HLTAP301A ANATOMY & PHYSIOLOGY
LECTURE 9 - THE LYMPHATIC & IMMUNE SYSTEM
Every day the body is constantly under attack from hordes of bacteria, viruses
and other pathogens, yet most of the time, we remain very healthy. The
Lymphatic System plays a very important role in fighting disease and
maintaining homeostatic balance within the body. A key component of the
body’s protective mechanism is the ability to recognize self from non-self –
and non-self organisms cause the body defense systems to kick in.
There are two semi-independent parts of the lymphatic system – the complex
network of lymphoid vessels and the lymphoid tissues and organs that are
scattered throughout the body.
The Lymphatic Vessels
The Blood vessels form a closed loop or circuit, whereas lymphatic vessels
are an open, one-way system. While arteries have the heart to pump
fluids along, the lymphatic vessels have no pump. In a similar manner to
veins, they are both thin walled and have one-way valves, and rely on the
‘milking’ action of muscles and pressure changes in the thorax to move fluids
along.
What is lymph? Fluid (essentially blood plasma) is forced out of the
circulation because it is under pressure and it moves into the interstitial
spaces, where it is now considered to be interstitial fluid. It becomes lymph
when it enters the lymphatic vessels, to be returned to the blood, where it is
again considered to be plasma. It is essentially all the same fluid, but is called
different things depending upon its location at any given time, and the
composition of various solutes changes at the different locations.
Approximately 24 liters of fluid per day are pushed out of the capillaries to
bathe the cells, and if all 24 liters are not reabsorbed, the fluid left behind
would lead to edema.
This fluid needs to be filtered before being returned to the blood circulation,
and this is where the lymph nodes come in.
Lymph Nodes
Little pockets of lymphatic tissue are scattered throughout the body and play
an important protective function. Lymph nodes are clustered along the
lymphatic vessels and they contain macrophages, whose role is to engulf and
destroy bacteria, viruses and other foreign substances that may have entered
the lymph fluid. Lymphocytes are also found here to respond to any foreign
substances found in the fluid. There are more afferent lymphatic vessels
entering the lymph node than leaving – this slows down the flow, giving the
macrophages and lymphocytes time to ensure the fluid is ‘cleaned’. Fluid
usually passes through several lymph nodes before this occurs.
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OTHER LYMPHOID ORGANS
The Thymus functions at peak levels during youth, and its major role is to
produce thymosin. This hormone encourages immature T cells to divide and
replicate and then undergo the maturation process to become immunecompetent T cells.
The Spleen is a blood filled organ whose role is to filter blood, cleansing it of
viruses, bacteria and any other foreign substances. The spleen’s most
important function is to destroy old red blood cells, returning the iron to the
liver for hemoglobin production. Plates are also stored here and like the liver,
the spleen acts as a reservoir for blood, which can help maintain blood
volume during a hemorrhage.
Peyers Patches are small pockets of lymphoid tissue, commonly known as
MALT (mucosa-associated lymphoid tissue), whose main role is to capture
and destroy bacteria in the small intestine, before it can penetrate the
intestinal wall.
The Tonsils are located in the mucosa of the pharynx. Their job is to trap
bacteria and other foreign substances as they enter the throat. Sometimes,
when they are overloaded with bacteria, they become swollen, red and sore, a
condition commonly called tonsillitis.
BODY DEFENSE MECHANISMS
Innate Body Defenses are both external and internal. They include the skin
and mucous membranes, the inflammatory response and certain cells.
Surface Membrane Barriers
The skin is a specialised epithelial membrane, which acts as a physical barrier
to most microorganisms and their enzymes and toxins. Mucous membranes
line all body cavities open to the exterior and function as an additional
physical barrier. Secretions of the epithelial tissues include acidic secretions,
sebum, hydrochloric acid, saliva, and mucous.
Internal Defenses – Cells and Chemicals
Once microorganisms breach the external barriers, they are confronted by
phagocytes (phago = to eat). Macrophages and Neutrophils are the main
phagocytes of the body, engulfing a foreign particle with flowing cytoplasmic
entensions and then breaking it down or digesting it.
The Inflammatory Response
Inflammation occurs any time the body tissues are injured by physical trauma,
intense heat, irritating chemicals, or infection by viruses, fungi, or bacteria.
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The four cardinal signs of acute inflammation are:
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Redness
Heat
Swelling
Pain
When cells are injured, a chemical alarm is sounded. Chemicals cause
dilation of surrounding blood vessels to increase blood flow to the area and
increase permeability, which allows fluid containing clotting factors and
antibodies to enter the tissues.
Pain receptors are also activated and soon after inflammation, the damaged
site is invaded by neutrophils and macrophages. The inflammatory response
(1) prevents the spread of damaging agents to nearby tissues, (2) disposes of
cell debris and pathogens, and (3) sets the stage for repair.
Fever
Fever is the body’s systemic response to invading microorganisms. If white
blood cells and macrophages are exposed to foreign cells in the body, they
produce chemicals known as pyrogens, that cause the hypothalamus to raise
body temperature. As temperature rises, the liver and spleen take up iron and
zinc, ions that bacteria need to multiply. It also increases the metabolic rate,
speeding up the repair process of tissue cells in general.
Specific Body Defenses (The Immune System)
The immune system recognizes something as foreign and acts to inactivate it
or remove it. Immune response is antigen-specific, is systemic (that is, not
limited to the initial infection site) and has memory.
Antigens are any substance (toxins, foreign proteins or bacteria) that are
recognized as foreign and activate the immune response. The body’s
adaptive defenses recognize and destroy the specific antigen that initiated the
response. After an initial exposure the immune response is able to recognize
the same antigen and mount a faster and stronger defensive attack.
There are two different types of cells that fight off foreign invaders. B Cells
produce antibodies and T Cells recognize and fight off cells that are ‘non-self’.
Both of these types of cells can divide and reproduce identical ‘clones’ that
will continue to fight off pathogens as long as they are needed. Both keep
some memory cells that will ‘remember’ a specific antigen, and if exposed to it
again, will fight it off more quickly and with a stronger response than during
the original exposure.
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ORGAN TRANSPLANTS AND REJECTION
Some conditions are best treated with an organ transplant – such as kidney or
heart transplant and bone marrow transplants. Often this operations are
successful however the body’s immune system can ‘reject’ the graft as it
recognizes the new organ as non-self.
Autografts are tissue grafts from one site to another by the same person
whereas Isografts are donated by an identical twin – these two are ideal organ
donors and are almost always successful.
Xenografts are tissues
transplanted from another animal species, such as a pig heart valve. An
allograft is usually taken from a recently deceased person, in the case of heart
transplants. Tissue and blood group antigens must be closely matched with
at least 75 percent match required before a graft is considered. After surgery,
immunosuppressive therapy must continue to prevent rejection by the body.
Drugs such as corticosteroids and immunosuppressants are commonly used
and unfortunately these also have long term side effects.
DISORDERS OF IMMUNITY
Autoimmune diseases occur when the body fails to differentiate between self
and non-self, and the body produces antibodies and sensitized T cells that
attack and damage its own tissues.
Conditions such as Multiple sclerosis (MS) which destroys the myelin sheaths
of the nervous system, Graves disease, where the thyroid produces excessive
amount of thyroxine and rheumatoid arthritis (RA) which destroys the joints
are all examples of autoimmune diseases.
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