Unit 2: Antigens and Antibodies

MMSC 415

Antigen (Ag)

any substance that has the ability to interact with a B-cell receptor (BCR), T-cell receptor (TCR), or antibody (Ab)

Immunogen (all are antigens)

substance capable of inducing an immune response

Hapten

a molecule that is too small alone to be immunogenic

When does Hapten elicits immune response

only when attached to a carrier molecule (e.g. protein)

What is an epitope

an antigenic determinant, the site on a complex antigenic molecule that combines with an antibody (Ab), B-cell receptor (BCR), or T cell receptor (TCR)

What receptors or molecules can an epitope bind to

An antibody (Ab), B-cell receptor (BCR), or T-cell receptor (TCR)

How small are epitopes

Epitopes are very small, typically consisting of 6-12 amino acids or monosaccharide molecules

Where are epitopes located on an antigen

They are specific sites on a larger, complex antigenic molecule where immune receptors or antibodies bind.

What does it mean when an antigen is multivalent

it carries two or more different epitopes or has multiple copies of the same epitope

What is an epitope

The specific part of an antigen that is recognized and bound by an antibody

Can an antigen have multiple copies of the same epitope

Yes, the same epitope can be repeated several times on one antigen

What is valency

The number of epitopes per molecule

What does a multivalent antigen contain

Multiple epitopes, which can be different epitopes or repeated copies of the same epitope

What is the relationship between an epitope and an antibody

an antibody recognizes and binds to a specific epitope on an antigen

Antigen binding

two parts interaction, Epitope and Paratope

Epitope on antigen

binds to Ab paratope

Paratope (antigen-binding site)

region on antibody that binds to an epitope (only to a specific epitope)

What is immunogenicity

The inherent ability of an immunogen to stimulate an immune response

what can immunogenicity result in

the formation of antibodies or activation of immune lymphocytes

What are examples of immune lymphocytes that can be activated

T cells

What is an immunogen

A substance that can stimulate an immune response

Antigenicity

capacity of Ag or hapten to bind to Ab BCR, or TCR (specific reactivity)

what are heterologous (Heterophile) antigens

Related antigens from another species

What can heterologous antigens stimulate

The production of antibodies

What can the antibodies produced by heterologous antigens react with

cells or tissues of other animals or plants

Physical nature of immunogens

• Foreignness

• Size/ degradability

• Complexity

Foreignness

Degree recognized as non-self, the greater the phylogenetic difference, the greater the immune response

Size/ Degradability

• Large antigens must be able to be processed

• Must be stable in vivo (sufficient retention time) to stimulate immune response

Complexity

• larger molecules are better immunogens

• must contain amino acid or monosaccharide sequence variation

Why are larger molecules better immunogens (antigens)

have more antigenic sites (multivalent), the more complex a molecule is the more effective it will be as an immunogen

Immunoglobulins

also known as antibodies and gamma (y) globulins

What are Immunoglobulins produced

produced by B cells (B lymphocytes, plasma cells)

Immunoglobulins Electrophoretic separation

most antibodies, but not all, are isolated in the gamma globulin fraction of serum

Proteins in electrophoresis

larger molecules to the right and smaller molecules to the left

Antibodies

produced in response to immunogenic stimulation

Five human classes (isotypes) of antibodies:

IgG, IgM, IgA, IgD, and IgE

Chemical structure of Immunoglobulins

• Monomer

• Two light chains

• Two heavy chains

Monomer

basic 4-polypeptide chain structure that resembles the letter Y

Two light chains (L chains)

k (kappa) & lambda

Two heavy chains (H chains)

gamma,

Variable (V) region: amino-terminal region (N-terminus)

• On Fab fragments (fragment antigen binding)

• Sequence differences on both chains VH and VL

Three small hypervariable regions:

HV1 (CDR1), HV2 (CDR2), & HV3 (CDR3)

Constant (C) region: carboxy-terminal region (C-terminus)

• Includes Fc fragment (fragment crystallizable)

• Biological activity, mediates effector functions

How many FAB does each monomer have

2 FAB

what is FAB made up

partial heavy chain and partial light chain

Fc fragment

consisting entirely on heavy chains

Ag binding sites

where antigens will bind to

light chain

found on the outside (in orange), identical on both sides

disulphide bonds

keep two heavy chains connected

hinge region

allows antibodies to flex + bend to bind with antigens

variable region

upper most domain consist of the 4 top blocks, made up of heavy + light chains

constant region

rest of the body, lost of Domain of heavy chain and some light chain

J chain (joining chain)

IgM and dimeric (meaning 2) IgA only

Secretory component:

IgA only

what is the process of Secretory component

wraps around the dimeric IgA to protect it from proteolytic enzymes

How does IgA stays

associates itself with mucus, embed or loosely bind to mucus so it’s not moved out or cleared by our mucus injection

J Chain functions

• used to secrete the antibodies into the mucosa

• help protect it when its in the mucosa

• very small power peptide usually just used for linking

IgA

• highest antibody, or the most frequent antibody in mucosal sites

• there to protect us in our respiratory tract, genital tract, and in our gut from pathogens

Determinants on Immunoglobulins

• Isotypes

• Allotypes

• Idiotypes

Isotypes

Five main types or classes and some subclasses

Allotypes

genetic variations in antibodies that differ between individuals of the same species

Idiotypes

Unique amino acid sequence of VH and VL domains

Classes (Isotypes) of Antibodies

• IgG

• IgM

• IgA

• IgD

• IgE

IgG

Most abundant (75%), longest half-life, monomer, best anamnestic (memory) Ab

IgG Activates complement

• IgG3 is most effective, then IgG1 & IgG2

• IgG4 does not activate complement

• C1q must cross-link two or more IgG

What is something only IgG can do

Crosses the placenta (at birth human babies haev IgG levels as high as their mothers

IgG - Very good neutralizing Ab

Neutralizes pathogens and particles so they are no longer infectious (e.g. viruses)

Opsonization in IgG

IgG1 and IgG3 have high affinity for FcR on phagocytes

What does the Fc portion binds to in IgG

Staphylococcus aureus

• protein A

• IgG1, IgG2, & IgG4

IgM

5-10% of total serum, a pentamer (highest possible valence of all antibodies)

Why is IgM generally confined to intravascular spaces

because of large size due to it being pentamer

J chain role in IgM

enhance transport across mucous membranes

IgM

First Ab produced, early after antigenic exposure (primary response), little anamnestic response

why is IgM better than IgG for activating and fixing complement pathway

a single IgM can activate C1 bonding to the pathogen causing the monomers to bind to it and their FC receptors stick up, only requiring one molecule

IgA

10-15%, monomers and dimers

Most predominant Ab in secretions IgA

tears, saliva, breast milk, intestinal secretions, genital secretions, and other mucous membranes

Secretory component IgA

protects against proteolytic enzymes

IgD

1%, found on the surface of b lymphocytes (as a BCR)

IgD functions in the activation of B Cells

exists as a monomer, as a receptor on a B cell (just like IgM)

When B cell is maturing early on what does it express

both a monomer of IgM and IgD on the surface of the B cell as a receptor

When IgD binds to a specific antigen what will it cause

a pathway to activate that b lymphocyte which will then allow it to mature into a plasma cell

IgE

0.01%, very potent activator of allergic reactions

Mast cells and Basophils have high affinity receptor for who

IgE

If exposed to an allergen

produce IgE antibodies against that allergen

IgE

does not disassociate once its bound to that epsilon receptor it will remain there for weeks to months time

Hydrogen bonding

hydrogen atom shared between two electronegative atoms

electrostatic forces

oppositely charged groups attract

Van der Waals forces

positive or negative charged regions of molecules

Hydrophobic bonds

Association of hydrophobic groups

Affinity

the strength of a single antigen-antibody interaction

how is affinity determined

by the summation of the attractive and repulsive forces described previously

Avidity

the strength with which a multivalent antibody binds a multivalent antigen

what is avidity

the accumulated strength of multiple interactions

Between IgM and IgG which one has greater avidity

IgM rather than IgG because it has 10 binding sites, it has a higher capacity for binding that makes it have higher avidity

Two antibodies with different antigenic specificities usually

have different amino acid sequences in their hypervariable regions

Monoclonal Antibody

rationale for use in diagnosis and treatment of disease

Hybridoma Technology Principle

Purification of mAb from heterogeneous mixture not feasible

Normal B cells

Expose to immunogen

Fuse B cells (plasma cells) with myeloma cells

creates a hybrid cell called a hybridoma

Uses of Monoclonal Antibodies (mAbs)

• labeled mAbs detect Ags in vitro

• mAb immunotherapy