Antigens, Immunogens , Epitopes, and Haptens

Innate and adaptive immunity:

Innate (Natural) Immunity:

Inborn resistance.present the first time a pathogen is encountered (does not require prior exposure)

Usually not specificnot modified by repeated exposure to the pathogen.

Adaptive (Acquired) Immunity:

weak or absent at the first exposure.

specific increases dramatically with subsequent exposures to the same pathogen (Memory)

•Types of innate immunity:Humoral (soluble)immunityCell mediated immunity

• Types of adaptive immunity Humoral (soluble)immunity Cell mediated immunity

Soluble mediators of innate immunity:

•Proteins and enzymes•Cytokines & type 1 interferons.•Complement

Proteins and enzymes

oA variety of proteins and enzymes (present in human blood and tissue fluids) are considered the first part of innate immunity.oExample: C reactive protein (CRP), lysozymes, and defensins.

nCytokines, and Type I interferons:

Cytokines: IL-1, IL-6, IL-12, and TNFα produced by activated phagocytes.

Type I interferons:

oIFN- α produced by dendritic cells

oIFN-β Produced by fibroblasts.

-INFs are rapidly produced, within minutes by

the infected cells (virus infection).

Cells of Innate immunity:

• Phagocytes:

- Monocytes and macrophages: blood

stream and tissues.

- Dendritic cells: lung, and GIT mucosa.

•Granular leukocytes:

-Neutrophils, and eosinophils.

-Basophils and mast cells.• Natural killer cells (NK).

Adaptive Immunity:

• Soluble mediators of adaptive immunity:

- Cytokines: IL-2, IL-3, IL-10,IL-13.

- Interferons: INF-γ (Gamma).

- Complement.• Soluble and cell-bounded proteins:

-The Immunoglobulins.

• Immune cells:

- T lymphocytes

- B lymphocytes

- N.K cells.

• n

Phagocytes: monocytes, MQ & DC

Granular leukocytes

Cytokines: IL1, IL6, IL12 & TNFα.

Interferons: IFα and IFβ.

T lymphocyte & B lymphocytes

Cytokines: IL2, IL3, IL10 & IL13.Interferons: IFγImmunoglobulins.

Complement

NK cell

n

Antigens, Immunogens , Epitopes, and Haptens

Antigen:

Is a molecule or part of a molecule that is recognized by the immune system.

Immunogen:

Is an antigen that evokes a specific, positive immune response.

Example:

Injecting a laboratory animal by human

chromosome Anti-DNA

antibodies and anti-Histons Antibodies

will be produced.

Immunogenicity and Properties of immunogen:

Immunogenicity is the ability of immunogen to activate immune response.

Properties of Immunogen:o Foreignnesso Sizeo chemical nature and complexity.

•Foreignness:

Immune system can discriminate between foreign and self substance (self & non self)•Size:

Molecular weight of at least 10,000 Dalton. •Chemical nature:

-Mainly proteins (polypeptide)

-Carbohydrates, steroids, and lipids are poor immunogens.

nChemical complexity:

As the chemical complexity increases; the

immunogenicity increases.

Homopolymers are poor immunogens

regardless of size, while heteropolymers

are strong immunogens.

Proteins are heteropolymers (different

amino acids)

n

Epitopes

(The Antigenic determinants): The smallest part of an antigen or immunogen that can be recognized by the B & T cells receptors. So it is the basic recognition element.

A single antigen molecule may have several different epitopes.

Immunodominant epitope is an epitope that can stimulate a greater antibody response than others.

• N

epitopes

•B cell receptors can recognize epitopes whether they are:

free soluble molecules. Surface-bound molecules. Degraded fragments.

•T cell receptors can bind only to epitopes which are surface-bound molecules.

Superantigen:

• is a substance such as a bacterial

toxin that is capable of stimulating

multiple T lymphocytes, leading to

release of relatively large quantities of

cytokines.

n

Hapten:

Small, non-biologic molecules that bind

to immune cells receptors but cannot

by themselves induce a specific

immune response.

(Land-Steiner’s experiment).

Injected a mice with:

• hapten No immune

response• carrier (protein) Anti carrier

•Hapten + Carrier Anti-Hapten +

Anti-Carrier

T-cell dependent and T-cell independent Antigens:

•Antigens are classified as either T-cell dependent or T-cell independent.

•T cell dependent antigen activates immune system depending on T-lymphocytes.

T-cell dependent antigen

T-cell independent Antigen

Structural properties Complex Simple

Chemical nature Proteins, glycoprotein, and lipoproteins

Polysaccharide; bacterial

Lipopolysaccharide.

Antibody - class induced

IgG, IgM, IgA, IgD, and IgE

Only IgM

Immunological Memory

yes No

Present in most Pathogens

yes No