Immunology: Antibodies and Antigens

Antibodies

• Antibodies are proteins that bind specifically to antigens.
• Antibodies belong to the class of proteins called immunoglobulins (Igs).
• Antibodies are the secreted form of the B-cell receptor.
• Antibodies are secreted from differentiated B cells called plasma cells.

Antigens and Immunogens

• Antigens are molecules that can bind specifically to an antibody or generate peptide fragments recognized by T-cell receptors.
• Immunogens are substances that can elicit an adaptive immune response.
• All antigens have the potential to elicit specific antibodies, but some need to be attached to an immunogen.
• All immunogens are antigens, but not all antigens are immunogenic.

Requirements for Immunogenicity

• A substance must possess the following characteristics to be immunogenic:
  • Foreignness
  • High molecular weight
  • Chemical complexity
  • Degradability

Haptens

• Small organic molecules of simple structure that fail to induce immune responses.
• Immune response can be provoked if a hapten is attached to a protein carrier.

Antigen, Epitope and Paratope

• Epitope is a part of an antigen that binds to an antibody or an antigen receptor.
• Antibodies are specific for epitopes rather than the whole antigen.
• Paratope is the part of an antibody that recognizes and binds to the antigen (antigen binding site).

Antibody Structure

• Antibodies are composed of two types of protein chains:
  • Heavy chains (2)
  • Light chains (2)
• Antigen binding region varies extensively between antibody molecules and is known as the variable or V region.

Antibody Function

• Antibodies act as adaptor molecules for immune effector systems.

Fc Receptors

• Fc receptors (FcRs) are expressed on a number of different cells and bind the Fc region of antibodies:
  • Fcγ receptors on macrophages and neutrophils bind to the Fc portion of IgG.
  • Fcα receptors on myeloid cells bind to the Fc portion of IgA.
  • Fcε receptors on mast cells, basophils and activated eosinophils bind to the Fc portion of IgE.

Antibody Participation in Host Defenses

• Neutralization: Binds to pathogens and blocks access to cells.
• Opsonization: Coating of pathogens and foreign particles.
• Complement Activation: Activation of the classical complement pathway.

Neutralizing Antibodies and SARS-CoV-2

• Neutralizing antibodies bind to Spike proteins on the surface of SARS-CoV-2, preventing the virus from binding to and entering the host cell.

Phagocytosis

• Phagocytosis of bacteria is greatly enhanced in the presence of antibodies.

Immune Response and Secretion of Antibodies

• Seroconversion: The phase of an infection when antibodies against the infecting agent are first detectable in the blood.

• Primary response: First exposure to an antigen.

  • Slow, sluggish, and short-lived.
  • Low levels of antibodies.

• Secondary response: Second exposure to an antigen.

  • Rapid, strong, and long-lasting.
  • High levels of antibodies.

Antigens Activate Specific Clones of Lymphocytes

• The clonal selection hypothesis:
  • Each B cell carries a single type of antibody receptor.
  • When an antigen binds to the B cell receptor, it activates that B cell to proliferate and differentiate into plasma cells that secrete antibodies.

Fighting Infection by Clonal Selection

• The clonal selection hypothesis is a key concept in understanding how the adaptive immune system fights infection.

Development of the Antibody Repertoire by Gene Recombination

• The immune system creates billions of different antibodies with a limited number of genes.
• Diversity of the antibody repertoire is generated by:
  • Somatic recombination
  • Somatic hypermutation
  • Class switching

V(D)J Recombination is Responsible for the Initial Antibody Repertoire

• Somatic recombination: Rearrangement of segments of genomic DNA within immunoglobulin during B cell development in the absence of antigen.

Secondary Diversification of the Antibody Repertoire - Somatic Hypermutation

• Somatic hypermutation: Random mutation that occurs in the V region and diversifies the antibody repertoire.
• Alters the affinity of the antibody for antigens.
• Occurs at the same time as class switching, both involving the enzyme activation-induced cytidine deaminase (AID).

Somatic Hypermutation Enables Affinity Maturation

• Affinity maturation: Results in the increased affinity for an antigen.
• BCRs are selected for progressively higher affinity for an antigen during an immune response.

Secondary Diversification of the Antibody Repertoire - Class Switching

• Class switching: Involves the C region of the heavy chain only and increases functional diversity of antibodies.
• Antigen specificity is retained.
• Cytokines and T cells play a major role in class switching.
• Isotype switching is greatly affected by the tissue environment:
  • Partly due to different cytokines released by T cells at different sites.

Use of Antibodies as Diagnostic, Therapeutic, and Research Tools

• Due to their high specificity and selectivity, antibodies are of great use in:
  • Diagnostics: Detection of infections, measurement of biological markers, recognition of allergies.
  • Therapeutics: Treatment of cancer, infectious diseases, autoimmune diseases.
  • Research: Immunohistochemistry, Western blots, flow cytometry.

Types of Antibodies

• Polyclonal antibodies: Antibodies generated in a natural immune response or after immunization.
• Monoclonal antibodies: Antibodies produced by a single clone of B lymphocytes, resulting in a homogeneous population of antibodies.

Production of Polyclonal Antibodies

• Polyclonal antibodies are produced by immunizing an animal with an antigen.

Blood Antibody Levels After Immunization

• After immunization, IgM levels initially increase.
• With repeated immunization, IgG levels increase.

Preparation of Monoclonal Antibodies

• Production of monoclonal antibodies:
  • Hybridoma technology: Fusion between immortal myeloma cell and an antibody-producing B cell.
  • Recombinant technology: Synthetic antibody or antibody fragment generated by recombinant DNA technology.

Learning Objectives

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