LEC 3 - IMMUNOGENS, ANTIGENS & VACCINES

What is an antigen?

A substance that can combine or be recognized by an antibody

What is an immunogen?

A substance capable of inducing an immune response

What is antigenicity (specific reactivity)?

The ability of an antigen to react specifically with the antibodies or cells it provoked

What is immunogenicity?

The ability to provoke an immune response by stimulating antibody production, T cell proliferation, or both

What is an epitope?

The part of an antigen that reacts specifically with an antibody or T-lymphocyte receptor

What is another name for an epitope?

Antigenic determinant

What does the epitope dictate?

The shape of the antibody

What are the two types of epitopes?

Linear epitope and conformational epitope

What is a linear epitope?

A sequence of amino acids on a single polypeptide chain

What is a conformational epitope?

Formed by the folding of one or more polypeptide chains that bring distant amino acids together

What does "foreignness" mean in immunogenicity?

It is the degree to which antigenic determinants are recognized as nonself by the immune system

What molecular weight is too small to trigger antibody production?

Less than 5,000 Daltons

What molecular weight makes a potential antigen?

Greater than 10,000 Daltons

What molecular weight makes a good immunogen (example: albumin)?

Around 40,000 Daltons

What molecular weight makes an excellent immunogen (example: hemocyanin)?

Around 1 million Daltons

Which macromolecule is the most immunogenic?

Proteins (high MW and complex structure)

Which macromolecule is second in immunogenicity?

Polysaccharides (smaller and rapidly degraded)

Which macromolecule is the least immunogenic?

Lipids (low MW and simple structure)

Can nucleic acids be immunogenic?

Yes, especially single-stranded forms with molecular flexibility

What are common routes of antigen entry?

Intravenous, intradermal, subcutaneous, intramuscular, and oral

What is oral tolerance?

When antigens entering through the GI tract are ignored by the immune system

What happens if the antigen dose is too low?

It may not provide enough stimulus

What happens if the antigen dose is too high?

It may cause excessive inflammation or tolerance

Describe the dose-response relationship.

Immune response increases with antigen dose up to a certain point

What is the primary immune response?

The initial reaction to an antigen, producing antibodies and immune cells

What is the secondary immune response?

A faster and stronger reaction upon re-exposure due to memory cells

What is the "window of opportunity"?

The time when the immune response is most effective

What does degradability of an immunogen mean?

Its ability to be broken down into smaller fragments for presentation by APCs

What are adjuvants?

Substances added to vaccines to boost the immune response

What are other names for adjuvants?

Antigen delivery systems or immunopotentiators

Give examples of adjuvants.

Aluminum salts, oil-in-water emulsions, and microparticles

What does aluminum hydroxide do as an adjuvant?

Promotes antigen intake; most commonly used in vaccines

What do oil-in-water emulsions (like squalene) do?

Promote antigen intake and migration of APCs

What are virosomes?

Virus-like microparticles without genetic material used as adjuvants

What is a hapten?

A small molecule that cannot elicit an immune response on its own

What is a carrier?

A large molecule, usually a protein, that can elicit an immune response

What is a complete antigen?

A substance that can both stimulate antibody synthesis and react with the antibody

What is an incomplete antigen (hapten)?

A substance that cannot stimulate an immune response alone but can react with an antibody

What is an autologous antigen?

Also called an autoantigen; found within the same individual and not foreign

What is a sequestered antigen?

An autologous antigen hidden from antibody-producing cells, making it inaccessible to immune tissues

What is a tissue-specific antigen?

A protein or molecule mainly expressed in specific tissues or organs

What is a syngeneic antigen?

An antigen found in genetically identical individuals (e.g., identical twins or inbred strains)

What is an allogeneic (homologous) antigen?

Also called an alloantigen; found on cells of the same species but with different genetic backgrounds

What do alloantigens form in response?

Alloantibodies

What is a xenogeneic (heterologous) antigen?

Also called a heteroantigen; present in cells or tissues of a different species and can trigger an immune response if introduced into humans

What does the prefix "xeno-" mean?

Foreign or from another species

What is a superantigen?

A microbial protein (often a toxin) that causes massive, non-specific T-cell activation

What can superantigens lead to?

An exaggerated immune response and cytokine storm causing self-damage

What are the two main types of vaccines based on immunity?

Active immunization and passive immunization

What happens in active immunization?

The body produces its own antibodies after exposure to an antigen

What is natural active immunization?

When a person contracts a disease and naturally forms antibodies

What is artificial active immunization?

When a person is vaccinated with an antigen to produce immunity

What happens in passive immunization?

Antibodies are transferred to provide immediate protection

What is natural passive immunization?

Transfer of maternal antibodies to a baby through the placenta or breast milk

What is artificial passive immunization?

Injection of pre-formed antibodies into the body

What are the main types of active vaccines?

Live attenuated, inactivated (killed), and subunit vaccines

What are live attenuated vaccines?

Vaccines made from live bacteria or viruses that have been weakened

Who should avoid live attenuated vaccines?

Immunocompromised individuals and pregnant women

What are inactivated (killed) vaccines?

Vaccines made from pathogens killed by heat or chemicals but still antigenic

Why do inactivated vaccines require larger antigen doses?

To stimulate an effective immune response since the pathogen is not live

What do subunit vaccines contain?

Specific purified parts of the pathogen such as proteins, polysaccharides, or toxoids

What are toxoid vaccines made from?

Chemically inactivated bacterial exotoxins

What do polysaccharide vaccines target?

The virulence factors of encapsulated bacteria

What are purified protein vaccines composed of?

Proteins derived from a pathogen

What are recombinant protein vaccines?

Highly purified protein vaccines produced through gene coding of the protein