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Phsi2005 notes (immune) 1
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Immune System
Immune System
1. Differentiate between specific and non-specific immune defence mechanisms
Innate
- Non-specific
- Present from birth
- Rapid response, within minutes to hours
- Respond to a range of molecular signals, does not target a specific pathogen
- Inflammation is a common reaction/response
Adaptive
- Specific, directed at specific invaders
- Can distinguish between pathogens
- First exposure can take days
- With repeated exposure it “remembers” prior exposure to the pathogen and then reacts
more rapidly
- Can be divided into cell-mediated immunity and humoral immunity
2. List non-specific innate cell and mediator functions
Pathogen-associated Molecular Patterns (PAMPS): bind to leukocyte pattern recognition receptors
(PRR) that activate the nonspecific immune response. The initial response of these immune cells to
invaders is to kill them or ingest them.
3. Discuss the mechanisms producing the cardinal signs of inflammation
Inflammation is a hallmark reaction of innate immunity.’
Inflammation has 3 important roles in fighting infection in damage tissue:
1) Attracting immune cells and chemical mediators to the site
2) Producing a physical barrier to retard the spread of infection
3) Promoting tissue repair once the infection is under control (a non-immunological function)
The inflammatory response is created when activated tissue macrophages release cytokines. These
chemicals attract other immune cells, increase capillary permeability, and cause fever. Immune cells
attracted to the site in turn release their own cytokines.
4. Specify the main mediators of inflammation
Chemical substances, called mediators, released from injured or activated cells co-ordinate the
development of the inflammatory response.
• Plasma proteins such as complement and antibodies.
• Other proteins such as sPLA and acute phase reactants.
2
• Cytokines and chemokines.
• Lipids such as prostaglandins and PAF.
• Amines such as histamine.
-
• ‘Gasses’ such as NO and O2 .
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• Kinins such as bradykinin.
• Neuropeptides such as substance P
5. Outline the functions of complement
• A complex series of about 20 proteolytic enzymes in the blood.
• ‘Classical’ and ‘alternate’ pathways act in a cascade fashion.
• Accelerated in the presence of IgGs
• Lytic to many micro-organisms.
• ‘Opsonise’ others.
A complement is a group of plasma enzymes that are involves in the immune function.
The complement cascade is similar to the blood coagulation cascade. Various intermediate of the
compliment cascade act as opsonins, chemical attractants for leukocytes, and agent that cause mast
cell degranulation.
6. Outline the functions of neutrophils (margination, diapedesis, chemotaxis, amoeboid
motion, phagocytosis)
Neutrophil granulocytes are the most abundant type of white blood cells in mammals and form an
essential part of the innate immune system. They are subdivided into segmented neutrophils (or
segs) and banded neutrophils (or bands). They form part of the polymorphonuclear cell family
(PMNs) together with basophils and eosinophils.
Neutrophils are normally found in the blood stream. During the beginning (acute) phase of
inflammation, particularly as a result of bacterial infection, environmental exposure, and some
cancers., neutrophils are one of the first-responders of inflammatory cells to migrate towards the
site of inflammation. They migrate through the blood vessels, then through interstitial tissue,
following chemical signals such as Interleukin-8 (IL-8), C5a, fMLP and Leukotriene B4 in a process
called chemotaxis (the phenomenon in which somatic cells, bacteria, and other single-cell or
multicellular organisms direct their movements according to certain chemicals in their environment).
They are the predominant cells in pus, accounting for its whitish/yellowish appearance.
Neutrophils are recruited to the site of injury within minutes following trauma and are the hallmark
of acute inflammation.
Neutrophils undergo a process called chemotaxis, which allows them to migrate toward sites of
infection or inflammation. Cell surface receptors allow neutrophils to detect chemical gradients of
molecules such as interleukin-8 (IL-8), interferon gamma (IFN-gamma), C5a, and Leukotriene B4,
which these cells use to direct the path of their migration.
Neutrophils have a variety of specific receptors including complement receptors, cytokine receptors
for interleukins and interferon gamma (IFN-gamma), receptors for chemokines, receptors to detect
and adhere to endothelium, receptors for leptins and proteins, and Fc receptors for opsonin.
Neutrophils are phagocytes, capable of ingesting microorganisms or particles. For targets to be
recognised, they must be coated in opsonins—a process known as antibody opsonisation. They can
internalize and kill many microbes, each phagocytic event resulting in the formation of a phagosome
into which reactive oxygen species and hydrolytic enzymes are secreted. The consumption of oxygen
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during the generation of reactive oxygen species has been termed the "respiratory burst", although
unrelated to respiration or energy production.
The respiratory burst involves the activation of the enzyme NADPH oxidase, which produces large
quantities of superoxide, a reactive oxygen species. Superoxide dismutates, spontaneously or
through catalysis via enzymes known as superoxide dismutases (Cu/ZnSOD and MnSOD), to
hydrogen peroxide, which is then converted to hypochlorous acid HClO, by the green heme enzyme
myeloperoxidase. It is thought that the bactericidal properties of HClO are enough to kill bacteria
phagocytosed by the neutrophil, but this may instead be a step necessary for the activation of
proteases.
Margination: is the movement of leukocytes out of the circulatory system, towards the site of
tissue damage or infection. This process forms part of the innate immune response, involving
the recruitment of non-specific leukocytes. Monocytes also use this process in the absence of
infection or tissue damage during their development into macrophages.
7. Describe the mechanism of opsonisation
Opsonin:
- Any molecule that targets an antigen for an immune response.
- Antibodies that tag encapsulated bacteria, along with additional plasma proteins
The term is usually used in reference to molecules that act as a binding enhancer for the process of
phagocytosis, especially antibodies, which coat the negatively-charged molecules on the membrane.
In the body, opsonins convert unrecognisable particles into “food” for phagocytes. They also act as a
bridge between pathogen and phagocytes by binding to receptors on the phagocytes. Molecules
that activate the complement system are also considered opsonins.
Both the membrane of a phagocytosing cell and its target have a negative charge (zeta-potential),
making it difficult for the two cells to come close together. Once the opsonins attach to the target,
the negative charged is masked. Take note that the negative charge of the target doesn't disappear.
The opsonin simply overrides the charge, making it easier for white blood cells (phagocytic cells), to
undergo phagocytosis. During the process of opsonisation, antigens are bound by antibody or
complement molecules. Phagocytic cells express receptors, CR1 and Fc receptors, that bind opsonin
molecules, C3b and antibody, respectively. With the antigen coated in these molecules, binding of
the antigen to the phagocyte is greatly enhanced. In fact, most phagocytic binding cannot occur
without opsonisation of the antigen.
Furthermore, opsonisation of the antigen and subsequent binding to an activated phagocyte will
cause increased expression of complement receptors on neighboring phagocytes.
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8. Outline the functions of natural killer cells and interferon
Natural killer cells (NK cells):
- A type of cytotoxic lymphocyte that attacks certain tumour and virus-infected cells.
- Critical to the innate immune system.
- Function: The role NK cells play is analogous to that of cytotoxic T cells in the vertebrate
adaptive immune response. NK cells provide rapid responses to virally infected cells and
respond to tumor formation, acting at around 3 days after infection. Typically immune cells
detect MHC presented on infected cell surfaces, triggering cytokine release causing lysis or
apoptosis.
- NK cells are unique, however, as they have the ability to recognise stressed cells in the
absence of antibodies and MHC, allowing for a much faster immune reaction. They were
named “natural killers” because of the initial notion that they do not require activation in
order to kill cells that are missing “self” markers of major histocompatibility complex (MHC)
class 1.
Interferon:
- Named for their ability to interfere with viral replication.
- They are cytokines secreted by lymphocytes that aid in the immune response.
- Function: Interferon-alpha (IFN-α) and interferon-beta (IFN-β) target host cells and promote
synthesis of antiviral proteins to prevent viral replication.
Inferferon-gamma (IFN-ϒ) activates macrophages and other immune cells
9. Distinguish between active and passive immunization
Active immunity occurs when the body is exposed to a pathogen and produces its own antibodies.
Active immunity can occur naturally, when a pathogen invades the body, or artificially, as when we
are given vaccinations containing dead or disabled pathogens.
Passive immunity occurs when we acquire antibodies made by another animal. The transfer of
antibodies from mother to foetus across the placenta is one example. Injections containing
antibodies are another. Travellers going abroad may be injected with gamma globulin (antibodies
extracted from donated human plasma), but this passive immunity lasts only about three months.
10. Define immunological memory and understand its mechanism
Memory cells are long lived and continue reproducing themselves. Second and subsequent
exposures to the antigen activate the memory cells and cause rapid clonal expansion, creating a
quicker and stronger secondary response to the antigen.
11. Specify the characteristics of antigens and antibodies
Antigens: substances that trigger an immune response from the body and that can react with
products of that response.
Antibodies: molecules keyed to a particular pathogen that helps target it for destruction.
Antigen is a substance that evokes the production of one or more antibodies. Each antibody binds to
a specific antigen by way of an interaction similar to the fit between a lock and a key. The substance
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may be from the external environment or formed within the body. The immune system will try to
destroy or neutralize any antigen that is recognized as a foreign and potentially harmful invader.
Antibodies refers to any molecule or molecular fragment that can be bound by a major
histocompatibility complex (MHC) and presented to a T-cell receptor. "Self" antigens are usually
tolerated by the immune system; whereas "Non-self" antigens can be identified as invaders and can
be attacked by the immune system.
12. Outline the anatomy of the lymphatic system
The immune system is probably the least anatomical identifiable system of the body because most
of it is integrated into the tissues of other organs, such as the skin and gastrointestinal tract. The
immune system has 2 anatomical components: lymphoid tissues and the cells responsible for the
immune response.
Lymphoid tissues are found all over the body. The 2 primary lymphoid tissues are the thymus gland
and the bone marrow, both sites where cells involved in the immune response form and mature.
Some types of mature immune cells do not specialise until their first exposure to the pathogen they
will fight. These mature but unspecialised immune cells are said to be naïve cells.
In the secondary lymphoid tissues, mature immune cells interact with pathogens and initiate
response. Secondary tissues are divided into encapsulated tissues and unencapsulated diffuse
lymphoid tissues.
The encapsulated lymphoid tissues are the spleen and lymph nodes. Both have an outer wall formed
from fibrous collagenous capsules. These spleen contains immune cells positioned so that they
monitor the blood for foreign invaders. Phagocytic cells in the spleen also trap and remove aging red
blood cells. The lymph nodes are associated with the lymphatic circulation. Here there is a net flow
of fluid out of capillaries and into the interstitial space. This filtered fluid is picked up by lymph
capillaries and passes through the encapsulated lymph nodes on its journey back to the circulation.
Inside lymph nodes, clusters of immune cells intercept pathogens that have entered the interstitial
fluid through breaks in the skin or though mucous membranes. Once these microbes have been
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swept into the lymph, immune cells in the nodes help prevent their spread throughout the body. For
example, when you have a sore throat, lymph nodes in your neck become swollen which result from
the presence of active immune cells that have collected in the nodes to fight infection.
The unencapsulated diffuse lymphoid tissues are aggregations of immune cells that appear in other
organs of the body. They include the tonsils at the posterior nasopharynx; the gut-associated
lymphoid tissue (GALT), which lies just under the epithelium of the oesophagus and intestines; and
clusters of lymphoid tissue associated with the skin and the respiratory, urinary, and reproductive
tracts. In each case, these tissues contain immune cells positioned to intercept invading pathogens
before they get into general circulation. Because of the large surface area of the digestive tract
epithelium, some authorities consider the GALT to be the largest immune organ in the body.
Anatomically, the cells of the immune system can be found in highest concentrations wherever they
are most likely to encounter antigens that penetrate the epithelium.
Primary Secondary
(Responsible for maturation of Ag-
(Sites for Ag contact and response)
reactive cells)
Thymus Bone Lymph Spleen
(T-cell marrow nodes
maturation)
(Expansion of
lymphatic system,
separate from blood (Similar to lymph
circulation. Deep nodes but part of
(T-cell maturation) (B-cell maturation) cortex harbors blood circulation.
mostly T-cells, Collects blood-borne
superficial cortex Ags)
harbors mostly B-
cells)
13. List specific acquired cell and mediator functions
Naïve Cells: a lymphocyte that has not yet been exposed to a specific antigen.
Clonal Expansion: reproduction of one type of lymphocyte following expansion to an antigen
Effector Cells: the cell or tissue that carries out the homeostatic response
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Memory Cells: lymphocyte responsible for creating stronger and more rapid immune response
following second exposure to an antigen
14. Describe the development of immunocompetence and MHC restriction
Immunocompetence: the ability of the body to produce a normal immune response following
exposure to an antigen. Immunocompetence is the opposite of immunodeficiency or immuno-
incompetent or immuno-compromised.
MHC restriction: refers to the fact that a given T cell will recognize a peptide antigen only when it is
bound to a host body's own MHC molecule. Normally, as T cells are stimulated only in the presence
of self-MHC molecules, antigen is recognized only as peptides bound to self-MHC molecules.
MHC restriction is particularly important when primary lymphocytes are developing and
differentiating in the thymus or bone marrow. It is at this stage that T cells die by apoptosis if they
express high affinity for self-antigens presented by an MHC molecule or express too low affinity for
self MHC. This is ensured through two distinct developmental stages: positive selection and negative
selection.
Developing T cells in the primary lymphoid organs (thymus) first express neither CD4, CD8 nor TcR (T
cell receptor). This is referred to as double negative selection. After differentiation, the T cell
expresses both CD4, CD8 and TcR. This is referred to as double positive selection. It is at this stage
that select T cells undergo apoptosis if they are found to select for self-antigen. This is a necessary
step as it prevents T cells from cascading an autoimmune response against its host tissues.
Ultimately, the T cells differentiate and mature to express either CD4 and TcR or CD8 and TcR. At this
point the T cells leave the primary lymphoid organ and enter the blood stream.
Conversely, it is thought that MHC Restriction plays a pivotal role in the antiretroviral therapy used
to treat HIV/AIDS as it can increase the CD4 cell count thus increasing the likelihood for an immune
response to be prompted
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15. Distinguish between humoral and cellular immunity (antibodies and lymphokines)
Adaptive/acquired immunity can be divided into cell-mediated immunity and humoral
immunity
Cell-mediated: uses contact-dependent signaling in which an immune cell receptor
binds to a receptor on its target cell.
Humoral: uses the secreted proteins known as antibodies to carry out the immune
responses.
The cellular response is mediated by T-cells and the humoral response is mediated by B-cells
(that produce plasma cells that produce antibodies).
16. Discuss the clonal selection hypothesis
The clonal selection hypothesis has become a widely accepted model for how the immune system
responds to infection and how certain types of B and T lymphocytes are selected for destruction of
specific antigens invading the body.
The clonal selection hypothesis states that the germline encodes many different antigen receptors -
one for each antigenic determinant to which an individual will be capable of mounting an immune
response. Antigen selects those clones of cells that have the appropriate receptor. The four basic
principles of the clonal selection hypothesis are:
- Each lymphocyte bears a single type of receptor with a unique specificity.
- Interaction between a foreign molecule and a lymphocyte receptor capable of binding that
molecule with a high affinity leads to lymphocyte activation.
- The differentiated effector cells derived from an activated lymphocyte will bear receptors of
an identical specificity to those of the parental cell from which that lymphocyte was derived.
- Lymphocytes bearing receptors for self molecules are deleted at an early stage in lymphoid
cell development and are therefore absent from the repertoire of mature lymphocytes.
The clonal selection hypothesis is now generally accepted as the correct hypothesis to explain how
the adaptive immune system operates. It explains many of the features of the immune response:
1. The specificity of the response
2. The signal required for activation of the response (i.e. antigen)
3. The lag in the adaptive immune response (time is required to activate cells and to expand
the clones of cells)
4. Self/non-self discrimination
17. Outline the functions of B- and T-lymphocytes
B lymphocytes develop in the bone marrow
The precursors of T lymphocytes are also produced in the bone marrow but leave the bone marrow
and mature in the thymus (which accounts for their designation).
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B cells (bursa-derived cells) and T cells (thymus cells) are the major cellular components of the
adaptive immune response. B cells are primarily responsible for humoral immunity (relating to
antibodies) whereas T cells are involved in cell-mediated immunity. The function of B cells and T cells
is to recognize specific “non-self” antigens, during a process known as antigen presentation. Once
they have identified an invader, the cells generate specific responses that are tailored to maximally
eliminate specific pathogens or pathogen infected cells. B cells respond to pathogens by producing
large quantities of antibodies which then neutralize foreign objects like bacteria and viruses. In
response to pathogens some T cells, called T helper cells, produce cytokines that direct the immune
response while other T cells, called cytotoxic T cells, produce toxic granules that contain powerful
enzymes which induce the death of pathogen infected cells. Following activation, B cells and T cells
leave a lasting legacy of the antigens they have encountered, in the form of memory cells.
Throughout the lifetime of an animal these memory cells will “remember” each specific pathogen
encountered, and are able to mount a strong and rapid response if the pathogen is detected again
Each B cell and T cell is specific for a particular antigen. What this means is that each is able to bind
to a particular molecular structure.
Both BCRs (B cell receptors) and TCRs (T cell receptors) share these properties:
They are integral membrane proteins.
They are present in thousands of identical copies exposed at the cell surface
They are made before the cell ever encounters an antigen.
They are encoded by genes assembled by the recombination of segments of DNA
They have a unique binding site.
This site binds to a portion of the antigen called an antigenic determinant or epitope.
The binding, like that between an enzyme and its substrate depends on complementarity of
the surface of the receptor and the surface of the epitope.
The binding occurs by non-covalent forces (again, like an enzyme binding to its substrate).
Successful binding of the antigen receptor to the epitope, if accompanied by additional
signals, results in:
o stimulation of the cell to leave G0 and enter the cell cycle.
o Repeated mitosis leads to the development of a clone of cells bearing the same
antigen receptor; that is, a clone of cells of the identical specificity.
BCRs and TCRs differ in:
their structure;
the genes that encode them;
the type of epitope to which they bind.
18. Distinguish the roles of helper, suppressor/regulator and cytotoxic T-cells
Helper: immune cells that secrete cytokines to help other immune cells
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Regulator (formally suppressor): are crucial for the maintenance of immunological tolerance. Their
major role is to shut down T cell-mediated immunity toward the end of an immune reaction and to
suppress auto-reactive T cells that escaped the process of negative selection in the thymus.
Cytotoxic: a lymphocyte that kills its target cells
19. Describe the mechanisms of antibody- and cell-mediated immune responses, including
antigen processing and presentation, cell collaboration, cytokines
Antibody: a molecule keyed to a particular pathogen that helps target it for destruction.
Antibodies are also called immunoglobulins, and this alternative name describes what the molecules
are: globular proteins that participate in the humoral immune response.
Cell-mediated immunity: an immune response that does not involve antibodies but rather involves
the activation of macrophages, natural killer cells (NK), antigen-specific cytotoxic T-lymphocytes, and
the release of various cytokines in response to an antigen. Cell-mediated immunity is directed
primarily at microbes that survive in phagocytes and microbes that infect non-phagocytic cells. It is
most effective in removing virus-infected cells, but also participates in defending against fungi,
protozoans, cancers, and intracellular bacteria. It also plays a major role in transplant rejection.
Antibodies are effective only against extracellular pathogens. Once a pathogen gets inside a host
cell, it can no longer be “seen” by the humoral immune system. Defending the body against
intracellular pathogens is the role of T lymphocytes, which carry out cell-mediated immunity. In this
process, T cells bind to cells that display foreign antigen fragments as part of a major
histocompatibility complex (MHC) on their surface.
Antigen processing is a biological process that prepares antigens for presentation the special cells, T
lymphocytes. This process involves two distinct pathways for processing of antigens from an
organism's own (self) proteins or intracellular pathogens (e.g. viruses), or from phagocytosed
pathogens (e.g. bacteria); subsequent presentation of these antigens on class I or class II MHC
molecules is dependent on which pathway is used. Both MHC class I and II are required to bind
antigen before they are stably expressed on a cell surface.
Cytokines are small cell-signaling protein molecules that are secreted by numerous cells and are a
category of signaling molecules used extensively in intercellular communication
20. Compare and contrast defence mechanisms against bacteria and viruses
When microorganisms— such as bacteria or viruses— invade the body, nonspecific defense
mechanisms provide the first line of defense.
These are the primary deterrents which ensure protection from numerous germs. There are physical
deterrents (including the skin and nasal hairs), chemical deterrents (enzymes found in perspiration
and saliva), and inflammatory reactions. These particular mechanisms are named appropriately
because their responses are not specific to any particular pathogen. Think of these as a perimeter
alarm system on a house. No matter who trips the motion detectors, the alarm will sound.
In cases where microorganisms get through the primary deterrents, there is a back-up system— the
specific defense mechanisms— which consists of two components: the humoral immune system and
the cell mediated immune system.
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The humoral immune system protects against bacteria and viruses present in the fluids of the body.
This system uses white blood cells, called B cells, which have the ability to recognize organisms that
don't belong to the body. In other words, if this isn't your house, get out! Intruders are referred to as
antigens. B cells produce antibodies that recognize and bind to a specific antigen to identify it as an
invader that needs to be terminated.
The cell mediated immune system protects against foreign organisms that have managed to infect
body cells. It also protects the body from itself by controlling cancerous cells. White blood cells
involved in cell mediated immunity are called T cells. Unlike B cells, T cells are actively involved with
the disposal of antigens. They make proteins called T cell receptors that help them recognize a
specific antigen. There are three classes of T cells that play specific roles in the destruction of
antigens: Cytotoxic T cells (which directly terminate antigens), Helper T cells (which precipitate the
production of antibodies by B cells), and Suppressor T cells (which suppress the response of B cells
and other T cells).
Differences b/w Bacteria and Viruses
Bacteria Viruses
Structure Cells. Usually surrounded by Not cells. Nucleic acid core with
cell wall protein coat
Living Conditions Most can survive and Parasitic. Must have a host cell
reproduce outside a host to reproduce
Susceptibility to Drugs Most can be killed or inhibited Cannot be killed with
by antibiotics antibiotics. Some can be
inhibited with antiviral drugs
These differences require the body to have a variety of immune responses.
21. Describe immunological surveillance against tumours and transplants
This theory proposes that cancerous cells develop on a regular basis, but they are detected and
destroyed before they can spread. Immune surveillance appears to recognise and control some
virus-associated tumours. In addition, some types of endogenous tumours lack surface antigens,
which allows NK cells to recognise these cells as abnormal and destroy them.
22. Describe the breakdown of immunological tolerance in auto-immune disease
Self-tolerance: the lack of immune response by lymphocytes to cells of the body.
This appears to be caused due to the elimination of self-reactive lymphocytes in a process known as
clonal deletion. During development, some lymphocyte clones develop with antibodies that can
combine with MHC-self antigen complexes. The primary lymphoid tissues contain self antigens that
can combine with these self-reactive lymphocytes and eliminate them by producing apoptosis.
When self-tolerance fails, the body makes antibodies against its own components through T cell-
activated B lymphocytes. The bodies attack on its own cells leads to autoimmune diseases. The
antibodies produced in autoimmune diseases are specific against a particular antigen and are usually
restricted to a particular organ or tissue type.
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It is not known why self-tolerance suddenly fails but we have, however, learned that autoimmune
diseases often begin in association with an infection. One particular trigger for autoimmune diseases
is foreign antigens that are similar to human antigens. When the body makes antibodies to the
foreign antigen, those antibodies have enough cross-reactivity with human tissues to do some
damage.