Immunosero Mod 2.1

Antigen

is a substance that is specifically recognized by the adaptive immune system

Immunogen

is a substance capable of causing an adaptive response

1. Macromolecular size

2. Foreignness

3. Chemical composition and molecular complexity

4. Ability to be processed and presented within MHC (Major Histocompatibility Complex) molecules

CHARACTERISTICS OF ANTIGENS & IMMUNOGENS

Macromolecular size

An immunogen must have a MW of at least 10,000 daltons (Da

Foreigness

• Un-relatedness to the host.

• The more taxonomically distant an antigen’s source is from the host, the greater the likelihood that host lymphocytes will react to it

• Plant protein is a better immunogen for an animal than is material from a related animal

• Exception: autoantibodies

Chemical composition and molecular complexity

• Proteins and polysaccharides are the most effective immunogens. • Synthetic polymers (nylon or Teflon) are nonimmunogenic. Used in making artificial heart valves, elbow replacements and other medical devices • Proteins are highly immunogenic compared to polysaccharides (smaller size and T cells do not recognize carbohydrates), nucleic acids and lipids

Ability to be processed and presented within MHC molecules

Must be subjected to antigen processing and can be complexed to MHC molecules for presentation to responsive T lymphocytes

Epitopes or antigenic determinant

are precise molecular shapes or configurations recognized by B cells, or the peptide sequences detected by T cells. The portion of a molecule(i.e. antigen) that binds to an antibody or T receptor

Linear (continuous/sequential)

Conformational (discontinuous) epitopes

B cells recognize 2 types of epitopes:

Haptens

is a substance that is nonimmunogenic by itself but is able to form new antigenic determinants when combined with a larger carrier molecule

Cathecol
Penicillin

Examples of haptens

Cathecols

from poison ivy plant. Upon contact with the skin, admixed with tissue proteins to form the immunogens resulting in contact dermatitis

Penicillin

drug-protein conjugates. Coupled with normal proteins in the body to provoke an immune response

Adjuvants

are substances delivered simultaneously with antigen for the purpose of potentiating, or enhancing, the immune response. Prevent antigen from diffusing away from the site of inoculation, which allows innate immune cells to accumulate in the site

Vaccines

Uses of Adjuvants

Vaccines

that consist of recombinant proteins antigens and NOT for vaccines that composed of live or killed pathogens

Aluminum gels or aluminum salts

Freund’s complete adjuvant

Lipopolysaccharide (LPS)

Squalene (MF59)

Examples of Adjuvants

Aluminum gels or aluminum salts

– only vaccine adjuvants currently licensed for use in the US (CDC); stimulates T-cells

Freund’s complete adjuvant

consists of mineral oil, emulsifier, and killed bacteria (M. butyricum, MTB, B. pertussis)

Lipopolysaccharide (LPS)

stimulates B-cells

Squalene (MF59)

from shark oil; HIV vaccine

Autoantigens

Alloantigens

Heteroantigens

Relationship of Antigens to the Host

Autoantigens

antigens that belong to the host.

Alloantigens

antigens from other members of the host’s species and are capable of eliciting an immune response

Heteroantigens

antigens from other species (other animals, plants, or microorganism)

Heterophile antigens

are heteroantigens that exist in unrelated plants or animals but is closely related in structure (crossreact)

Antibodies

are the main element in humoral arm of the adaptive immune response.

-Found on the surface of B cells, where they serve as receptors that specifically recornize foreign antigens to initiate the immune response

Immunoglobulins

another term for antibody. -These molecules are globular proteins that play a role in immunity. -They are glycoproteins that composed of 86% to 97% polypeptide and 2% to 14% carbohydrate. -They constitute approx. 20% of plasma proteins in healthy individuals and can be detected by serum protein electrophoresis.

4 chains

Glycoproteins

Bonds and other forces

STRUCTURE OF IMMUNOGLOBULINS

Heavy chains

Long arms- backbone of the antibody

Light chains

Short arms, together with HC form the paratope. Maintain the over-all Y-shape and structural integrity of antibody molecule

Variable region

Bind to a particular antigen

Constant

Gives signals to immune cells for phagocytosis. Trigger other immune responses to destroy the invader

Hinge

Allows the arms of the antibody to move and spread out

Fragment-antigen binding

Fab

Fragment crytallizable

Fc

Heavy chain

• Composed of 440-550 amino acids • Igs are defined by their heavy chains • Uniform, in comparison, from one antibody to another, within the same isotype

Light chain

• Composed of 220 amino acid residues • Either kappa (65%) or lambda (35%), but never both

Domains

Each type of immunoglobulin is made up of several regions called

Variable region

Constant region

2 types of region

β-pleated sheet

Within each of these domains, the polypeptide chain is folded back and forth forming

Immunoglobulin fold

The folded domains of HC line up with the LC producing a cylindrical structure:

hypervariable regions

Antigen captured within the fold by binding to a small number of AA at strategic locations on each chain is known as

complementarity-determining regions (CDRs

Three small hypervariable regions consisting of approximately 30 amino acid residues are found within the variable regions of both heavy and light chains. Each of these regions, called _______, is between 9 and 12 residues long

Paratope

is determined by the apposition of 6 hypervariable loops, three from each chain

Variable region

located in the first 110 AA of the molecule at the N-terminal

Constant regions

from 111 to C-termina

Hinge region

The segment of heavy chain located between the CH1 and CH2 regions □ Rich in hydrophobic residues and high proline content – allows flexibility of the molecule

Carbohydrate

Localized between the CH2 domains of the two HC

□ Functions: ○ Increase solubility of Immunoglobulins

○ Provide protection against degradation of the molecule

○ Enhance functional activity of the Fc domains

■ Important because glycosylation is critical for recognition of Fc receptors found in phagocytic cells

Gerald Edelman

• Performed analytic ultracentrifugation to separate Igs on the basis of molecular weight; unfolded the IgG molecule with urea and disrupted the S=S bonds with mercaptoethanol

• Indicated that the formula for IgG had to be H2L2

Rodney Porter

• Use of the proteolytic enzyme papain to cleave IgG into fragments Fab and FC

• IgG = Fab and Fc

Alfred Nisonoff

• Use of the proteolytic enzyme pepsin to cleave IgG to produce the fragment F(ab’)2

Pepsin

■ Cleaves below hinge region

■ 2 fragments ( 1 Fab & 1 Fc)

Papain

■ Cleaves above hinge region

■ 3 fragments (2 Fab & 1 Fc)

Isotype

Allotype

Idiotype

Antigenic determinants classification:

Isotype

refers to the H chain that is unique to each immunoglobulin class (determines the Ig class)

Allotype

minor variations(constant regions) of AA sequences present in some individuals of same species but not others (constant region)

Idiotype

variations in variable regions that give individual antibody molecules specificity.

1. CH2 region of IgG is able to bind complement.

2. Important mediator of Opsonization

3. Participates in antibody-dependent cellular toxicity

4. Bind to bacterial toxins and viruses and neutralizes their activity

5. Cross the placenta

6. Participates in agglutination reactions and precipitation

MAJOR FUNCTIONS OF IMMUNOGLOBULIN G (IgG)

IgM

• Macroglobulin- the largest of all Igs, 19 S and with MW of 900,000 da.

IgM

is known as the primary response antibody as it is the first to appear after antigenic stimulation and the first to appear in the maturing infant

IgA

Contains one variable and three constant regions

is an anti-inflammatory agent

IgA2

a dimer found in the respiratory, urogenital, and intestinal mucosa • Appears in breast milk, colostrum, saliva, tears, and swea

Secretory IgA

Synthesized in plasma cells found mainly in mucosal-associated lymphoid tissue and is released as in dimeric form

Secretory component (SC)

attached to the Fc region around the hinge portion of the alpha chains - Serves as a specific receptor for IgA -Once bound (IgA and SC precursor)- taken inside the cell and released at he opposite surface (Transcytosis)

IgD

• Consist of 58 AA.

• Scarce in serum, <0.001% of total Igs.

• It has unusually long hinge region-IgD is more susceptible to proteolysis than other Igs.

• It does not appear to serve as protective function.

Most is found on surface of immunocompetent but unstimulated B lymphocytes

IgE

• Very low concentration in serum (0.0005%)

• The epsilon heavy chain is composed of one variable and four constant domains.

• A single disulfide bond joins Є chain to a light chain, and 2 S=S bonds link the heavy chains to one another

• Most heat-labile. Heating to 56ºC for 30 minutes to 3 hours results in conformational changes and loss of ability to bind to target cells

Primary antibody respon

Produced when host first encounters antigen -On the average, Antibody usually appears in the serum from the 5th to 10th day after antigen injection

Latent period

-aka Induction period -Period after the injection of Ag and before Ab is detected in serum -After the latent period, the titer of the Ab rises, reaches a plateau, and begins to drop

IgM

The 1st antibody class to appear is

Secondary Immune response

aka Memory, Anamnestic, or Booster response • Produced after the host has previously been exposed to an antigen - Promoted by the introduction of the same Ag for the 2nd time

Paul ehrlich

Side chain theory

Felix Haurowitz

Template theory

Niels Jerne

Clonal selection

Macfarlane Burnet

Clonal selection

Side chain theory

Formulated by Paul Ehrlich in the early 1900s • Ehrlich postulated that: 1. Certain cells had specific surface receptors for antigen that were present before contact with antigen occurred. 2. Once antigen was introduced, it would select the cell with the proper receptors, combination would take place. 3. Receptors would break off and enter the circulation as antibody molecules. 4. New receptors would form in place of those broken off, and this process could be repeated. • Laid the foundation for further hypotheses

Template theory

• “Instructive theory” • Formulated by Felix Haurowitz in the early 1930s • Theory states that: 1. Antibody-producing cells are capable of synthesizing a generalized type of antibody 2. When contact with antigen occurs, the antigen serves as molds/templates (alters protein synthesis so that antibody with a specific fit is made) 3. This now-specific antibody enters the circulation. 4. Antigen remains behind to direct further synthesis

Clonal selection

• Supported by Niels Jerne and Macfarlane Burnet in the 1950s • Most acceptable theory Antigen B A B A • Individual lymphocytes are genetically reprogrammed to produce Ig receptor with a single antigen specificity. • This process occurs before contact with antigen, during maturation of the B cells in the bone marrow • During maturation of the T cells in the thymus, antigen specific T cell receptors are generated • Mature lymphocytes that have seeded the lymphoid tissues are prepared to respond to a diversity array of potentially harmful antigens long before the body encounters them

Chromosome 14

All heavy chains are derived from a single region on the long arm of

Monoclonal antibodies

• Derived from a single parent antibody-producing cell that has reproduced many times to form a clone

• Directed against a specific epitope on an antigen