Immunogens and Antigens Quiz

Immunogens, Antigens, and Antibodies

• Immunogens induce an immune response
• Antigens bind to antibodies or T cells
• All immunogens are antigens, but not all antigens are immunogens

Requirements for Immunogenicity

• Foreignness: Do not respond to self (except in autoimmune diseases). More foreign = more immunogenic
• Size (molecular weight): Larger than 6kDa are generally immunogenic; smaller than 1kDa are not (e.g., penicillin, aspirin)
• Chemical Complexity: Homopolymers of amino acids are not immunogenic
• Degradability: Susceptibility to enzyme breakdown is important for antigen presentation
• Stability: Must be stable enough for antigen presentation
• Dosage: Insufficient doses can lead to tolerance. Repeated administration may be needed for a sufficient response

Hapten

• Low molecular weight; antigen but not immunogenic
• Non-immunogenic by itself
• Conjugation with an immunogenic compound (carrier) creates an immunogen
  • Antibodies to hapten
  • Antibodies to carrier
  • Antibodies to conjugate

Epitopes Recognized by Lymphocytes

• B-lymphocytes recognize epitopes on surface antigens
• T-lymphocytes recognize epitopes on peptides presented by MHC molecules

Major Classes of Antigens

• Carbohydrates (polysaccharides): Immunogenic as part of glycoproteins
• Lipids: Rarely immunogenic
• Nucleic Acids: Generally not immunogenic; seen in systemic lupus erythematosus (SLE)
• Proteins, Glycoproteins: Very immunogenic

Major Classes of Antigens

• One epitope: Examples are haptens
• Many epitopes of the same specificity: Examples include many polysaccharides and homopolymers
• Many epitopes of different specificities: Examples are proteins

Cross-reactivity

• Ability of an antibody specific for one antigen to react with a second antigen
• Common with closely related antigens (ex. toxin and toxoid)

Adjuvant

• Substance mixed with an immunogen to enhance its immunogenicity
• Mechanisms:
  • Physically prolonging antigen persistence/release
  • Causing local inflammation and immune cell proliferation
  • Improving antigen uptake and processing
• Human Adjuvants: aluminum hydroxide/phosphate (FDA approved)
• Animal Adjuvants: Freund's complete adjuvant

Antibodies

• Gamma globulins
• Produced by plasma cells (B lymphocytes)
• Exist in secreted and membrane-bound forms
• Form the B cell receptor on the surface of B cells

Why do we need antibodies?

• Neutralization of toxins/viruses
• Immobilization of bacteria/parasites
• Activation of complement
• Initiation of opsonization
• Antibody-dependent cell-mediated cytotoxicity (ADCC)

Antibody Structure

• Antigen binding, located in hypervariable regions
• Light chain
• Heavy chain
• Disulfide bonds
• Fc region

Hypervariable Regions

• Highly variable amino acid sequence
• 3 Hypervariable regions in light and heavy chains

Five Major Classes of Antibody

• IgM, IgG, IgA, IgD, IgE
  • Detailed information about each isotype is included in separate tables regarding; -molecular weight- -half life- -distribution- -important features

Distribution of various Ig isotypes in body fluids

• Shows distribution of antibody in serum and secretions.

IgG- Four Subclasses

• IgG1, IgG2, IgG3, IgG4.
• Specific details about the features of each.
• Includes placental passage
• Opsonization
• Fc receptor

IgG - Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

• Activation of complement
• Toxin/viral neutralization
• Bacterial immobilization

IgG - Antibody Levels Early in Life

• Shows the progression of antibody levels in early human development, from conception to adulthood.

IgM

• First immunoglobulin produced after infection or immunization
• Most efficient immunoglobulin for complement fixation

IgA

• Primarily found in secretions (tears, saliva, sweat, mucus)
• Critical component of respiratory and mucosal surfaces (first line defense)
• Roles include antibacterial and antiviral activity

IgE

• Mediates type I hypersensitivity reactions
• High levels seen during parasitic infections

Test your knowledge on immunogens, antigens, and antibodies. This quiz covers the requirements for immunogenicity, characteristics of haptens, and more. Understand the distinctions between immunogens and antigens and how they interact with the immune system.