Immunology (Innate and adaptive immune systems) ANTIGENS (Ag)

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Immunology
Contents
o Immunology introduction
o Immune system
o Cells of Immune system
o T lymphocytes
o B lymphocytes
o Antigens
o Antibodies
o Complement system
o Cytokines
o Histocompatibility complex (MHC)
o Immune response
o Hypersesitivity
o Autoimmunity
o Immune deficiency, Immune deficiency diseases and
AIDS
o Monoclonal Antibodies (mAbs) Treatment for
Cancer Cells
o Nutritional Aspects
◘ Immunology
Is the branch of biomedical science that deals with the response of an
organism to antigenic challenge and its recognition of what is self and
what is not.
It deals with the defense mechanisms including all physical, chemical
and biological properties of the organism that help it to combat its
susceptibility to foreign organisms, material, etc.
• The word immunity was derived from the Lain word (immunis) meaning
to be free.
• Now, in medical terms, it denotes resistance to reinfection /free of disease.
Lec. 1 & 2

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♦ Some definitions:
Immunity • Is the capability of multicellular organisms to resist harmful
microorganisms from entering it.
- Immunity involves both specific and nonspecific
components.
The immune
system
• Is a complex defense system, physiological function is to:
a- Prevent infections
b- Eradicate established infection and
c- Self/Nonself discrimination.
• Is the collection of cells, tissues and molecules that mediate
resistance to infection.
Immune
response
• Is the coordinated reaction of these cells and molecules to
infectious microbes.
Immunology • Is the study of the immune system and its response to invading
pathogens.
Antigens • Substances recognized by the cells and molecules of the
immune system and to which the system responds
Innate and adaptive immune systems
• The immune system is divided into those which are:
- Innate or static to the organism, and those which are responsive
- Or Adaptive to a potential pathogen or foreign substance.

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I-The innate system of immunity (natural, native, non specific immunity)
• is the first line of defense present in healthy individuals.
a- It is non- specific,
b- The resistance is static (it does not improve with repeated exposure),
c- There is no memory on subsequent exposures.
• Innate (natural) immunity includes soluble chemicals
- e.g. complement, fibronectin, interferons and cells (NK cells, PMN,
Monocytes, Macrophages and Eosinophils).
Barriers of the innate immune system
Types Function
Physical/ anatomical
barriers
Tight junctions In the epidermal skin layer:
water-proof and blocks UV
Subcutaneous glands In the dermis: contain fatty
acids maintaining a pH of 3-5
Mucous membranes Traps microbes
Mechanical removal Mucus In the respiratory tract:
microorganism trapping and
attachment prevention by the
mucus
Cilia Cilia propels the mucus and
trapped microbes towards the
sites of removal
Cough and sneeze reflex Respiratory tract: removal of
microorganisms from the
body
Vomiting and diarrhea GI tract: removal of toxins
and pathogens from the GI
tract
Flushing of body fluids Systemic: fluids such as tears,
urine, saliva and sweat also
flush microbes from the body

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♦ The innate immune response
• When tissue damage occurs as a result of trauma or infection, a complex
series of cellular and biochemical events occur, which are designed to:
a) Limit the spread of infection,
b) Limit the degree of tissue damage,
c) Eliminate any microorganisms
d) Repair the damaged tissue.
Types Function
Physiological barriers Temperature Normal body temperature
slows down the growth of
some pathogens
pH Low stomach, skin and
vaginal pH inhibit
microbial growth
Chemical barriers Lysozymes Bactericidal enzyme
secreted by the cells, found
in tears &at the mucosal
surfaces
Lactoperoxidase Mucosal secretion that
stimulates cells to produce
toxic radicals
Cryptidins and α-defensins Base of crypt cells in the
small intestine: damage cell
membranes
β-defensins Produced within the skin,
respiratory tract: damage
cell membranes
Surfactant proteins A and
D
Present in lungs: function
as opsonins, enhancing the
phagocytic activity of cells.

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Innate immune response and inflammation
 These activities are limited to the innate immune response,
- i.e. they do not require specific identification of the participating
organism but often a successful outcome requires the combined activity
of both the innate and specific immune responses.
 If microbes do breach epithelia and enter the tissues or circulation, they
are attacked by phagocytes, specialized lymphocytes, natural killer (NK)
cells and several plasma proteins (complement system).
 It has memory and is called the adaptive or specific or acquired
immunity.
II- The adaptive system (specific, acquired immunity)
• Is the second line of defense.
1-It is activated once the innate system has been overwhelmed.
2-It is specific to the infective or non-self antigens.
3-It can store the information about the invader as memory to show an
enhanced response to subsequent challenge (it has memory).
4- It develops more slowly and mediates, more effective, defense against
infection.

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• It is often sub-divided into two major types depending on how the
immunity was introduced:
a) Naturally acquired immunity occurs through contact with infection
(a disease causing agent), when the contact was not deliberate.
b) Artificially acquired immunity develops only through actions such as
vaccination.
♣ Both naturally and artificially acquired immunity can be further subdivided into:
i- Active acquired immunity is induced in the host itself by antigen and lasts much
longer, sometimes for lifelong (infection or vaccine).
ii- Passive acquired immunity is acquired through transfer of antibodies or activated
T-cells from an immune host, and is short lived, usually lasting only a few months,

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• Cells and molecules of the innate and adaptive immune responses work as an
integrated host defense system to eliminate infectious agent and provide long lasting
protective immunity.
♣ Adaptive immunity includes:
1- Humoral (antibodies produced by B lymphocytes).
2- Cellular reactions by T cells (they also produce cytokines).

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◘ The origin and the cells of the immune system
♣ The cells involved in the immune reactions:
1- Lymphocytes:
 There are many types; important in both humoral and cell-mediated
immunity.
 B cells produce antibodies (humoral response), while T cells are sub-
grouped into cytotoxic T cells (Tc), helper T cells (Th1,Th2),
suppressor T cells (Ts) and memory cells.
2- Null cells (Natural Killer cells NK and Killer cells K ells):
 NK cells are non phagocytic and non adherent to surfaces.
 They are programmed to kill foreign,
tumor cells and cells infected with certain
viruses by releasing perforin which forms
pores in the cell membrane of the target
cell through which granzymes and other
associated molecules can enter inducing
apoptosis.
 Granzymes are serine proteases that are released by cytotoxic T cells
and natural killer (NK) cells.
 They induce programmed cell death in the target cell, thus eliminating
cells that have become cancerous or are infected with viruses or
bacteria.
 NK cells are Ab dependent and have receptors for the Fc region of Ab.
They are involved in both innate and adaptive immune reactions and
antigen presentation.

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♦ Granukocytes
1-Neutrophils:
 They are abundant type of granulocytes found in peripheral blood.
 They are motile and phagocytosing cells.
 They are very important at clearing bacterial infections.
 They have short life span (hours) and are part of Innate Immunity.
2-Eosinophils:
 Their function is extravasations and phagocytosis during inflammation.
 Kills Ab-coated parasites (worms) through degranulation and are
involved in allergic inflammation reactions, their granules contain a
unique toxic protein called cathepsin.
 They regulate functions of other immune cells (CD4+
T-cells, dendritic
cells, B-cells neutrophils etc.).

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3-Basophils:
 They are non phagocytic, their granules contain histamine,
sertonine, heparine, peroxidase etc.
 They are toxic and inflammatory compounds.
 They might be blood Mast cells’, a cell-killing cells and contain and
they are important in allergic reactions.
4-Accessory cells
1-Mast cells:
 They are found in most of the organs.
 They are absent in circulation.
 They cause expulsion of parasites through release of granules,
histamine, leukotrienes, chemokines, cytokines and are also involved in
allergic responses as they have high affinity receptors to antibodies of
type IgE.
3-Monocytes and macrophages (Mϕ):
 Monocytes are the largest mononucleated young Mϕ in blood.
 They are antigen presenting cells (APC), phagocytosing and killing of
microorganisms, activation of T cells and initiation of immune
response.
 There are tissue-specific macrophages e.g. Liver- Kuffer cells,
connective tissue-Histocytes, Bone-Osteoclasts etc.
 They cause activation of T cells and initiation of immune response.
4-Plasma cell:
 (in tissue) are fully differentiated to B
cells, which is the only cell that secretes
antibodies (Ab).

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I-Function of phagocytic cells:
 Phagocytes of humans and other animals are called "professional" or
"non-professional" depending on how effective they are at
phagocytosis.
 The main difference between professional and non-professional
phagocytes is that the professional phagocytes have molecules called
receptors on their surfaces that can detect harmful objects, such as
bacteria.
 The professional phagocytes include cells called neutrophils,
monocytes, macrophages (Mϕ), dendritic cells, and mast cells.
 Phagocytes function to take up pathogens, foreign materials and cell
debris and break it down.
 This function is known as phagocytosis.
 Phagocytosis is the critical function of phagocytes that is either
performed within the phagocyte (intracellular killing) or outside of the
phagocyte (extracellular killing).

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◘ 1-Intracellular killing:
A - Oxygen-dependent intracellular killing,
B - Oxygen-independent intracellular killing.
 When a phagocyte ingests bacteria (or any material), its oxygen
consumption increases (respiratory burst), produces reactive oxygen-
containing molecules that are toxic to both the invader and the cell
itself, so they are kept in compartments inside the cell.
 This method oxygen-dependent intracellular killing, of which there
are two types.

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1- Oxygen-dependent
 Production of a superoxide, which is converted to hydrogen peroxide
(H2O2) and singlet oxygen by an enzyme called superoxide dismutase
(SOD).
 Superoxides also react with the hydrogen peroxide to produce
hydroxyl radicals which assist in killing the invading microbe.
 In the sequential univalent process by which O2 undergoes reduction,
several reactive intermediates are formed, such as superoxide (O2
–
),
hydrogen peroxide (H2O2), and the extremely reactive hydroxyl radical
(OH.): collectively termed as the reactive oxygen species (ROS)as in
the equation.
♣The use of the enzyme myeloperoxidase from neutrophil granules.
 When granules fuse with a phagosome, this enzyme is released into the
phagolysosome, and this enzyme uses H2O2 and chlorine to create
hypochlorite which is extremely toxic to bacteria.
2- Oxygen-independent intracellular killing
 There are three main types.
 It is not as effective as the
oxygen-dependent ones.
1- Using electrically charged
proteins that damage the bacterial
membranes.
2- Using lysozymes to break down
the bacterial cell wall.
3- Using lactoferrins, to remove
essential iron from bacteria

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◘ 2-Extracellular killing:
 Interferon-γ can be produced by CD4+ Th cells, CD8+ Th cells, NK
cells, B cells, other T cells, monocytes, Mϕ, or dendritic cells stimulates
Mϕ to produce toxic nitric oxide (NO) which is then released, kills
microbes near the macrophage.
 Activated macrophages produce and secrete tumor necrosis factor
(TNF) which kills cancer cells and cells infected by viruses.
3-Viruses
 Mϕ and other components of the innate immune system can, to a
limited extent, control viruses, once a virus is inside a cell, the adaptive
immune responses particularly the lymphocytes, are more important for
defense.
 Virus-infected cells that have been killed by lymphocytes e.g. NK are
cleared from the body by phagocytes.
4-Antigen presentation
 Antigen presentation (by Mϕ macrophages and
dendritic cells) is a process in which some
phagocytes move parts (peptides) of engulfed
materials back to the surface of their cells and
"present" them to other cells of the immune
system.
II-Function of B cells
 Production of both specific and non specific antibodies (Abs).

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III-Functions of T cells:
♣ T cells have many subpopulations they
act to:
1- The subpopulation cells T-cells called
T-helper 2 (Th2) cooperate with B-cells
to stimulate antibodies production.
2- Inflammatory effects:
The subpopulation cells called
T-helper 1 (Th1), on activation, they induce
the activation of monocytes and macrophages
leading to the so-called delayed-type
hypersensitivity reactions.
3- Cytotoxic effect:
 The T cells in this subset
become cytotoxic killer cells.
 These cells are called T
cytotoxic cells (Tc).
4- Regulatory effect:
 Some T cells are able to
suppress the immune response leading to
downward modulation, or a shutoff in the
reactivity of other cells.
 These cells are called T-suppresser cells
(Ts).
5- Signal via cytokines:
• Cytokines are soluble mediators released by cells (e.g. Th1 and Th2).

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◘ CD (cluster of differentiation):-
 The label CD was coined to define surface molecules that are
recognized by a given set of monoclonal antibodies.
 They receive a number e.g. CD markers on leukocytes e.g.

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Comparison between innate and acquired immunity
Innate Adaptive
Present at birth Acquired response to antigens
Rapid 0-6 hours Slow initiation (days), rapid thereafter
Leukocytes involved:
Polymorphnucleus, monocytes,
macrophages, eosinophils, NK cells.
Specific B cells, specific T cells
Mechanical barriers:
Skin and mucous membranes and cells Skin and mucosal immune systems
Soluble molecules:
Complement(C) Specific antibodies (Ab)
Cells involved:
Phagocytes (macrophages, neutrophils)
and
natural killer (NK) cells
Both B and T lymphocytes
Mediators are:
Macrophage-derived cytokines (e.g.
interferon α- and β-) and tumor necrosis
factor (TNF)
Lymphocte-derived cytokines (e.g.
interferon γ)
ANTIGENS (Ag)
• Historically antigen named as antibody generators molecule which
stimulates production of and binds specifically to an antibody.
• Antigens are classified into immunogens and haptens.
1-Hapten:
 Molecule which can bind to specific
antibody but cannot elicit adaptive
immune response.(small MW)
2-Immunogen:
 Molecule which can stimulate
adaptive immune response.
 Best immunogens are proteins with
MW > 10,000.

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♣ General properties of Antigens:
1- Foreignness:
 any substance (including self antigens e.g., cornea, the eye lens, sperm cells, and
skin that does not come in contact with the immune system before the system
reaches functional maturity would be recognized as antigenic)
2- Molecular weight:
 small molecules (M<1,000Da).
3- Chemical composition:
 All major classes of biological compounds (lipids, CHO, nucleic acids and
proteins) can act as antigens.
 Homo-polymers of a single amino acid are poor immunogens.
 Only protein antigens are recognized by T-cells and thus induce T-helper cell.
 High complexity increase immunogenicity.
4- Physical form
 In general, particulate antigens are more immunogenic than soluble antigens,
and denatured antigens are more immunogenic than native ones.
5- Degradability
 Antigens that are easily phagocytosed are generally more immunogenic.
6- Nativity:
 A native antigen is an antigen that is not yet processed by an APC to smaller
parts.
 T-cells cannot bind native antigens, but require their processing by APCs,
whereas B-cells can be activated by native ones.

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A hapten is a small molecule that can
elicit an immune response only when
attached to a large carrier such as a
protein (a hapten-carrier).
Once the body has generated antibodies
to the small-molecule, hapten may be
able to bind to the antibody.
Examples of haptens, o-, m- and p-
aminobenzoic acid
Some haptens can induce autoimmune
disease e.g. penicillin-class drugs cause
autoimmune hemolytic anemia.
◘ Some immunological definitions:-
a- An antigen
 Is a foreign molecule that, when introduced into the body, triggers the
production of antibodies by the immune system.
 The immune system will then kill or neutralize the antigen that is
recognized as a foreign invader.
 "Self" antigens are usually tolerated by the immune system; whereas
"non-self" antigens are identified as invaders and attacked by the
immune system.
b-An immunogen
 Is a specific type of antigen which is able to induce an immune
response.
 Whereas an antigen is able to combine with the products of an immune
response once they are made.
 The concepts of immunogenicity and antigenicity are different.
c- Immunogenicity
 Is the ability of Ag to induce a humoral and/or cell-mediated immune
response.

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d- Antigenicity
 Is the ability of Ag to combine specifically with the final products of the
immune response (i.e. secreted Abs and/or surface receptors on T-cells).
♣ All molecules that have the property of immunogenicity also have the
property of antigenicity, the reverse is not true.
e- Antigenic specificity
 It is the ability of the host cells to recognize an antigen specifically as a
unique molecular entity and distinguish it from another with exquisite
precision.
An antigen (epitope) is characterized by its ability
to be "bound" at the antigen-binding site (paratope)
of an antibody.
Antibodies tend to discriminate between the specific molecular structures
presented on the surface of the antigen.
Antigens are usually proteins OR polysaccharides.
Lipids and nucleic acids are antigenic only when combined with proteins
and polysaccharides.
♣ Microbial antigens include parts (coats, capsules, cell walls, flagella,
fimbrae, and toxins) of bacteria, viruses, and different microorganisms.
♣ Non-microbial exogenous antigens include pollen, egg white and
proteins from transplanted tissues and organs or on the surface of transfused
blood cells.
♣ Vaccines are examples of immunogenic antigens intentionally
administered to induce acquired immunity in the recipient.

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◘ Origin of Antigens
 Antigens can be classified in order of their class into:
1- Exogenous antigens,
 Are antigens that have entered the body from the outside, for example
by inhalation, ingestion, skin contact or injection.
 By endocytosis or phagocytosis exogenous antigens are taken into the
antigen-presenting cells (APCs) and processed into fragments.
 APCs then present the fragments to T-helper cells (CD4+) by the use of
class II major histocompatibility complex (MHC) molecules on APC
surface.
 Some T-cells are specific for such peptide.
 These T-cells become activated and start to secrete cytokines.
♣ Cytokines are substances that can activate cytotoxic T-lymphocytes (CTL,
Tc), antibody-secreting B-cells, macrophages, and others.
♣ Some antigens (e.g. intracellular viruses) start out as exogenous
antigens, and later become endogenous where viruses can be released back
into circulation upon the destruction of the infected cell.
i- Bacterial antigens:

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A - Antigens related to bacterial cells;
i- Somatic antigen (O): part of cell wall of Gram –ve bacteria.
ii- Capsular antigen: usually polysaccharide.
iii- Flagellar Ag (H): a protein made of flagellin.
iv- Fimbrial Ag: surface antigens in fimbriated bacilli.
v-Lipopolysaccharides (LPS).
B - Antigen secreted by bacteria:
- Exotoxins.
- Enzymes.
C - Antigens related to viruses:
- protein coat viral antigens.
- Soluble antigens (soluble nucleoproteins as in influenza).
2- Endogenous antigens,
 Are antigens that have been generated within previously-normal cells as a result
of normal cell metabolism, or because of viral or intracellular bacterial infection.
 The fragments are then presented on the cell surface in the complex with peptide
of major histocompatibility complex MHC class I molecules.
♦ Endogenous antigens include: autologous, syngeneic, allogenic (homologous)
and xenogenic (heterologous) antigens.
a- Autologous
 are found within the same individual (e.g. a skin graft from an individual’s thigh
to his chest); that is, they are not foreign.
b -Syngeneic
 are found in genetically identical individuals (e.g. identical twins); so, they are
not foreign.
c-Allogeneic (alloantigens)
 are found in genetically dissimilar members of the same species (e.g. a kidney
transplant from mother to daughter); it is foreign.
d-Xenogeneic (heterogeneic)
 are found in different species (e.g. a transplant of monkey kidneys to human); it
is foreign.

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◘ Human tissue antigens:
I- Blood group antigens: A, B and Rh antigens on red blood cells.
II- Histocompatibility antigens:
 They are glycoprotein molecules on all nucleated cells,
- Major histocompatibility complex antigens (MHC class I and II).
- Human leukocyte antigen (HLA).
III- Autoantigens
 An autoantigen is usually a normal protein or complex of proteins
sometimes DNA or RNA that is recognized by the immune system of
patients suffering from a specific autoimmune disease.
IV- Tumor antigens
 Tumor antigens or neoantigens are those antigens that are presented by
MHC class I or class II molecules on the surface of tumor cells and
never by the normal ones.
 In this case, they are called tumor-specific antigens (TSAs) as a result
from a tumor-specific mutation.
 More common are antigens that are presented by tumor cells and
normal cells, and they are called tumor-associated antigens (TAAs).
 Tumor antigens can also be on the surface of the tumor in the form of a
mutated receptor.

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♦ T-independent antigens:- Polysaccharides, lipids & nucleic acids
♣ Properties
• Polymeric structure.
• Polyclonal B cell activation.
• Repetition of the same antigenic determinant.
• Resistance to degradation
♣ Examples
o Pneumococcal polysaccharide, lipopolysaccharide (LPS)
o Flagella.
♦ T-dependent antigens:- Proteins
♣ Examples
o Microbial proteins.
o Non-self or Altered-self proteins.
o Few copies of different antigenic determinants.
o They do not stimulate Ab production without the help of T cells.
♦ Hapten-carrier conjugates
 It contains both native determinants and haptenic determinants.
♣ Super-antigens (Sags)
- They are a class of antigens that cause non-specific activation of
T-cells resulting in polyclonal T-cell activation and massive cytokine
release.
- SAgs are produced by some pathogenic viruses and bacteria as a
defense mechanism against the immune system.

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♣ Examples, Bacterial antigens
- Staphylococcal enterotoxins,
- Staphylococcal toxic shock toxin,
- Staphylococcal exfoliating toxin,
- Streptococcal pyrogenic exotoxins.
◘ Adjuvant (Latin. adjuvare = to help)
 Adjuvant is a substances that, when mixed with antigens, enhance the
antibody response to antigen itself.
♦ Adjuvant differs considerably in terms of composition and mechanism of
action and include:
i- Aluminium salts
 Are used extensively as an adjuvant.
 They enhance the immune repines by increasing antigen persistence and
by recruiting antigen-presenting cells.
ii- Emulsions:
a- Freund's incomplete mineral oil
 Which typically increase antigen persistence and recruit macrophages to
the site of injection.
b- Freund's complete adjuvant
 Is Freund's incomplete adjuvant mixed with a suspension of killed
Tubercle bacilli.
iii-Liposomes.
 A liposome is an artificially-prepared vesicle composed of a lipid
bilayer that can be used as a vehicle.
 They have been used to encapsulate antigens and enhance antigen
presentation.
♣ B-cell non-specific mitogens
 Such as Bortidella pertussis and bacterial lipopolysaccharide (LPS),
phytohemaglutinin (PHA) act as non-specific B-cell mitogens.
 A mitogen is a chemical substance that encourages a cell to commence cell
division, triggering mitosis.
 A mitogen is usually some form of a protein.
 Mitogenesis is the induction (triggering) of mitosis, typically via a mitogen.

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◘ Factors Influencing Immunogenicity
♣ Contribution of the Biological System
1. Genetic factors:
 Some substances are immunogenic in one species but not in another.
 Some substances are immunogenic in one individual but not in others.
 The species or individuals may lack or have altered genes that code for
the receptors for antigen on B- or T- cells or they may not have the
appropriate genes needed for the APC to present antigen to the helper
T-cells.
2. Age:
 Age can influence immunogenicity.
 Very young and very old individuals have a diminished ability to mount
and immune response to an immunogen.
♦ Methods of administration:
1. Dose:
 There is a dose of antigen above or below which the immune response
will not be optimum.
2. Route:
 Subcutaneous route is better than intravenous or intra-gastric.
3. Adjuvant:
 Substances that can enhance the immune response to an antigen.
 They, sometimes, have undesirable side effects e.g. fever, inflammation.