The lymphatic system
The lymphatic system actually consists of two parts:
(1) a network of lymphatic vessels and
(2) various lymphoid tissues and organs scattered
throughout the body.
The lymphatic vessels transport back to the blood any
fluids that have escaped from the blood vascular
system.
The lymphoid organs house phagocytic cells and
lymphocytes, which play essential roles in the body’s
defense mechanisms and its resistance to disease.
lymphatic vessels
• As explained before, the hydrostatic and colloid osmotic
pressures operating at capillary beds force fluid out of the
blood at the arterial ends of the beds and cause most of it to
be reabsorbed at the venous ends .
• The fluid that remains behind in the tissue spaces, as much
as 3 L daily, becomes part of the interstitial fluid. This leaked
fluid, plus any plasma proteins must be carried back to the
blood.
• This is resolved by the lymphatic vessels, or lymphatics, that
collect the excess protein-containing interstitial fluid and
return it to the bloodstream. Once interstitial fluid enters the
lymphatics, it is called lymph (lymph = clear water).
Distribution and Structure of Lymphatic
Vessels:
The lymphatic vessels form a one-way
system in which lymph flows only
toward the heart.
This transport system begins in
microscopic blind-ended lymphatic
capillaries ,which is present between
the tissue cells and blood capillaries .
• lymphatic capillaries are so remarkably
permeable that they were once thought to
be open at one end like a straw. When fluid
pressure in the interstitial space is greater
than the pressure in the lymphatic capillary,
a minivalve flaps gape open, allowing fluid
to enter the lymphatic capillary. However,
when the pressure is greater inside the
lymphatic capillary, the endothelial
minivalve flaps are forced closed, preventing
lymph from leaking back out.
•
Proteins in the interstitial space enter
lymphatic capillaries easily. When tissues
are inflamed, lymphatic capillaries develop
openings that permit uptake of even larger
particles such as cell debris, pathogens and
cancer cells. The pathogenic agents and
cancer cells can then use the lymphatics to
travel throughout the body. This threat to
the body is partly resolved by the fact that
through the lymph nodes it is cleansed of
debris and “examined” by cells of the
immune system.
• Highly specialized lymphatic capillaries called
lacteals (lak′te-alz) are present in the
fingerlike villi of the intestinal mucosa. The
lymph draining from the digestive viscera is
milky white (lacte = milk) rather than clear
because the lacteals play a major role in
absorbing digested fats from the intestine.
• The lymphatic collecting vessels have the
same three tunics as veins, but the lymph
vessels are thinner-walled, have more
internal valves, and anastomose more.
Lymph is eventually delivered to :
- The right lymphatic duct drains lymph from the
right upper arm and the right side of the head and
thorax .
-The much larger thoracic duct receives lymph from
the rest of the body. As the thoracic duct runs
superiorly, it receives lymphatic drainage from the
left side of the thorax, left upper limb, and the
head region.
Each terminal duct empties its lymph into the
subclavian vein on its own side of the body .
Lymph Transport
The lymphatic system lacks a pump. The same mechanisms
that promote venous return in blood vessels act here as
well—
-the milking action of active skeletal muscles,
-pressure changes in the thorax during breathing,
- and valves to prevent backflow.
Also:-pulsations of nearby arteries also promote lymph flow.
- contraction of smooth muscle in the walls of the
lymphatic vessels .
Notice that when physical activity or passive movements
increase, lymph flows much more rapidly .Hence, it is a good
idea to immobilize a badly infected body part to hinder flow
of inflammatory material from that region.
HOMEOSTATIC IMBALANCE
Anything that prevents the normal return of lymph to
the blood, such as blockage of the lymphatics by
tumors or removal of lymphatics during cancer surgery,
results in short-term but severe localized edema
(lymphedema).
• Usually, however, lymphatic drainage is eventually
reestablished by regrowth from the vessels remaining
in the area.
• To summarize lymphatic vessels functions, they
(1) return excess tissue fluid to the bloodstream,
(2) return leaked proteins to the blood, and
(3) carry absorbed fat from the intestine to the blood
(through lacteals).
Lymphoid Cells
1- Lymphocytes, they arise in red bone marrow ,
then mature into one of the two main varieties of
immunocompetent cells—T cells (T lymphocytes)
or B cells (B lymphocytes)—that protect the body
against antigens.
• (Antigens are anything the body perceives as
foreign, such as bacteria and their toxins, viruses,
mismatched RBCs, or cancer cells.)
2- Macrophages play a crucial role in body
protection and in the immune response by
phagocytizing foreign substances and by helping to
activate T cells.
Lymphoid tissues and organs
Lymph Nodes
The principal lymphoid organs in the body are the lymph
nodes, which cluster along the lymphatic vessels .Lymph
nodes vary in shape and size, but most are bean shaped and
less than 2.5 cm (1 inch) in length.
• There are hundreds of these small organs, but because they
are usually embedded in connective tissue, they are not
ordinarily seen.
• Large clusters of lymph nodes occur near the body surface in
the inguinal, axillary, and cervical regions .
• Lymph nodes have two basic functions :
(1) They act as lymph “filters.” Macrophages effectively
preventing pathogens from being delivered to the blood.
(2) They help activate the immune system. Lymphocytes
monitor the lymphatic stream for the presence of antigens
and mount an attack against them.
Circulation in the Lymph Nodes
Lymph enters the convex side of a lymph node through
a number of afferent lymphatic vessels. It then finally
exits the node at its hilum (hi′lum), the indented region
on the concave side, via efferent lymphatic vessels.
• Because there are fewer efferent vessels draining the
node than afferent vessels feeding it, the flow of lymph
through the node stagnates somewhat, allowing time
for the lymphocytes and macrophages to carry out
their protective functions.
• Lymph passes through several nodes before it is
completely cleansed.
HOMEOSTATIC IMBALANCE
-When large numbers of pathogen are trapped in the
nodes, the nodes become inflamed, swollen, and
tender to the touch( swollen glands).
-Lymph nodes can also become secondary cancer
sites, particularly in metastasizing cancers that
enter lymphatic vessels and become trapped there.
The fact that cancer-infiltrated lymph nodes are
swollen but usually not painful helps distinguish
cancerous lymph nodes from those infected by
microorganisms.
Spleen
The soft, blood-rich spleen is about the size of a fist and is the
largest lymphoid organ. Located in the left side of the
abdominal cavity just beneath the diaphragm, it curls around
the anterior aspect of the stomach .It is served by the large
splenic artery and vein, which enter and exit the hilum on its
slightly concave anterior surface.
The spleen performs four functions:
1. Its macrophages remove debris and foreign matter from
blood flowing through its sinuses.
2. It stores some of the breakdown products of red blood
cells for later reuse (for example, iron for making hemoglobin)
and releases others to the blood for processing by the liver.
3. It is a site of erythrocyte production in the fetus (a
capability that normally ceases after birth).
4. It stores blood platelets.
HOMEOSTATIC IMBALANCE
Because the spleen’s capsule is relatively thin, a
direct blow or severe infection may cause it to
rupture, spilling blood into the peritoneal cavity.
Splenectomy was the standard treatment .
-During the past 20 years, surgeons have
discovered that, if left alone, the spleen can often
repair itself so splenectomies has decreased .
-If the spleen is removed, the liver and bone
marrow take over most of its functions.
-In children younger than 12, the spleen will
regenerate if a small part of it is left in the body
Thymus
The bilobed thymus (thi′mus) is found in the inferior neck
and extends into the superior thorax, where it partially
overlies the heart deep to the sternum.
- By secreting the hormones thymopoietin and the
thymosins, the thymus causes T lymphocytes to become
immunocompetent; that is, it enables them to function
against specific pathogens in the immune response.
-Prominent in newborns, the thymus continues to
increase in size during the first year, when it is highly
active.
-After puberty, it starts to atrophy gradually and by old
age it has been replaced almost entirely by fibrous and
fatty tissue and is difficult to distinguish from surrounding
connective tissue.
Tonsils
The tonsils are the simplest lymphoid organs. They
form a ring of lymphatic tissue around the entrance to
the pharynx (throat), where they appear as swellings of
the mucosa .
The tonsils are named according to location:
-The paired palatine tonsils are located on either side
at the posterior end of the oral cavity. These are the
largest of the tonsils and the ones most often infected.
- The lingual tonsil , a lumpy collection of lymphoid
follicles at the base of the tongue.
-The pharyngeal tonsil (referred to as the adenoids if
enlarged) is in the posterior wall of the nasopharynx.
-The tonsils gather and remove many of the pathogens
entering the pharynx in food or in inhaled air.
Aggregates of Lymphoid Follicles
Peyer’s patches (pi′erz) are large isolated clusters of lymphoid
follicles, structurally similar to the tonsils, that are located in the
wall of the distal portion of the small intestine .Lymphoid
follicles are also heavily concentrated in the wall of the appendix.
• Peyer’s patches and the appendix are in an ideal position :
(1) to destroy bacteria (which are present in large numbers in
the intestine)and
(2) to generate many “memory” lymphocytes for long-term
immunity.
• Peyer’s patches, the appendix, and the tonsils—all located in
the digestive tract—and lymphoid follicles in the walls of the
bronchi (organs of the respiratory tract) and in the mucosa of
genitourinary organs, are part of the collection of small
lymphoid tissues referred to as mucosa-associated lymphatic
tissue (MALT).
• MALT protects passages that are open to the exterior from
foreign matter entering them.
BODY DEFENSES
Two intrinsic defense systems act both independently and
cooperatively to provide resistance to disease, or immunity
(immun = free).Together they are called immune system.
1. The innate (nonspecific) defense system.This system has two
lines.
a-The first line of defense is the external body membranes—
intact skin and mucosae.
b-The second line of defense ,called into action whenever the
first line has been penetrated, uses antimicrobial proteins,
phagocytes, and inflammation to inhibit the invaders’ spread
throughout the body.
2. The adaptive (or specific) defense system provides the
body’s third line of defense. This defensive response takes
considerably more time to mount than the innate response.
Although we consider them separately, the adaptive and
innate systems always work hand in hand.
PART 1: INNATE DEFENSES
Are the mechanical barriers that cover body surfaces
and the cells and chemicals that act on the initial
internal battlefronts to face pathogens (harmful or
disease-causing microorganisms) and infection.
• Many times, our innate defenses alone are able to
destroy pathogens
• In other cases, the adaptive immune system is called
into action to reinforce and enhance the innate
mechanisms.
• In other words , the innate defenses reduce the
workload of the adaptive system by preventing the
entry and spread of microorganisms in the body.
The body’s first line of defense ,Skin and Mucosae
-As long as the epidermis is unbroken(keratin is a
physical barrier and also resistant to most weak acids
and bases and to bacterial enzymes and toxins) and
mucosae are intact, these physical barriers, produce a
variety of protective chemicals:
1. The acidity of skin secretions (pH 3 to 5) inhibits
bacterial growth, and sebum contains chemicals that
are toxic to bacteria. Vaginal secretions of adult females
are also very acidic.
2. The stomach mucosa secretes a concentrated
hydrochloric acid solution and protein-digesting
enzymes. Both kill microorganisms.
3. Saliva, which cleanses the oral cavity and teeth,
4. lacrimal fluid of the eye contain lysozyme, an
enzyme that destroys bacteria.
5. Sticky mucus traps many microorganisms that
enter the digestive and respiratory passageways.
6. Tiny mucus-coated hairs inside the nose trap
inhaled particles, and cilia on the mucosa of the
upper respiratory tract sweep dust- and bacterialaden mucus toward the mouth, preventing it from
entering the lower respiratory passages.
Although the surface barriers are quite effective,
they are breached occasionally ,when this happens
and microorganisms invade deeper tissues, the
internal innate defenses come into play.
Second line of defense
• Phagocytes
1-macrophages (“big eaters”), the chief phagocytes which
derive from white blood cells called monocytes that leave
the bloodstream, enter the tissues, and develop into
macrophages.
Free macrophages, like the alveolar macrophages of the
lungs, wander throughout the tissue spaces in search of
cellular debris or “foreign invaders.” Fixed macrophages like
Kupffer cells in the liver and microglia of the brain are
permanent residents of particular organs. Whatever their
mobility, all macrophages are similar structurally and
functionally.
2- Neutrophils, the most abundant type of white blood
cell, become phagocytic on encountering infectious material
in the tissues.
3- Eosinophils, another type of white blood cell, are only
weakly phagocytic, but they are important in defending the
body against parasitic worms.
• Natural Killer Cells
Natural killer (NK) cells, which “police” the body in
blood and lymph, are a unique group of defensive cells
that can lyse and kill cancer cells and virus-infected
body cells before the adaptive immune system is
activated. Unlike lymphocytes of the adaptive immune
system, which recognize and react only against specific
virus-infected or tumor cells, NK cells are far less picky.
The name “natural” killer cells reflects this
nonspecificity of NK cells.
NK cells are not phagocytic. They attack the target
cell`s membrane and release a lytic chemical which
induces the target cell to undergo apoptosis
(programmed cell death).
• NK cells also secrete potent chemicals that enhance the
inflammatory response
• Inflammation
The inflammatory response is nonspecific and triggered
whenever body tissues are injured by physical trauma (a
blow), intense heat, irritating chemicals, or infection by
viruses, fungi, or bacteria.
The inflammatory response has several beneficial effects:
1. Prevents the spread of damaging agents to nearby tissues
2. Disposes of cell debris and pathogens
3. Sets the stage for repair.
The four cardinal signs of short-term, or acute, inflammation
are redness, heat ,swelling, and pain.
-If the inflamed area is a joint, joint movement may be
hampered temporarily. This forces the injured part to rest,
which aids healing.
-Some authorities consider impairment of function to be the
fifth cardinal sign of acute inflammation.
Summary of nonspecific body defenses
• Fever
Fever, or abnormally high body temperature, is a systemic
response to invading microorganisms. Body temperature is
regulated by the hypothalamus, (body’s thermostat).
Normally set at approximately 37°C (98.6°F), the thermostat
is reset upward in response to chemicals called pyrogens
(pyro = fire), secreted by leukocytes and macrophages
exposed to foreign substances in the body.
-High fevers are dangerous because it denatures enzymes.
-Mild or moderate fever, however, seems to benefit the
body. In order to multiply, bacteria require large amounts of
iron and zinc,but during a fever the liver and spleen
sequester these nutrients, making them less available.
Fever also increases the metabolic rate of tissue cells in
general, speeding up repair processes.
• Antimicrobial proteins : Complement and interpheron (see
before).
PART II:ADAPTIVE DEFENSES
Unlike the innate system, which is always ready and
able to react, the adaptive system must “meet” or be
primed by an initial exposure to a specific foreign
substance (antigen) before it can protect the body
against that substance.
Three important aspects of the adaptive immune
response:
1. It is specific: It recognizes and is directed against
particular pathogens or foreign substances.
2. It is systemic: Immunity is not restricted to the initial
infection site.
3. It has “memory”: After an initial exposure, it
recognizes and mounts even stronger attacks on the
previously encountered pathogens.
• Two separate but overlapping arms of adaptive
immunity were recognized, each using a variety of
attack mechanisms :
1-Humoral immunity (hu′mor-ul), also called
antibody-mediated immunity, is provided by
antibodies present in the body’s “humors,” or
fluids (blood, lymph, etc.). Though they are
produced by lymphocytes (or their offspring),
antibodies circulate freely in the blood and lymph,
where they bind primarily to bacteria, to bacterial
toxins, and to free viruses, inactivating them
temporarily and marking them for destruction by
phagocytes or complement.
2-Cellular or cell-mediated immunity
-lymphocytes(cells) themselves rather than
antibodies defend the body.
-Also has cellular targets—virus-infected or
parasite-infected tissue cells, cancer cells, and cells
of foreign grafts.
The lymphocytes act against such targets either
directly, by killing the foreign cells, or indirectly, by
releasing chemical mediators that enhance the
inflammatory response or activate other
lymphocytes or macrophages.
The antigens are the triggers of the activity of the
cells involved in these immune responses.
Antigens
Antigens (an′tĭ-jenz) are substances that can mobilize
the immune system and provoke an immune response.
Most antigens are large, complex molecules that are
not normally present in the body. Consequently, as far
as our immune system is concerned, they are intruders,
or nonself.
An almost limitless variety of foreign molecules can act
as complete antigens, including virtually all foreign
proteins, many large polysaccharides, and some lipids
and nucleic acids. Of these, proteins are the strongest
antigens.
Pollen grains and microorganisms—such as bacteria,
fungi, and virus particles—are antigens because their
surfaces bear many different foreign macromolecules.
Incomplete Antigens or Haptens
As a rule, small molecules—such as peptides,
nucleotides, and many hormones—are not complete
antigens. But, if they link up with the body’s own
proteins, the adaptive immune system may recognize
the combination as foreign and mount an attack that is
harmful rather than protective. (These reactions, called
hypersensitivities.)
• In such cases, the troublesome small molecule is called
a hapten (hap′ten; haptein = grasp) or incomplete
antigen.
• Besides certain drugs (particularly penicillin), chemicals
that act as haptens are found in poison ivy, animal
dander, detergents, cosmetics, and a number of
common household and industrial products.
Self-Antigens: MHC Proteins
The external surfaces of all our cells are dotted with a
huge variety of protein molecules.
Assuming our immune system has been properly
“programmed,” these self-antigens are not foreign or
antigenic to us, but they are strongly antigenic to
other individuals. (This is the basis of transfusion
reactions and graft rejection.)
Among the cell surface proteins that mark a cell as self
is a group of glycoproteins called MHC proteins, the
major histocompatibility complex .
Because millions of combinations of these genes are
possible, it is unlikely that any two people except
identical twins have the same MHC proteins.
Cells of the Adaptive Immune System: Three cell types :
• B lymphocytes or B cells contribute to humoral immunity.
• T lymphocytes or T cells are non-antibody-producing
lymphocytes that constitute the cell-mediated arm of
adaptive immunity.
• macrophages do not respond to specific antigens but has
essential roles.
Lymphocytes
Like all other blood cells, lymphocytes originate in red bone
marrow from hematopoietic stem cells.
During development, lymphocytes are “educated.” The
aim of this education is twofold:
1) Each lymphocyte must become able (competent) to
recognize its one specific antigen by binding to it. This
ability is called immunocompetence.
(2) Each lymphocyte must be unresponsive to
self-antigens so that it does not attack the body’s
own cells. This is called self-tolerance.
-B cells become immunocompetent and selftolerant in the bone marrow.
-T cells are educated in the thymus under the
direction of thymic hormones.
The lymphoid organs where the lymphocytes
become immunocompetent (thymus and bone
marrow) are called primary lymphoid organs .
All other lymphoid organs are referred to as
secondary lymphoid organs.
Humoral (antibody-mediated)Immune Response
An immunocompetent but but as yet immature
B lymphocyte is activated (stimulated to
complete its differentiation) when antigens bind
to its surface receptors. These events trigger
clonal selection ,that is, stimulate the B cell to
grow and then multiply rapidly to form an army
of cells all exactly like itself and bearing the
same antigen-specific receptors .The resulting
family of identical cells, all descended from the
same ancestor cell, is called a clone and clone
formation is the primary immune response.
Most cells of the clone become plasma cells, the
antibody-secreting effector cells of the humoral
response.
Although B cells secrete limited amounts of
antibodies, plasma cells secrete antibodies at the
unbelievable rate of about 2000 molecules per
second.
Each plasma cell functions for 4 to 5 days and then
dies.
Clone cells that do not differentiate into plasma
cells become long-lived memory cells that can
mount an almost immediate humoral response if
they encounter the same antigen again at some
future time.
Memory
The cellular proliferation and differentiation
just described constitute the primary immune
response, which occurs on first exposure to a
particular antigen.
• The primary response typically has a lag
period of 3 to 6 days after the antigen
challenge. This lag period mirrors the time
required for the few B cells specific for that
antigen to proliferate and for their offspring to
differentiate into plasma cells.
• After the mobilization period, plasma
antibody levels rise, reach peak levels in about
10 days, and then decline .
If (and when) someone is reexposed to the same
antigen, whether it’s the second or the twenty-second
time, a secondary immune response occurs. Secondary
immune responses are faster, more prolonged, and
more effective, because the sensitized memory cells
are already in place . These memory cells provide what
is commonly called immunological memory.
Within hours after recognition of the “old enemy”
antigen, a new army of plasma cells is being generated.
Within 2 to 3 days the antibody titer (concentration) in
the blood rises steeply to reach much higher levels than
were achieved in the primary response. Secondary
response antibodies not only bind with greater affinity
(more tightly), but their blood levels remain high for
weeks to months. Memory cells persist for long
periods in humans and many retain their capacity to
produce powerful secondary humoral responses for
life.
Active and Passive Humoral Immunity
When your B cells encounter antigens and produce
antibodies against them, you are exhibiting active
humoral immunity .
• Active immunity is
(1) naturally acquired when you get a bacterial or viral
infection, during which time you may develop
symptoms of the disease and suffer a little (or a lot),
and
(2) artificially acquired when you receive vaccines.
Indeed, once it was realized that secondary responses
are so much more vigorous than primaries, the race
was on to develop vaccines to “prime” the immune
response by providing a first meeting with the antigen.
Most vaccines contain pathogens that are dead or
attenuated (living , but extremely weakened).
Passive humoral immunity
Instead of being made by your plasma cells, the antibodies are
harvested from the serum of an immune human or animal donor.
As a result, your B cells are not challenged by antigens,
immunological memory does not occur, and the protection
provided by these antibodies ends when they naturally degrade in
the body.
-Passive immunity is conferred naturally on a fetus or infant
when the mother’s antibodies cross the placenta or are ingested
with the mother’s milk. For several months after birth, the baby is
protected from all the antigens to which the mother has been
exposed.
-Passive immunity is artificially conferred via a serum such as
gamma globulin, which is administered after exposure to hepatitis.
Other immune sera are used to treat poisonous snake bites
(antivenom), botulism, rabies, and tetanus (antitoxin) because
these rapidly fatal diseases would kill a person before active
immunity could be established.
The donated antibodies provide immediate protection, but their
effect is short-lived (two to three weeks).
Antibodies
Antibodies, also called immunoglobulins (Igs) (im″uno-glob′u-linz), constitute the gamma globulin part
of blood proteins. As mentioned earlier, antibodies
are proteins secreted by activated B cells or plasma
cells in response to an antigen, and they are
capable of binding specifically with that antigen.
They are formed in response to an incredible
number of different antigens. Despite their variety,
all antibodies can be grouped into one of five Ig
classes, each slightly different in structure and
function.
Basic Antibody Structure
Regardless of its class, each antibody has a basic
structure consisting of four looping polypeptide chains
linked together by disulfide (sulfur-to-sulfur) bonds .
-Two of these chains, the heavy (H) chains, are identical
to each other and contain more than 400 amino acids
each.
-The other two chains, the light (L) chains, are also
identical to each other, but only about half as long as
each H chain.
The four chains combined form a molecule, called an
antibody monomer (mon′o-mer), with two identical
halves, each consisting of a heavy and a light chain. The
molecule as a whole is T or Y shaped.
• Each chain forming an antibody has a variable (V)
region at one end and a much larger constant (C)
region at the other end. Antibodies responding to
different antigens have very different V regions, but
their C regions are the same (or nearly so) in all
antibodies of a given class.
• The V regions of the heavy and light chains in each
arm of the monomer combine to form an antigenbinding site shaped to “fit” a specific antigenic
determinant. Hence, each antibody monomer has
two such antigen-binding regions.
Antibody Classes
The five major immunoglobulin classes are
designated IgM, IgA, IgD, IgG, and IgE, on the basis
of the C regions in their heavy chains. (Remember
the name MAGED to recall the five Ig types.)
• IgD, IgG, and IgE have the same basic Y-shaped
structure and thus are monomers.
• IgA occurs in both monomer and dimer (two linked
monomers) forms.
• IgM is a huge antibody, a pentamer (penta = five)
constructed from five linked monomers.
• The antibodies of each class have different biological
roles and locations in the body.
IgM is the first antibody class released to the blood by
plasma cells.
IgG is the most abundant antibody in plasma and the
only Ig class that crosses the placental barrier.
IgA dimer, also called secretory IgA, is found primarily
in mucus and other secretions that bathe body
surfaces. It plays a major role in preventing pathogens
from gaining entry into the body.
IgE antibodies, found in minute quantities in blood,
are the “troublemaker” antibodies involved in some
allergies.
IgD is important in activation of B cells.
Cell-Mediated Immune Response
Antibodies are fairly useless against intracellular pathogens
(such as viruses, and the tuberculosis bacillus that quickly slip
inside body cells to multiply there), fungi, malignant cells,
and grafts of foreign tissue.
In these cases, the cell-mediated arm of adaptive immunity
comes into play . No antibodies are produced.
The T cells that mediate cellular immunity are much more
complex than B cells in both classification and function.
There are Four populations of T cells :
- CD4 cells are primarily helper T cells (TH),
- CD8 cells are cytotoxic T cells (TC), whose role is to destroy any
cells in the body that harbor anything foreign.
- Regulatory T cells (TReg),
- Memory T cells, and some fairly rare subgroups.
The first step is the recognition of the foreign antigen
by macrophages (antigen presenter) that present
the antigen to helper T cells which become
activated.
• These activated helper T cells, which are antigen
specific, divide many times to form memory T cells
and cytotoxic (killer) T cells .
• the helper T cells direct or help complete the
activation of all other immune cells(B cells) so
defect in T helper is very serious as it affects both T
and B immunity.
-The memory T cells will remember the specific foreign
antigen and become active if it enters the body again.
- Cytotoxic T cells are able to chemically destroy foreign
antigens by disrupting cell membranes. This
is how cytotoxic T cells destroy cells infected with
viruses and prevent the viruses from reproducing.
These T cells also produce cytokines, which are chemicals
that attract macrophages to the area and activate
them to phagocytize the foreign antigen and cellular
Debris.
-regulatory T (TReg) cells (formerly known as suppressor T
cells) dampen the immune response either by direct
contact or by releasing inhibitory cytokines.
Organ Transplants and Prevention of Rejection
Immune rejection presents a particular problem when
the goal is to transplant organs from a living or
recently deceased donor.
Essentially, there are four major varieties of grafts:
1. Autografts are tissue grafts transplanted from one
body site to another in the same person.
2. Isografts are grafts donated to a patient by a
genetically identical individual, the only example
being identical twins.
3. Allografts are grafts transplanted from individuals
that are not genetically identical but belong to the
same species.
4. Xenografts are grafts taken from another animal
species, into a human being.
Homeostatic Imbalances of Immunity
1-Immunodeficiencies
Congenital or acquired condition that causes immune cells, to
behave abnormally.
The most devastating congenital conditions are a group of
related disorders called severe combined immunodeficiency
(SCID) syndromes, which may affect both B and T.
• There are various acquired immunodeficiencies. For example,
Hodgkin’s disease, cancer of the B cells, Immunosuppressive
drugs and certain drugs used to treat cancer also suppress
the immune system.
• The most devastating of the acquired immunodeficiencies is
acquired immune deficiency syndrome (AIDS), which
cripples the immune system by interfering with the activity of
helper T cells so produce a marked deficit of B and T cells.
2-Autoimmune Diseases
Occasionally the immune system loses its
ability to distinguish friend (self) from foe
(foreign antigens). When this happens, the
immune system, turns against itself.
• The body produces antibodies
(autoantibodies) and sensitized TC cells that
destroy its own tissues.
• This puzzling phenomenon is called
autoimmunity. If a disease state results, it is
referred to as autoimmune disease.
3-Hypersensitivities
Result when the immune system causes tissue damage
as it fights off a perceived threat (such as pollen or
animal dander) that would otherwise be harmless to
the body.
• People rarely die of hypersensitivities; they are just
miserable with them.
• The different types of hypersensitivity reactions are
distinguished by:
(1) their time course, and
(2) whether antibodies or T cells are involved.
-Hypersensitivities mediated by antibodies are the
immediate and subacute hypersensitivities.
-T cells cause delayed hypersensitivity.
Developmental Aspects of the Immune System
1. Development of the immune response occurs
around the time of birth. The ability of the immune
system to recognize foreign substances is
genetically determined.
2. The nervous system plays an important role in
regulating immune responses, possibly through
common mediators. Depression impairs immune
function.
3. With aging, the immune system becomes less
responsive. The elderly more often suffer from
immune deficiency, autoimmune diseases, and
cancer.