Immunogen vs. Antigen Quiz

Immunogen vs. Antigen

• An immunogen induces an immune response, while an antigen binds to specific antibodies.
• Immunogens are typically larger and more complex than antigens.
• All immunogens are antigens, but not all antigens are immunogens.

Features that determine immunogenicity

• Recognition of foreignness: The immune system responds to substances that are not part of the body.
• Size: Larger antigens (over 10,000 Daltons) tend to elicit stronger immune responses.
• Chemical and structural complexity: More complex antigens trigger stronger immune responses.
• Genetic constitution of the host: Individuals with different genes may respond differently to the same antigen.
• Dosage, route, and timing: The amount of antigen, the way it enters the body, and the timing of administration affect immune response strength.

Factors that increase or decrease immunogenicity

• Size: Larger proteins are more immunogenic than smaller ones.
• Dose: Moderate amounts of protein are more immunogenic than very high or very low amounts.
• Route: Subcutaneous or intraperitoneal injections are more immunogenic than intravenous or intragastric administration.
• Composition: Complex proteins are more immunogenic than simple proteins.
• Form: Particulate or denatured proteins are more immunogenic than soluble or native proteins.
• Similarity to self-proteins: Proteins that are very different from the body’s own proteins are more immunogenic.
• Adjuvants: Slow protein release or the presence of bacteria enhance immunogenicity.
• Interaction with MHC: Effective interaction with MHC molecules increases immunogenicity.

Classification of Antigens based on Chemical Nature (Structural)

• Proteins: Major type, highly immunogenic (e.g., enzymes, structural proteins).
• Polysaccharides: Often found on the surface of bacteria (e.g., capsular polysaccharides).
• Lipids and nucleic acids: Less immunogenic, but can elicit responses in certain contexts.

Classification of Antigens based on Origin

• Exogenous antigens: Come from outside the body (e.g., pathogens).
• Endogenous antigens: Originate from within the body (e.g., cancer cells, autoantigens).

Epitope

• Antigenic determinant: Small, specific region of an antigen recognized by an antibody.
• Size: Typically 4-5 amino acids or monosaccharides.
• Types:
  • Topographical: Located on the surface of the antigen.
  • Internal: Exposed after processing by antigen-presenting cells (APCs).
• Valency: Number of epitopes on an antigen (can vary from one to hundreds).
• Configuration: The shape of an epitope can be altered by interactions with other substances.

Autoantigens

• Normal proteins/protein complexes: Recognized by the immune system in autoimmune diseases.
• Loss of immune tolerance: Autoantigens are normally not targeted by the immune system, but tolerance may be lost due to genetic and environmental factors.

Tumor Antigens

• Presented by MHC I on the surface of tumor cells.
• Tumor-specific antigens (TSAs): Presented only by tumor cells, often the result of mutations.

Superantigens

• Trigger intense immune responses by activating large numbers of T cells.
• Bind directly to MHC class II molecules and T cell receptors (TCRs) outside of the conventional antigen-binding site.
• Activate up to 20% of T cells, leading to massive cytokine release.
• Examples: Staphylococcal enterotoxins (SEs), Toxic shock syndrome toxin-1 (TSST-1).

Superantigen Mechanism of Action

• Binding to MHC and TCR: Bypass the need for antigen processing and activate T cells directly.
• Massive T cell activation: Lead to activation of a larger proportion of T cells than conventional antigens.
• Cytokine release: Release of pro-inflammatory cytokines (IL-2, TNF-α), causing systemic inflammatory responses, toxic shock syndrome, and autoimmune reactions.
• Potential for immunopathology: Can cause tissue and organ damage, leading to severe clinical manifestations.

Haptens

• Small molecules that are not immunogenic on their own.
• Become immunogenic when combined with carrier proteins, forming a complete antigenic structure.
• Characteristics:
  • Low molecular weight (usually < 1000 Da).
  • Derived from various chemical structures (e.g., small organic molecules, peptides, drug metabolites).
• Example: Penicillin.