Presentation6 Acquired Immunity

7/28/2019 Presentation6 Acquired Immunity

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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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Presentation6 Acquired Immunity

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