Antigens: Types and Features

Antigens

• Antigens are molecules that trigger an immune response when introduced into the body.
• They bind to antibodies, MHC molecules, or lymphocyte receptors (TCRs and BCRs).
• There are two main types: immunogens and haptens.

Immunogens and Haptens

• Immunogens: Substances that stimulate an immune response on their own.
• Haptens: Substances that cannot induce an immune response by themselves but can do so when combined with a larger molecule, such as a host protein.

Cardinal Features of Antigens

• Immunogenicity: The ability of an antigen to induce an immune response (T cells, B cells, antibodies).
• Antigenicity: The ability of an antigen to bind to the products of the immune response it triggered (lymphocytes or antibodies).

Epitope or Antigenic Determinant

• The specific part of an antigen responsible for inducing the immune response and binding to immune response products (lymphocyte receptors and antibodies).

Sources of Antigens

• Infectious agents (microbial antigens): Bacteria, viruses, parasites, fungi.
• Altered self-antigens: Tumor cells.

Challenges of Parasitic Antigens

• Parasites often have complex life cycles, which can present challenges in developing effective vaccines.
• Example: Malaria parasites have multiple stages in their life cycle, making it difficult to target a single antigen for vaccination.

Non-microbial Antigens

• Cell Surface Antigens:
  • Blood group antigens (A, B, AB, O)
  • Rh Factor
  • MHC class I (found on all nucleated cells)
  • MHC class II (primarily found on antigen-presenting cells).

Factors Affecting Immunogenicity

• Foreignness: The immune system primarily recognizes and targets non-self substances.
• Size: Larger antigens tend to be more immunogenic.
• Chemical composition: Complex molecules with diverse antigenic determinants are often more immunogenic.
• Physical properties: Particulate antigens are generally more immunogenic than soluble antigens, and denatured antigens tend to be more immunogenic than native forms.
• Degradability: Easily phagocytosed and degraded antigens are typically highly immunogenic.
• Genetic factors: Variations in antigen receptor genes can influence an individual's immune response against a specific antigen.
• Age: Young and elderly individuals may have a weaker immune response compared to middle-aged individuals.

Chemical Nature of Antigens

• Proteins: The most common and potent antigens.
• Polysaccharides: Can be strong immunogens.
• Nucleic acids: Weak antigens, but can become highly immunogenic when combined with proteins.
• Lipids: Generally not immunogenic, but some can act as haptens.

Routes of Administration (Vaccine Antigens)

• Per os (oral): Polio vaccine
• Intranasal: FluMist
• Intramuscular: Most vaccines
• Intravenous: Rarely used
• Intradermal: BCG for tuberculosis vaccine
• Subcutaneous: Most vaccines

Adjuvants

• Substances that enhance the immune response to an antigen.

Types of Antigens

• T-independent antigens: Directly stimulate B lymphocytes to produce antibodies without the need for helper T cells.
  • Examples: Lipopolysaccharides
• T-dependent antigens: Indirectly stimulate B lymphocytes to produce antibodies.
  • Helper T cells are required.
  • Examples: Proteins
• Superantigens: Activate a large number of T cells non-specifically.
  • Primarily derived from bacteria or viruses.
  • Can lead to excessive cytokine production and systemic toxicity.
• Haptens: Small, non-immunogenic substances that require conjugation to a carrier molecule to elicit an immune response.
• Autoantigens: Self-antigens that the immune system normally does not recognize. They play a role in autoimmune diseases.
• Exogenous antigens: Enter the body from external sources.
• Endogenous antigens: Generated inside cells, often due to viral or bacterial infection.

This quiz covers the fundamentals of antigens, including their role in the immune response, types such as immunogens and haptens, and key concepts like immunogenicity and antigenicity. Test your understanding of how these molecules interact with the immune system and the importance of epitopes.