presentation6 acquired immunity
TRANSCRIPT
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Dr. A. A. Wegdan
Acquired Immunity
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Acquired Immunity
Definition
- Acquired immunity is a specific immunity
occurring in certain individuals after exposure
to a particular antigen
Types:
- Humoral (i.e. due to production of antibodies)
- Cell mediated (i.e. due to activation of Tlymphocytes).
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Antigen (Immunogen)
An immunogen is a substance that can induce
an immune response and reacts specifically with
the components of this response
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Hapten
It is a low molecular weight substance which is
non immunogenic by itself, but if it is carried on a
large molecule i.e. if it is bound to a protein
carrier, it becomes immunogenic. e.g.polysaccharide capsule of pneumococci, drugs,
aspirin.
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Epitopes
These are small chemical groupings (aminoacids or oligosaccharides) present on thesurface or within the antigen molecules.
These epitopes are responsible for thespecificity of the immune response because theyare the sites with which the antibodies combine.
Epitopes are usually multiple and the valency of
the antigen equals the number of its epitopes. Removal or addition of epitopes may alter the
antigen
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Immunogenicity
1- Foreignness: In general, molecules recognizedas self are not immunogenic. To be immunogenic,molecules must be recognized as non-self i.e. foreign
2 - Molecular size: The most potent 2 are proteinswith high molecular weights i.e. above 100,000.Generally, molecules with molecular weight below10,000 are weakly immunogenic e.g. amino acids.Substances of low molecular weight may act as
haptens.3 - Chemical-structural complexity:A certain
amount of chemical complexity is required. Proteins
are more immunogenic than carbohydrates and lipids
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Immunogenicity
4- Dose of the antigen:Within an appropriate dose, optimumimmunogenicity is obtained. Extremely low dose results in a state oflow-zone tolerance and extremely high dose results in a state ofhigh-zone tolerance.
5- Route of administration:Antigens administered by S.C., I.M.,
I.V. or by inhalation are more immunogenic than those administeredorally.
6- Adjuvants: These are substances which can enhanceimmunogenicity of antigens without altering their chemical nature,for example,
Mineral gels e.g. aluminium hydroxide. Bacterial products e.g. endotoxin, killed tubercle bacilli, killed
Bordetella pertussis.
Oil in water emulsion
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Common Types of Antigen
1 - Bacterial antigense.g. cell wall (somatic or O) antigens, flagellar(H) antigens, capsular antigens, fimbrial antigens, virulence (Vi) antigens,intracellular antigens e.g. nucleoproteins and extracellular antigens such asbacterial toxins.
2- Other microbial antigenssuch as viral, fungal and parasiticantigens.
3- Tissue antigens e.g. Blood group antigens (ABO antigens and Rh antigens).
Major histocompatibility complex (MHC) antigens or human, leucocyteantigens (HLA): These are glycoprotein antigens classified into two classes;
4- Drugs:Some drugs acts as antigens while others can act as haptens.
5 - Food antigenssuch as proteins of milk, eggs and other foods.
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Classes of Antigens
1- T-dependent Antigens These are antigens require the aid of T-helper cells to
stimulate B-lymphocytes to produce the antibody.
These are called T-dependent antigens, e.g. complexproteins (serum proteins).
2- T-independent Antigens Some antigens can elicit a humoral response without the
cooperation of T-cells. These are called T-independent antigens, e.g.
lipopolysaccharides, polymers of D-amino acids,dextran.
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THE HUMORAL IMMUNE RESPONSE
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Humoral Immune Responce
FUNCTIONS
Destruction of extracellular pathogens
Prevention of spread of intracellular pathogens
How?
By: ANTIBODIES (IMMUNOGLOBULINS
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Resting nave B cells Plasma cells Antibodies
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Immunoglobulins
Glycoproteins
Bind specifically to antigens that induced their
formation
Present in gamma globulin fraction of plasmaproteins
Present in extravascular compartment
Five classes: IgG IgA IgM IgE IgD Subclasses also present: IgG1, IgG2, IgG3,
IgG4
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Functions of Antibodies
1. Agglutination
2. Neutralization
3. Opsonization4. Complement activation
5. Antibody-dependent-cell-mediated
cytotoxicity (ADCC)
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Immune Response
Primary Immune Response
Long induction period
Low antibody level
Short duration Predominantly IgM
No memory cells were
present
Secondary Immune Response
Short induction period
High antibody level
Long duration Predominantly IgG
Memory cells were present
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T-Cell Mediated Immunity
Main functions of T cells :
Destruction of Intracellular pathogens
Helping other cells of the immune system
T cells cannot recognize antigen directly
It must be presented to them by special cells
called: Antigen Presenting Cells (APCs)
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