Immunology Basics Quiz

Antibodies

• Antibodies are proteins that bind to specific substances called antigens.
• Antibodies are also known as immunoglobulins (Igs).
• Antibodies are secreted by differentiated B cells called plasma cells.

Antigens and Immunogens

• Antigens can bind to antibodies or be recognized by T-cell receptors.
• Immunogens can elicit an adaptive immune response.
• All antigens have the potential to elicit specific antibodies, but some need to be combined with an immunogen.
• All immunogens are antigens, but not all antigens are immunogenic.

Requirements for Immunogenicity

• Substances must possess the following characteristics to be immunogenic:
  • Foreignness
  • High Molecular Weight
  • Chemical Complexity
  • Degradability

Haptens

• Small organic molecules that cannot trigger immune responses on their own.
• Can trigger an immune response when attached to a protein carrier.

Antigen, Epitope, and Paratope

• Epitope is a specific region on an antigen that binds to an antibody.
• Antibodies are specific to epitopes and not the entire antigen.
• Paratope is the region on an antibody that recognizes and binds to the epitope.

Antibody Structure and Fragments

• Antibodies are made of two heavy chains and two light chains.
• The variable region of the antibody determines its antigen specificity.
• Proteolytic enzymes can cleave antibodies into three fragments:
  • Two identical Fab fragments (antigen-binding)
  • One Fc fragment (crystallisable)

Immunoglobulins

• Five different classes of immunoglobulins:
  • IgM
  • IgD -IgG (4 subclasses - IgG1-4)
  • IgA (2 subclasses - IgA 1,2)
  • IgE
• Different antibody isotypes activate diverse effector systems.
• Selected properties of human antibodies can be found in Table 4-1 of Helbert's Immunology for Medical Students 3rd edition.

Selected Features of Human Antibodies

• IgM:
  • Predominant in early immune responses.
  • Has 10 potential antigen-binding sites.
  • Efficient at agglutinating bacteria and activating complement.
• IgD:
  • Involved in B-cell activation.
  • Serum function unknown.
• IgG:
  • Most prevalent in serum.
  • Able to cross the placenta to protect newborns.
• IgE:
  • Evolved to combat parasitic infections.
  • Also involved in allergic reactions.
• IgA:
  • Found in serum and seromucous secretions
  • IgA1 is primarily found in serum.
  • IgA2 is predominantly found in seromucous secretions like saliva, colostrum, tracheobronchial, and genitourinary secretions.

Antibody Variations

• Isotype: Differences in the constant region of the heavy chain, creating the different antibody classes.
• Allotype: Variations between individuals due to minor sequence differences in the heavy or light chains of antibodies.
• Idiotype: Variations in the antigen-binding region of different antibodies.

Antigen-Antibody Interactions

• Affinity: Strength of the interaction between a single antigen-binding site (paratope) and its epitope.
• Valence: Number of molecules an antigen or antibody can bind simultaneously.
• Avidity: Strength with which a multivalent antibody binds to a multivalent antigen.
• Antibodies form multiple non-covalent bonds with antigens like:
  • Hydrogen bonds
  • Electrostatic bonds
  • Van der Waals forces
  • Hydrophobic forces
  • Cation-pi interactions

Specificity, Cross-reactivity, and Non-reactivity

• Reactions can exhibit high specificity.
• Reactions can be cross-reactive, binding to structurally related yet different antigens.

Immune Response and Secretion of Antibodies

• Seroconversion: Phase where antibodies against an infectious agent become detectable in the blood.
• Primary response: First exposure to an antigen. Slow, weak, and short-lived with low antibody levels.
• Secondary response: Second exposure to an antigen. Faster, stronger, and longer-lasting with higher antibody levels.

Antigens Activate Specific Clones of Lymphocytes

• The clonal selection hypothesis explains how specific lymphocytes are activated by antigens.

Postulates of the Clonal Selection Hypothesis

• Each lymphocyte bears a unique receptor.
• Lymphocytes with receptors that bind to antigens are selectively activated.
• Activated lymphocytes proliferate and produce more cells with the same specificity.
• Lymphocytes differentiate to become effector cells and memory cells.

Development of the Antibody Repertoire by Gene Recombination

• The immune system creates billions of antibodies from a limited number of genes.
• The diversity of the antibody repertoire is generated through:
  • Somatic recombination: Rearrangement of DNA segments during B-cell development.
  • Somatic hypermutation: Random mutations in the V region, altering antibody affinity.
  • Class switching: Changes in the C region of the heavy chain, altering antibody function.

V(D)J Recombination Responsible for Initial Antibody Repertoire

• Somatic recombination rearranges segments of genomic DNA within immunoglobulin during B-cell development, independent of antigen exposure.
• The variable region of an immunoglobulin heavy and light chain is encoded by multiple gene segments:
  • Heavy chain: Three segments.
  • Light chain: Two segments.

Secondary Diversification of the Antibody Repertoire - Somatic Hypermutation

• Somatic hypermutation occurs in activated B cells, randomly mutating the V region and diversifying the antibody repertoire.
• It occurs simultaneously with class switching and both processes involve the enzyme Activation Induced Cytidine Deaminase (AID).

Somatic Hypermutation Enables Affinity Maturation

• Affinity maturation is the increased affinity for an antigen caused by somatic hypermutation.
• B-cell receptors with higher affinity for antigens are selected during the immune response.

Secondary Diversification of the Antibody Repertoire - Class Switching

• Class switching alters the C region of the heavy chain, enhancing antibody functional diversity while maintaining antigen specificity.
• Cytokines and T cells have a significant role in class switching.
• Isotype switching is heavily influenced by tissue environment due to different cytokine release by T cells in different areas.

Use of Antibodies as Diagnostic, Therapeutic, and Research Tools

• Antibodies are remarkably useful due to their high specificity and selectivity:
  • Diagnostics: Detection of infections, measurement of biological markers, identification of allergies.
  • Therapeutics: Treatment of cancer, infectious diseases, and autoimmune diseases.
  • Research: Immunohistochemistry, Western blots, Flow cytometry.

Production of Polyclonal Antibodies

• Polyclonal antibodies are generated in natural immune responses or after immunization.

Blood Antibody Levels After Immunization

• IgM levels increase first after immunization.
• IgG levels increase with repeated immunizations.

Preparation of Monoclonal Antibodies

• Monoclonal antibodies are produced in unlimited quantities using:
  • Hybridoma technology: Fusion of an immortal myeloma cell with an antibody-producing B cell.
  • Recombinant technology: Synthetic antibody or antibody fragment produced by recombinant DNA technology.

Learning Objectives

• Describe the basic structure of antibodies.
• Understand the functions and properties of antibodies.
• Define: antibody, antigen, epitope, paratope, immunogen, hapten, affinity, avidity, valency, idiotype, isotype, and allotype.
• Discuss antigen-antibody interactions.
• Discuss the generation of the antibody repertoire.
• Discuss the use of antibodies in therapeutics and diagnostics.
• Discuss the preparation of monoclonal antibodies.

Required Reading

• Antibodies - Chapter 10 - 9th Edition (omit section on Fc receptors and antibody engineering).
• B cell development and the antibody response - Chapter 9 - 9th Edition (read section on somatic hypermutation and class switch recombination).

Additional Reading

• Janeway Immunobiology 9th Edition - Chapter 4 - Antigen Recognition by B and T Cell Receptors.
• Helbert - Immunology for medical students 3rd edition - Chapters 4, 5, and 6.