Antigen and Immunogens

What is an antigen (Ag)?

Any substance that can interact with a B-cell receptor, T-cell receptor, or antibody.

What is an immunogen?

A substance capable of inducing an immune response.

What is the relationship between antigens and immunogens?

All immunogens are antigens, but not all antigens are necessarily immunogens.

What is a hapten?

A molecule that is too small by itself to cause an immune response.

What does a hapten need to cause an immune response?

It must be attached to a larger molecule, such as a protein.

What are antibodies made against in a hapten-carrier complex?

The hapten-carrier adduct.

What is an epitope?

The specific part of an antigen that binds to an antibody, BCR, or TCR.

What is another name for an epitope?

An antigenic determinant.

How small are epitopes usually?

About 6–12 amino acids or monosaccharide molecules.

What does it mean when an antigen is multivalent?

It has two or more different epitopes or multiple copies of the same epitope.

What is valency?

The number of epitopes on a molecule.

What are the two parts involved in antigen-antibody binding?

The epitope and the paratope.

What is the epitope?

The part of the antigen that binds to the antibody.

What is a paratope?

The antigen-binding site on an antibody that binds to an epitope.

What does an epitope bind to?

A paratope on an antibody.

What is immunogenicity?

The ability of an immunogen to stimulate an immune response.

What can immunogenicity result in?

Formation of antibodies or activation of immune lymphocytes, such as T cells.

What is antigenicity?

The ability of an antigen or hapten to bind to an antibody, BCR, or TCR.

What is another way to think about antigenicity?

The ability to specifically react with an antibody, BCR, or TCR.

What is the difference between immunogenicity and antigenicity?

Immunogenicity means causing an immune response, while antigenicity means being able to bind to an antibody, BCR, or TCR.

What are heterologous antigens?

Related antigens that come from another species.

What can heterologous antigens stimulate?

Antibodies that react with cells or tissues of other animals or plants.

What are three important physical factors that affect how well something works as an immunogen?

Foreignness, size/degradability, and complexity.

What does foreignness mean?

The degree to which something is recognized as non-self.

How does phylogenetic difference affect the immune response?

The greater the phylogenetic difference, the greater the immune response.

Why does the size of an antigen matter?

Large antigens need to be able to be processed by phagocytic cells.

What does degradability mean in this context?

The antigen must be able to be processed or digested by phagocytic cells.

What does an antigen need to be able to do in the body to stimulate an immune response?

It must be stable enough to remain in the body for enough time to stimulate an immune response.

Are size and foreignness alone enough to make a good immunogen?

No

What size molecules are usually better immunogens?

Larger molecules, usually greater than 10 kDa.

Why are molecules with more antigenic sites better immunogens?

They can have multiple sites that stimulate an immune response.

How does complexity affect immunogenicity?

The more complex a molecule is, the more effective it tends to be as an immunogen.

What type of molecules are examples of complex immunogens?

Complex proteins.

What must a molecule contain to be a good immunogen in terms of sequence?

Variation in its amino acid or monosaccharide sequences.

How does cross-linking affect polysaccharide immunogenicity?

Cross-linking improves immunogenicity.

Are lipids strong or weak immunogens?

Generally weak immunogens.

Are nucleic acids strong or weak immunogens?

Generally weak immunogens.

Why are synthetic polymers generally poor immunogens?

They have only a few simple repeating units.

What is an example of something that can involve synthetic polymers?

Foreign body implants.