the spleen), and other lymphatic tissues
• This system helps in circulating body fluids
and defending the body against diseasecausing organisms or substances
Lymph
• Most components of blood plasma filter
out through capillary walls to form
interstitial fluid
• This fluid then passes into lymphatic
capillaries and is called lymph; lymph is
usually a clear, pale yellow fluid
• Interstitial fluid is located in between cells;
lymph is located within lymphatic vessels
and in lymphatic tissue
Lymphatic Vessels
• Lymphatic capillaries—tiny vessels
located in the spaces between cells;
closed at one end
• Lymphatic capillaries unite to form larger
lymphatic vessels, which are similar to
veins, except they have thinner walls and
more valves
Lymphatic Vessels (continued)
• As lymphatic vessels exit lymph nodes, they
unite to form lymphatic trunks and then
lymphatic ducts
• There are 2 main lymphatic ducts:
– Thoracic duct (or left lymphatic duct)—the main
duct for the return of lymph to the blood; receives
lymph from the left side of the body and the entire
lower body and drains it into venous blood near the
left internal jugular and left subclavian veins
– Right lymphatic duct—receives lymph from the
upper right side of the body and drains it into venous
blood near the right internal jugular and right
subclavian veins
Red Bone Marrow
• Stem cells in red bone marrow give rise to
B cells and pre-T cells
• Pre-T cells migrate to the thymus, where
they develop into mature T cells
• Some of these T cells leave the thymus
and travel through the bloodstream to
lymph nodes, the spleen, and other
lymphatic tissues
Thymus
• The thymus is a lymphatic organ located
in the mediastinum between the aorta and
the sternum
• It is large in infants, but it atrophies after
puberty is reached
• Most T cells are formed before puberty,
but some continue to develop in the
thymus throughout an individual’s lifetime
Lymph Nodes
• Lymph nodes are bean-shaped organs
consisting of masses of B cells and T cells
that have migrated through the blood
• They are scattered throughout the body
and are located at intervals along
lymphatic vessels
• Lymph passes through lymph nodes and is
filtered as it travels through the lymphatic
vessels
Spleen
• The spleen is a lymphatic organ located in the
left side of the abdominal cavity near the
stomach and diaphragm
• It contains B cells and T cells and carries out
immune functions related to these cells
• In addition, the macrophages in the spleen
remove pathogens and worn out/dead/damaged
blood cells from the blood
• The spleen stores up to 1/3 of the body’s supply
of platelets
• During fetal life, it produces blood cells
Other Lymphatic Tissues
• The appendix contains some lymphatic tissue
• The tonsils are also composed of lymphatic
tissue and are located in the pharynx
• Usually there are 5 tonsils:
– Adenoid (or pharyngeal tonsil)—a single tonsil in
the nasopharynx
– Palatine tonsils—two tonsils located at the rear of
the oral cavity; the ones usually removed during a
tonsillectomy
– Lingual tonsils—two tonsils located at the base of
the tongue
Resistance
• Resistance is the natural ability of the
body to fight damage or disease
• There are 2 types of resistance:
– Nonspecific resistance
– Specific resistance or immunity
Nonspecific Resistance
• Nonspecific resistance consists of defense
mechanisms that are present at birth and give
immediate protection against pathogens and
foreign substances
• It includes the barriers provided by the skin and
mucous membranes, lacrimal fluid (or tears),
stomach acid, saliva, perspiration, phagocytes
(neutrophils and macrophages), natural killer
cells, inflammation (causes redness, pain, heat,
and swelling), and fever
Specific Resistance: Immunity
• The ability of the body to defend itself against
specific invading organisms or substances is
called specific resistance or immunity
• Substances that are recognized as foreign and
cause immune responses are called antigens
• In autoimmune diseases, the immune system
attacks the body’s own tissues (examples—
multiple sclerosis, type I diabetes, rheumatoid
arthritis, and lupus)
Major Histocompatibility Complex
• Most body cells contain “self antigens” in
their plasma membranes
• These antigens make up the major
histocompatibility complex (MHC) and
are also called human leukocyte
antigens (HLA)
• This complex of antigens consists of
thousands of molecules that vary from one
individual to another
MHC (continued)
• Only identical twins have the same
antigens of this type
• When “matching” is done for bone marrow
or other organ transplants, a donor and
recipient are compared with respect to
their MHC antigen compositions
T Cells
• Depending on the antigen receptors they
have, T cells are classified as either CD4
or CD8 cells
• They must recognize an antigen and be
“activated” before they can eliminate
foreign invaders
T Cells (continued)
• Once activated, T cells divide, differentiate, and
produce 3 specific types of cells:
– Helper T cells—most CD4 cells develop into these
cells, which cause both B cells and T cells to
proliferate
– Cytotoxic T cells—CD8 cells develop into these
cells, which leave lymphatic tissues to destroy
infected cells (or cancerous cells) by digesting them
or causing them to rupture and die
– Memory T cells—these cells “remember” antigens
that have previously invaded the body and can
quickly eliminate them
B Cells
• In addition to the MHC antigens, the plasma
membranes of B cells and T cells have antigen
receptors that enable them to recognize foreign
antigens and respond to them
• After recognizing an antigen and being
“activated”, B cells transform into plasma cells
and begin secreting antibodies (or
immunoglobulins)
• Memory B cells can recognize antigens that
have previously invaded the body and quickly
eliminate them
Antibodies
• Antibodies are chemically glycoproteins
(carbohydrates attached to proteins) also known
as immunoglobulins
• The structure of an antibody matches its antigen
much as a lock accepts a specific key
• Antibodies travel through lymph and can
inactivate specific foreign antigens by binding
with them
• Some antigen-antibody complexes are
destroyed by phagocytic cells
Antibodies
• There are 5 classes of antibodies:
– IgG—most abundant; about 80% of all antibodies in
the blood; can cross the placenta from mother to
fetus, giving newborns immune protection
– IgA—makes up 10-15% of antibodies in the blood;
levels decrease during stress
– IgM—makes up 5-10% of antibodies in the blood; first
secreted by plasma cells
– IgD—makes up only about 0.2% of antibodies in the
blood
– IgE—makes up less than 0.1% of all antibodies in the
blood; involved in allergic reactions